Introduction

Trap cropping (TC) is a pest control strategy where the pest species is diverted from the main crop with attractive plants and controlled there (Hokkanen 1991; Shelton and Badenes-Perez 2006). At the end of the 1960s, Lygus spp. were the first crop pests that were successfully controlled with the help of TC on cotton (Stern et al. 1969). These plant bugs were shown to prefer lushly growing alfalfa over cotton, and strips of this crop interspersed in cotton fields virtually eliminated the need to spray the main crop for Lygus control (Stern et al. 1969). In recent years, interest in trap cropping has increased when sustainable pest control methods are preferred to reduce the use of synthetic pesticides (Li et al. 2024). Lygus bugs (Hemiptera: Hemiptera: Miridae) are known to be polyphagous species that could be severe pests on dozens of different crops (Young 1986; Holopainen and Varis 1991). Therefore, trap cropping, using a crop plant higher on Lygus preference scale, could be one solution to control Lygus bugs on crop plants (Swezey et al. 2013; Zhang et al. 2020a, b). When implementing area-wide pest management, it will be important to note that natural vegetation surrounding agricultural fields, and weeds in the field ecosystems, are important alternative food sources for Lygus bugs and other Mirids (Lu et al. 2024). In addition, omnivorous Lygus species prefer to use other pest insects, such as aphids and moth larvae, as supplemental food (Woelke et al. 2023) and their availability could change the host plant preference of Lygus bugs.

When applying natural and biologically sustainable pest control methods, it is necessary to understand the biology of the target organism. Therefore, we first review the current knowledge of the biology of Lygus bugs and then analyse the environmental and host plant-related factors affecting host plant selection by Lygus adults and nymphs, and how these factors influence the importance of Lygus bugs as plant pests. Finally, we summarise the TC studies that have shown the efficiency of TC in Lygus pest control in field experiments.

Diversity and distribution of Lygus bugs

Mirids (Hemiptera: Miridae) or capsid bugs, also known as plant bugs or leaf bugs, comprise the largest family in the Heteroptera suborder of Hemiptera with the over 11,000 known species (Rintala and Rinne 2010; Cassis and Schuh 2012). The genus Lygus has 51 described species, including 29 Nearctic, 20 Palaearctic, and 2 Holarctic species (Schwartz and Foottit 1998). Typically, Lygus species are highly polyphagous, but some species are specialised on a few plant species, such as L. gemellatus, which prefers to feed on mugwort (Artemisia vulgaris) and related Artemisia spp. that are growing in ruderal sites (Rintala and Rinne 2010). However, despite the known habitat specificity of L. gemellatus, it is often found amongst other, more abundant Lygus species on several field crops (Racz and Bernath 1993; Varis 1995).

In agricultural ecosystems, the most economically important species are the tarnished plant bug L. lineolaris (Palisot de Beauvois) in the whole of North America from Canada to Mexico (Scholl and Medler 1947; Young 1986), and the western tarnished plant bug L. hesperus Knight in the western parts of North America from Alaska to Mexico (Schull et al. 1934; South 1991; Zhou et al. 2012), except that L. lineolaris dominates on canola fields in British Columbia and Alberta (Carcamo et al. 2002). The European tarnished plant bug L. rugulipennis is considered as the most common and the most widely spread economically important Lygus species (Saulich and Musolin 2020). It is found in most of Europe (Holopainen and Varis 1991; Woelke et al. 2023), in Asia south to Turkey (Kaya 2018) and Iran (Ghahramani et al. 2019), and in the whole of Russia to Japan (Saulich and Musolin 2020), and also reported from northern Canada (Schwartz and Foottit 1998). As the nearly identical common names indicate, these three species are morphologically and visually quite similar, difficult to identify, have a large host plant range, damage various crops, and are occasionally highly abundant within their distribution range (Saulich and Musolin 2020). Compared to previous species, L. pratensis looks shinier due to less pubescent forewings and has some reddish hue, and adult bugs are slightly longer (5.8–7.3 mm vs. 4.7–5.7 mm) than L. rugulipennis (Rintala and Rinne 2010). L. pratensis is distributed in most of Europe and Northern Asia, south to India (Saulich and Musolin 2020) and reported as an important and abundant pest of Bt cotton in the Northwestern region of China (Lu et al. 2008; 2024). Another shiny species is L. punctatus which is typically found on conifers in dry sandy soils (Rintala and Rinne 2010) and only occasionally on crop plants (Varis 1995). Similar feeding damage on crop plants as caused by Lygus species can be produced by some related species in the subfamily Miridae belonging to the genera Adelphocoris, Apolygus, Lygocoris, and Stenotus (Saulich and Musolin 2020; Lu et al. 2024). The species in these genera have similar life habits as Lygus species, except that hibernation takes place at the embryonic stage whilst Lygus bugs hibernate at the adult stage (Saulich and Musolin 2020).

Life cycle

The life cycle of Lygus and other mirid bug species is often related to two or more different habitats and annual or sequential migration between these (Lu et al. 2024). Adult mirids are capable of flying up to 25–45 km per day in search of suitable feeding and breeding sites (Lu et al. 2024). Lygus nymphs can walk up to 50 m when searching for a new host plant (George et al. 2023). There is also variability in the number of generations per year (Saulich and Musolin 2020). For example, Lygus spp. in Finland have only one annual generation (Varis 1972), two in Oregon (Schowalter 1987), three in Kazakhstan, and four in Spain and North-Western China (Saulich and Musolin 2020). In northern Europe, Lygus spp. hibernate at the adult stage with underdeveloped ovaries (Saulich and Musolin 2020). L. rugulipennis uses forest litter habitat for overwintering under snow, and in winters with thin snow cover and extremely cold weather conditions, mortality can reach over 80% (Varis 1972). In late May, when the weather is warm enough, flying adults of L. rugulipennis migrate to open field sites where the whole reproductive period takes place on herbaceous plants. Mated females drill their eggs into the soft tissues of herbaceous plants (Holopainen 1989; Romani et al. 2005) but also of conifer seedlings (Holopainen 1986). Female of L. lineolaris can lay more than 100 eggs in optimal conditions (George et al. 2021). In warmer climatic areas, the hibernating sites of Lygus adults during cold or dry periods can be variable. For example, L. lineolaris in Quebec, Canada, can overwinter in perennial strawberry fields or on wild plants such as mullein (Verbascum thapsus) (Dumont and Provost 2022). Snodgrass (2003) found that L. lineolaris in Mississippi does not have full hibernation. Still, part of the population has about one-month diapause in plant debris, and part of the population is winter-active on the winter host plant and starts oviposition about one month earlier on wild plants.

Lygus species have five nymphal instars (George et al. 2021). The duration of each instar of L. rugulipennis at + 20 °C is 5–8 days, and the whole nymphal period is 38–40 d (Varis 1972), whilst Lygus developmental rate seems to be species-dependent (Carcamo et al. 2006). Nymphs can cause similar damage to the seedlings of small host plants as adults (Varis 1972; Holopainen 1986; Xu et al. 2014). The mortality of nymphs is high, reaching over 80% in the first instar of L. rugulipennis (Varis 1972). High nymphal mortality can be partly a result of cannibalism amongst omnivorous nymphs in dense Lygus populations (Hagler et al. 2020).

Feeding behaviour and plant damage

As omnivorous species, Lygus spp. can act as a carnivore and predate and feed on other insects, or act as an herbivore and suck plant sap. However, Lygus species are not sap feeders in the sense of “real” sap feeders that feed passively on phloem sap like aphids (Walker 2022) or actively on xylem sap like spittlebugs (Bergman et al. 2021). Lygus nymphs and adults have piercing-sucking mouthparts and they can actively ingest even individual plant cell contents (Tuelher et al. 2020). When feeding on plants, Lygus species prefer “soft” meristematic cells in plant reproductive organs, and actively growing vegetive growing points, such as the apical meristem (Varis 1972). The behaviour of adults and nymphs could be different when choosing insect or plant food. For example, L. pratensis adults preferred to feed more on moth eggs and aphids than on bean pods when insect food was available, whilst nymphs still preferred bean pods (Ma et al. 2023). The known cannibalism of Lygus adults (Hagler et al. 2020) suggests that an herbivorous nymph may develop into an adult bug that, in some circumstances, can predate on Lygus nymphs and other herbivores as a biocontrol organism.

Adults and nymphs of Lygus damage the host plant tissues by their sucking-piercing mouth parts directly causing mechanical damage (Varis 1972; Laurema et al. 1985; Holopainen 1990) and indirectly through cell rupturing by secreting saliva that contains polygalacturonases, α-amylases, and proteases to degrade pectin of the plant cell walls, leading to the collapse of plant cells (Laurema et al. 1985; de la Paz Celorio-Mancera et al. 2008; Cooper et al. 2013; Tuelher et al. 2020). Additional damage, particularly on young seedlings, is caused by the ovipositor when female bugs insert eggs inside plant tissue so that only the opening end of the egg is visible (Holopainen 1986; Manninen et al. 1998). Feeding damage severity on the host plant depends on the plant growth stage during bug feeding as well as on the duration of feeding. For example, on small sugarbeet seedlings, L. rugulipennis feeding on the apical meristem can cause stunted and deformed growth, the development of multiple leaders, or the death of the seedling (Varis 1972). Seedling mortality was highest when fed on the cotyledon stage, but the development of multiple crowns was higher on 1–2 leaf stage seedlings, on which the feeding time per plant was nearly fourfold when compared to time on seedlings at the cotyledon stage (Varis 1972). Nymphal instars spend less time on ingestion and more time on cell rupturing than adults, probably due to the smaller amounts or less effective enzymatic saliva (Tuelher et al. 2020).

Typical for Lygus feeding damage is the delay between active feeding and the development of visible damage symptoms such as the formation of lateral buds and new shoots after damage to the apical meristem of small seedlings (Holopainen 1986), although some wilting can be observed just after feeding (Varis 1972; Holopainen 1986). The final cellular damage, such as necrosis of buds, deformed growth, and multiple leaders and crowns, will become visible several days after the actual damage. This has led to difficulties in defining the reason for the damage because the causal agent of the tissue damage and other symptoms might have moved to another plant community. For example, Lygus feeding on developing strawberry fruit prevents proper development of fruit tissue surrounding the achene (seed) (Handley and Pollard 1993). This will result in nearly identical deformation of strawberry fruits, as can result from poor pollination. However, Lygus feeding damage does not affect seed size, whilst in poorly pollinated areas of fruit, seeds are smaller than in healthy areas (Allen and Gaede 1963). Another example is the identification of Lygus feeding damage on small conifer seedlings (Holopainen 1986; South 1991). In experimental conditions, it took at least one week before the feeding damage on the apical meristem of young Scots pine seedlings became visible (Manninen et al. 1998) and later resulted in multiple-leader seedlings.

Host plant diversity

Species of the Lygus genus have one of the most diverse host plant diets known for insects. In the search of the world's most polyphagous insect herbivore species at the host plant species or subspecies level, two Lygus species were amongst the top nine (Thompson et al. 2023). When counting the reported host plant species in documented scientific or web sources, the top three species are the meadow spittlebug [Philaenus spumarius (L.)], the fall webworm [Hyphantria cunea (Drury)], and the brown soft scale (Coccus hesperidum L.), with 1311, 636, and 552 host plant species, respectively. L. rugulipennis had 402 and L. lineolaris had 333 in the same listing of host plant species (Thompson et al. 2023). The most preferred plant families for L. lineolaris are Asteraceae, Brassicaceae, and Fabaceae (Young 1986) and for L. rugulipennis, Brassicaceae, Asteraceae, and Fabaceae (Holopainen and Varis 1991).

Host plant choice by Lygus bugs

Visual cues

Visual cues play a crucial role in the host plant location for insects across various phytophagous orders (Reeves 2011). Recent studies, such as those on L. rugulipennis, highlight the significance of vision over olfaction in certain insect species (van Tol et al. 2022; George et al. 2023; Hetherington 2023). These studies emphasize the interplay between sensory modalities and show the dynamic nature of insect host-finding strategies. In fact, the ability of many insects to visually discriminate between host plant species indicates the importance of considering visual cues in studies of host finding (Reeves 2011). It is increasingly evident that insects likely use a combination of visual and olfactory cues to enhance their efficiency in locating hosts and mates (Pan et al. 2015a, b, c; George et al. 2020). Compared with either olfactory or visual stimuli alone, combining the visual and olfactory cues may augment the accuracy of discriminating between hosts from nonhosts, mates from non-mates, and specific parts of host plants. Synergistic responses of insects to host plants and mates using visual, olfactory, and tactile cues have been reported in the literature (Fukaya et al. 2005; Burger et al. 2010; Milet-Pinheiro et al. 2012; George et al. 2020).

The utility of visual cues for Lygus bugs is complicated by their dispersal and flight behaviour. Many Lygus spp. show a pronounced crepuscular flight behaviour, migrating at dusk or during the night. For example, Šedivý and Honěk (1983) concluded that the migration flights of L. rugulipennis appear to be between 6 pm and 1 am. For other related species, such as L. hesperus, peak flight was found to be at dusk (Butler 1972; Mueller and Stern 1973; Shrestha et al. 2022), or between 3 and 7 pm (Blackmer et al. 2008), with the most active flight occurring at 2.5 m above the soil surface (Butler 1972). However, for L. lineolaris in Canadian strawberry fields, Rancourt et al. (2000) found that the peak period of flight occurs usually at midday. However, in June, the peak was at dusk or at night (10 pm–6 am) (Rancourt et al. 2000). In higher latitudes, like in Finland, the nights are still rather cold after snowmelt, and therefore, in April and May, L. rugulipennis adults emerge from the litter in the warmest day-time hours (Varis 1972), and spring migration to nursery fields also takes place on warm sunny days (Holopainen and Rikala 1990).

Challenges arise in utilizing visual cues under low light conditions. Insects must ensure adequate photon absorption, efficient conversion into neural signals, and appropriate processing of visual information (Honkanen et al. 2017). Insects have evolved two basic compound eye types, apposition and superposition, that collect light in different ways. An apposition eye is well suited for vision in bright light, whilst superposition eyes are typically found amongst nocturnal insects. For excellent reviews on the problems and adaptations related to vision in dim light, see e.g., Honkanen et al. (2017), Warrant (2017), and Kelber et al. (2017). An extreme example is the tropical nocturnal bee Megalopta genalis, which has excellent night vision and can visually navigate to and from its nest in a dark rainforest, when fewer than five photons are absorbed by each of its photoreceptors every second (see Warrant 2017).

To our knowledge, photoreceptors in Lygus have not been studied, and in general, studies on vision in Hemiptera remain comparatively scarce, particularly in comparison to more extensively studied insect orders such as Hymenoptera, Diptera, and Lepidoptera. For examples, see Mahot et al. (2020), who found vision studies in Hemiptera only for aphids, backswimmers Notonecta spp., the stink bug Nezara viridula, and some psyllids. These studies demonstrate trichromatic sensitivity, with distinct peaks in UV, blue, and green spectra, enabling discrimination of subtle variations in visual cues (Fennell et al. 2019; Mahot et al. 2020). The majority of Hemiptera use green:blue opponency or green:(blue + UV) opponency to locate host plants, as foliage reflectance peaks around 550 nm (Chittka et al. 1994).

Although few studies have evaluated spectral sensitivity amongst Hemiptera (Döring and Chittka 2007; van der Kooi et al. 2021), few insects possess photoreceptors that are maximally sensitive to red wavelengths (620–700 nm; Briscoe and Chittka 2001, van der Kooi et al. 2021). However, the green sensitive photoreceptors present in most insects exhibit modest sensitivity up to approximately 650 nm (Chittka and Waser 1997), wavelengths well within the red spectrum. Differences in photoreceptor sensitivity to light in the green and red ranges likely lead red wavelengths to be perceived as low-intensity green (Chittka and Waser 1997).

Identification and optimization of colour cues is important in the design and development of efficient traps, including trap crops, that combine visual and olfactory cues (Avé et al. 1978). Using colour as a visual cue is usually effective from a distance if the trap is large enough to be easily detected by insect eyes (Miller and Strickler 1984). Studies with Lygus bugs have demonstrated attraction to various colours, but the response appears to be species specific and, in some cases, habitat dependent (Blackmer et al. 2008).

L. lineolaris: Prokopy et al. (1979) found that non-UV-reflecting gloss white, zinc (Zn) white, Zoecon yellow, and clear Plexiglas rectangles captured equivalent numbers of L. lineolaris, adults, but significantly greater numbers than other hues of yellow, green, orange, blue, red, aluminium foil, black, and lead (Pb)-white rectangles. In peach orchards, Legrand and Los (2003) found that L. lineolaris was captured in higher numbers on pink sticky traps than on white traps, possibly due to their resemblance to peach flower petals. George et al. (2023) found in a field experiment that colour red was almost 16 times more attractive than white and nine times more attractive than blue or yellow to L. lineolaris adults. Previously, Prokopy et al. (1979) reported that darker colours were less attractive to L. lineolaris compared with non-UV reflecting white, yellow, and clear painted plexiglass rectangles, and Blackmer et al. (2008) found no clear differences in the numbers of L. lineolaris and L. hesperus caught on a range of different coloured traps. In this study, the majority of L. lineolaris collected on the sticky cards were males, and the male:female ratio was 9:1 for all the insects trapped on the different coloured sticky cards. This male bias has also been reported for L. lineolaris by Prokopy et al. (1979) and Blackmer et al. (2008), and for L. hesperus by Blackmer et al. (2008).

L. rugulipennis: Holopainen et al. (2001) found blue sticky traps much more attractive to L. rugulipennis than yellow sticky traps, as observed later by Blackmer and Byers (2009) with L. elisus. Using different coloured light sources in the greenhouse environment, van Tol et al. (2022) determined that the attraction of L. rugulipennis to different light sources was significantly affected by the wavelength spectrum and light intensity. There was no significant difference in the response of male and female L. rugulipennis to the differently coloured light sources. The visual response was largely restricted to wavelengths in the UV‐A/violet range, within which no specific wavelength was superior (van Tol et al. 2022). Other wavelengths in the visual range of 470–720 nm elicited a much lower response from the bugs. Amongst the two intensities examined, the bugs were more attracted by the higher intensity within the wavelength range of 365–420 nm. However, light intensity did not have a significant impact on attraction within the visual range of 470–720 nm (van Tol et al. 2022).

Concerning other Lygus species, Landis and Fox (1972) examined the responses of L. hesperus and L. elisus to coloured water-pan traps and found that significantly more Lygus spp. were trapped in light orange-yellow and deep chrome-yellow pans than in green, red, or pink water traps. Reflecting coloured traps are considered less appropriate during dusk because coloured traps rely on sunlight reflection, which is absent or limited in dusk conditions (Šedivý and Honěk 1983; Nowinszky and Puskás 2014). Therefore, brightness may likely explain the preference for certain colours by dusk flying insects (van Tol et al. 2021), even though many night insects appear to have colour vision (Kelber et al. 2017).

To our knowledge, all research concerning colour preferences of the various Lygus species has focused on physical traps, neglecting the potential influence of host plant coloration on trap crop attractiveness. Future studies should explore how properties such as colour, pattern, and shape of host plants or selected trap crop plant species can be manipulated to enhance their attractiveness to Lygus.

Olfactory cues

Host plant volatiles

Mobile herbivorous insects, such as Lygus bugs, commonly utilize plant-produced chemical cues, such as volatile organic compounds (VOCs), to locate their plant hosts (Bernays and Chapman 1994; Bruce et al. 2005). Exploiting this fundamental behaviour through the use of volatile cues can serve as a method for controlling economically significant insect pests using trap crops (Hokkanen 1991; Shelton and Badenes-Perez 2006). Lygus bugs have exhibited responses to various plant-produced compounds by orienting towards the sources of these compounds, indicating the significance of VOC blends in their host-finding behaviours in natural settings (Blackmer et al. 2004; Blackmer and Cañas 2005; Williams III et al. 2010). Moreover, herbivorous insect feeding can induce changes in the volatile blends released by their host plants, both locally at the feeding site and systemically in other parts of the plant (Röse et al. 1996; Paré and Tumlinson 1997;Rodriguez-Saona et al. 2002; Dicke and Van Loon 2000). These induced alterations in the volatile blend can render hosts either more attractive or less attractive to conspecifics (De Moraes et al. 2001; Blackmer et al. 2004).

For example, the volatiles emitted by plants and conspecifics influence the behaviour of L. rugulipennis, providing information to both sexes regarding the presence of suitable host plants that have been colonized by other conspecifics acting as pioneers, or indicating the presence of an already exploited host plant (the presence of eggs), thereby mitigating competition (Frati et al. 2008; Wynde and Port 2012). Males can also utilize this information to increase the probability of encountering mature females.

Mirids demonstrate volatile-mediated preferences for flowering plants. Throughout their life cycle, individual mirids undergo extensive host plant switching, with adults tracking a succession of flowering plant species in the agricultural landscape (Pan et al. 2015a, 2013a; 2013b). Adults of these mirids are predominantly found in the flowering stages of their preferred hosts, with host selection mediated by infochemicals (Pan et al. 2015b; Xiu et al. 2019). Exploiting mirid host plant use and habitat switching can be employed for preventative pest management strategies aimed at disrupting their annual life cycle. For example, the feeding preferences of Ap. lucorum served as the basis for designing trap crops using mung beans. Additionally, for other mirid species, strategically deploying flowering plants in or near cropping fields can divert colonization flows, enable spatially targeted interventions, and provide foraging resources for resident natural enemies (Li et al. 2019).

Several dozen plant-produced VOCs have been identified to attract various species of Lygus bugs (Table 1). Although typically, each species exhibits a set of approximately 5–10 specific VOCs identified in studies conducted thus far, it may be assumed that most of these compounds elicit a behavioural response in all the Lygus species considered. These attractive compounds include typical Green Leaf Volatiles (GLVs) such as (E)-3-hexen-1-ol, (Z)-3-hexen-1-ol, 1-hexenol, (E)-2-hexenyl acetate, and (E)-2-hexenal, as well as terpenes such as (E)-ß-ocimene, α-farnesene, and linalool (Williams III et al. 2010; Chen et al. 2010). However, after feeding damage by chewing herbivores, rapid emission bursts of some GLVs can be detected in minutes from plant leaves, whilst the induction of terpene emissions gradually increased during the herbivore feeding period (Maja et al. 2014). These induced compounds may influence, e.g., herbivorous moth females to avoid ovipositing on the “reserved” plants and possibly to avoid pressure from natural enemies (Merey et al. 2013). Many parasitoids and predators use induced GLVs and other induced compounds as orientation cues to find herbivore hosts (De Moraes et al. 1998; Dicke and van Loon 2000). The repellent effects of the GLVs on the orientation behaviour of L. hesperus males and females were shown by Williams III et al. (2010).

Table 1 Plant volatile organic compounds (VOCs) reported to affect behaviour of mirid bugs (Lygus rugulipennis, L. lineolaris, L. hesperus, L. pratensis and Apolygus lucorum)
Full size table

Behavioural responses in Ap. lucorum adults were elicited by volatile compounds extracted from 18 preferred host plants (Pan et al. 2015b). These volatiles reached concentrations 2–8 times higher during the flowering stage compared to the vegetative phase. Amongst them, only four compounds—m-xylene, butyl acrylate, butyl propionate, and butyl butyrate—strongly attracted adults in multi-year laboratory and field trials. These fragrant volatiles are emitted in greater amounts once plants begin to flower and appear to mediate A. lucorum’s preference for flowering host plants (Pan et al. 2015b).

In some cases, an interaction between sex pheromones and plant-produced VOCs has been observed. For example, Cross et al. (2008) found that for L. rugulipennis, the combination of sex pheromone and bean plant odour enhanced attractiveness to insects, suggesting an interaction between plant odour and pheromone. In another study, male and female L. rugulipennis were attracted to ß-caryophyllene but not in the presence of sex pheromone, whereas both sexes were attracted to pentyl butyrate in the presence of sex pheromone (Wynde and Port 2012).

Sex pheromones

Mirids depend on volatile compounds for sexual communication. The discovery and synthesis of their sex pheromones have enabled volatile-based mass trapping and population monitoring techniques for Lygus bugs (Yasuda and Higuchi 2012; Zhang et al. 2020a, b, 2021). Utilizing pheromone-based trapping has now become a cost-effective approach to determine the initiation of crop infestation and to evaluate the population dynamics of mirid insects at various scales, whether at the field, farm, or landscape levels (Lu et al. 2024). The potential and feasibility of using Lygus sex pheromones to enhance the attractiveness of trap crops remains to be determined.

All Lygus species covered in this review produce sex pheromone blends consisting of three compounds: hexyl butyrate, (E)-2-hexenyl butyrate, and (E)-4-oxo-2-hexenal (Fountain et al. 2014, 2017). These three compounds are emitted in species-specific ratios and are produced solely by female insects (see Table 2).

Table 2 Sex pheromone components identified for five different Lygus species (L. rugulipennis, L. lineolaris, L. hesperus, L. pratensis and Apolygus lucorum)
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Virgin females of L. rugulipennis emit these compounds in a ratio of 1.5:1:0.08 (Cross et al. 2008). In the case of L. lineolaris, a high abundance of (E)-4-oxo-2-hexenal is crucial for attraction (George et al. 2021), and an attractant lure expressing a ratio of 4:10:7 of hexyl butyrate, (E)-2-hexenyl butyrate, and (E)-4-oxo-2-hexenal was found to be most effective in capturing L. lineolaris (Parys and Hall 2017).

Whilst (E)-4-oxo-2-hexenal is identified as an essential sex pheromone component for several Lygus species, (E)-2-hexenyl butyrate is crucial for L. lineolaris, and hexyl butyrate is essential for L. hesperus.

Synthetic Lygus sex pheromones have been effectively employed as the 'pull' component in a push–pull strategy for controlling L. rugulipennis in strawberry cultivation, containing 10 mg hexyl butyrate + 0.3 mg (E)-2-hexenyl butyrate + 2 mg (E)-4-oxo-2-hexenal (Fountain et al. 2021).

All three sex pheromone components are recognized by each Lygus species, as ratios of the butyrate esters are crucial for conspecific attraction and heterospecific avoidance by males, thus contributing to reproductive isolation amongst the three species (Byers et al. 2013). Moreover, both male and female L. hesperus emit these major volatiles to repel ant predators, indicating an additional defensive function of the sex pheromones as allomones in Lygus bugs (Byers et al. 2013).

Host plant contact chemoreception and chemical quality

Plant surface quality

Some plant bug species may select their host plant on the basis of leaf cuticular wax composition, such as the ratio of long-chain aliphatic molecules and non-polar cyclic compounds, forming a hydrophobic layer on the leaf surface (Martinez et al. 2017). However, the overall chemistry of the plant surface layer is highly complicated, containing atmospheric oxidants, plant-released VOCs, and leaf-adsorbed chemicals, all of which may influence herbivore behaviour (Mofikoya et al. 2020; Ossola and Farmer 2024). The rostral tip of the tarnished plant bug contains two sensory fields, each containing eleven sensilla basiconica (Avé et al. 1978). Removal of the rostral tip or antennae abolishes a feeding preference for frego bract cotton squares, suggesting that receptors at these loci are essential for a normal behavioural choice. Electrical recordings from the rostral nerve show increased impulse activity to stimulation of the rostral tip sensilla with juice from frego bract and normal bract squares of three varieties of cotton, confirming their function as contact chemoreceptors (Avé et al. 1978).

Lygus preference to feed on soft meristems and to oviposit on soft young plant tissue (Varis 1972; Holopainen 1986; Handley and Pollard 1993; Kytö 1993) suggests that the plant cellular structure and toughness might affect Lygus host plant selection as much as the surface quality. For example, when softer herbaceous weeds are available, Lygus bugs avoid feeding and ovipositing on conifer seedlings and prefer to feed and oviposit on herbaceous plants (Schowalter et al. 1986; Holopainen 1989).

Nitrogen and other nutrients

Nitrogen (N) is an essential nutrient for herbivorous insects, and nitrogen availability from plants is the main predictor for herbivore performance (Li et al. 2016) and the whole insect community on plants (Fagan et al. 2002). Surprisingly, only very few field experiments with crop plants have shown that an increase in N fertilization will increase the size of Lygus populations on cotton (Carrillo et al. 2008; Samples et al. 2019). Rämert et al. (2001) found that all tested nitrogen-fixing plants were preferred by Lygus spp. in comparison with lettuce in search of suitable TC plants for lettuce. In potted Scots pine seedlings, increased nitrogen fertilization led to increased total nitrogen and free amino acid levels in a whole shoot analysis. The oviposition rate of L. rugulipennis increased with the increase in N input and affected the relative growth rate (RGR) of L. rugulipennis nymphs (Holopainen et al. 1995). However, on mycorrhizal and non-mycorrhizal silver birch (Betula pendula Roth) seedlings in the laboratory, L. rugulipennis oviposition or RGR did not correlate with the total N concentration of seedlings, whilst the RGR of Epirrita autumnata Bork. larvae correlated positively with leaf N concentration (Nerg et al. 2008). Similarly, total N or mycorrhizal infection level did not correlate with the number of eggs laid by L. rugulipennis on small Scots pine (Pinus sylvestris L.) seedlings (Manninen et al. 1998).

One explanation for the difference between the leaf-chewing herbivores and Lygus bugs in their responses to whole plant nitrogen levels and nitrogen availability changes could be in the N allocation process in plants. When N availability is improved, N is first allocated as free amino acids to foliage to maintain photosynthesis and from there to actively growing meristems in growing points such as shoot tips and flower buds (Yoneyama et al. 2003). These small, undifferentiated meristematic cells, with thin cell walls are the main target of Lygus feeding on plants. When N availability is limited, plants try to keep N concentration and amino acid flow to meristematic tissue constant, e.g., by mobilising and reallocating N to meristems from old senescent leaves during a shortage of soil nitrogen (Yoneyama et al. 2003). As a result, by focusing their feeding on meristems, Lygus bugs are finally relatively tolerant against temporal changes in plant N availability.

Secondary metabolites

Plant secondary metabolites (organic compounds that are not directly involved in their normal growth, development, or production) protected plants against herbivores and pathogens. Notable for non-food crops, such as cotton and conifer seedlings, which are damaged by Lygus bugs, is that they are strongly protected against their specialist herbivores by terpene-based secondary metabolites. Conifers have high concentrations of monoterpenes and resin acids (diterpenes), forming the oleoresin stored in resin canals in needles, bark, and xylem (Heijari et al. 2005). In young conifer seedlings, Lygus bugs prefer to feed on apical meristems (Holopainen 1986), and in older seedlings, on developing buds (Kytö 1993). With selective feeding and focusing sucking efforts on meristematic tissues in the growing point and buds, Lygus adults and nymphs can avoid the harmful effects of conifer resins. However, concentrations of some pine resin acids at the whole plant level were marginally correlated with the number of eggs laid by L. rugulipennis (Manninen et al. 1998). Holopainen et al. (1995) observed that the levels of total phenolics and resin acids, representing phenylalanine and mevalonic acid pathways, respectively, were both reduced by increased nitrogen availability in young Scots pine seedlings, suggesting improved nutritive value for Lygus bugs.

Cotton has terpenoid aldehydes stored in subepidermal pigment glands in all above- and below-ground external tissues, including seeds (Hagenbucher et al. 2013). Leigh et al. (1985) observed that Lygus bugs avoid puncturing these glands, which contain mainly the toxic sesquiterpene aldehyde gossypol. Other terpenoid aldehydes, hemigossypolone, and heliocides are also detected outside the gland tissues. In cultivars resistant to moth larvae, such as the tobacco budworm Heliothis virescens (F.), concentrations of terpenoid aldehydes are high, and larval growth is substantially reduced (Hedin et al. 1992). Presumably, lower concentrations of secondary compounds together with constant nitrogen concentration are the main reasons why Lygus damage in cotton is concentrated in the new, actively growing meristematic tissues in the tips of shoots, reproductive organs of plants, and margins of young leaves (Tuelher et al. 2020).

The plant family Brassicaceae (previously Crucifereae) is known for their distinctive secondary metabolites, glucosinolates. These are sulphur rich compounds that are in damaged plant cells and are converted by the myrosinase enzyme to herbivore-repellent and toxic compounds such as thiocyanate and isothiocyanate. Brassicaceae specialist herbivores may sequester glucosinolates for their own defence (Hopkins et al. 2009). Lygus species favour Brassica spp. and Sinapis spp. plants, and these plant species are preferred by Lygus spp. when compared, e.g., with Chrysanthemum spp. (Woelke et al. 2023) or alfalfa (Butts and Lamb 1990). The glucosinolate concentration of oilseed rape cultivars did not affect the developmental rate or survival of Lygus sp. nymphs, and regardless of the host plant glucosinolate content, nymphs developed faster and had a higher survival rate on oilseed rape than on alfalfa (Butts and Lamb 1990).

Medicago sativa L. (alfalfa or lucerne) is an important crop plant species with a high protein content and a secondary metabolism characterized by saponins and flavonoids (Rafińska et al. 2017). Flavonoids are a diverse group of polyfenolic compounds, including, e.g., flavonols, that are known to be important antioxidants. For example, Goławska et al. (2010) reported that flavonol group compounds, apigenin glycosides, were negatively correlated with pea aphid (Acyrthosiphon pisum Harris) abundance on alfalfa plants. So far, studies of alfalfa saponin or flavonoid effects on Lygus species have not been published, suggesting that the putatively harmful effects of saponins and flavonoids are compensated by the beneficial effects of the high nitrogen content of alfalfa plants.

Impacts of Lygus physiological stage on host plant preference

Insects use their olfactory abilities and visual cues to locate mates, food sources, and suitable plants for laying eggs. They exhibit remarkable sensitivity and precision in discerning complex odours amidst background noise, navigating these scents during flight towards the source. The plasticity of their olfactory system further enhances their adaptability, adjusting sensory responses to align with internal physiological conditions (Saveer et al. 2012).

It is plausible that at different stages in their life cycle, insects orientate towards different targets and seek host plants that best match their needs at each particular physiological stage. Therefore, for applications such as trap cropping, it is necessary to consider the physiological state and the corresponding needs of the insects targeted for trap cropping. For example, the process of mating induces significant physiological transformations in various insect species, prompting corresponding alterations in behaviour to match their internal state. Saveer et al. (2012) showed that the cotton leafworm Spodoptera littoralis (Boisduval) (Lepidoptera, Noctuidae) females altered their olfactory preferences post-mating. Prior to mating, unmated female cotton leafworm moths displayed strong attraction towards the scent of lilac (Syringa) flowers. However, post-mating, this attraction diminished, and instead, females oriented towards the odour of green leaves from the cotton plant, their preferred host for egg-laying. This shift in behaviour stemmed from changes in how floral and green odours are perceived in the moth's primary olfactory processing centre, the antennal lobe.

How the physiological stage (e.g., pre-mating, post-mating, post egg-laying, pre-overwintering) of Lygus bugs might affect their host plant orientation behaviour, has to our knowledge not been studied. In Northern Europe, for example, after hibernating for the winter L. rugulipennis emerges with undeveloped ovaries. It is possible that their primary interest at that time is to locate the best possible feeding sites which will allow rapid and full development of the ovaries for egg-laying.

The “mother knows best” hypothesis, based on the optimal oviposition theory, suggests that an ovipositing female selects the host plant where the performance of the offspring will be best, but the female may prefer to feed on another plant species (Martinez et al. 2017). In Lygus bugs, the egg stage lasts about 3 weeks, depending on temperature (Varis 1972). This means that the female had to select an oviposition plant that would provide optimal food for the nymphs several weeks later. This may explain why, e.g., ovipositing Lygus bugs feed particularly on the apical meristems of small sugarbeet (Varis 1972; Tamaki and Hagel 1978) and conifer (Holopainen 1986; Kytö 1992; Kohmann 2006) seedlings in the early growing season. The advantage of apical meristem damage in young seedlings is that it will lead to stunted growth of seedlings and activation of the growth of lateral buds, which lead to the development multiple leader shoots that can provide several new actively growing meristem tissues for nymphs to feed on.

Dong et al. (2013) showed that Apolygus lucorum females prefer to feed and oviposit on flowering vegetation than on plants that have the flower removed. The experiment was conducted on three different plant species: cotton (Gossypium hirsutum), balsam (Impatiens balsamina), and castor bean (Ricinus communis). The results showed that nymphal performance followed the selection by the female.

When the new Lygus generation(s) prepare for overwintering, it is likely that they seek host plants differently than Lygus bugs earlier in the season. It is interesting that the mullein plant (Verbascum) has been shown to be an excellent autumnal trap crop for Lygus (Dumont and Provost 2022), but it has not been mentioned as having potential at other times of the season.

Host plant resistance

Lygus-resistant cultivars of crop plants

For trap cropping, the ideal situation is that the push–pull factors in the crop plant and the trap crop are optimal for the defence against the key pest. Using the most attractive trap crop cultivar as a trap crop and the most resistant crop cultivar as a main crop, the highest probability of reduced damage should be expected. There are reports that indicate that some commercial cultivars are more resistant to Lygus damage than others, but extensive studies that analyse the host plant properties that support Lygus resistance or susceptibility in each crop plant species are still quite scarce.

Traditionally, Lygus bugs in cotton are secondary pests, whilst insecticides used to control the boll weevil (Anthonomus grandis Boh.) and folivorous moths such as Helicoverpa armigera (Hübner) and Heliothis virescens F. have normally reduced Lygus populations under economic thresholds (George et al. 2021; Lu et al. 2024). This has led to the insecticide resistance of Lygus spp. to multiple insecticide classes (Lu et al. 2024) and increased the need for Lygus-resistant cotton varieties. Amongst the properties that have reduced Lygus performance on cotton in certain varieties have been the trait of not producing extrafloral nectar, reduced hairiness on leaves, and increased gossypol content (Meredith 1998). The nectarliness variety had a significantly lower 40–60% oviposition rate by Lygus than the normal variety (Bailey et al. 1984), and nectarliness was considered to be the most consistent trait to suppress Lygus and produce the highest yield when Lygus population density is high (Meredith 1998). Glabrous cotton cultivars with the lowest number of trichomes were less favoured by Lygus than hairy cultivars (Wood et al. 2017; George et al. 2021). Gossypol is a terpenoid aldehyde stored together with other non-volatile terpenoids in subepidermal pigment glands throughout the cotton plant (Hagenbucher et al. 2013). Consequently, glandless varieties with low concentrations of these toxic compounds were less preferred by herbivorous insects and rodents (Bottger et al. 1964).

In cotton, transgenic Bt technology has been successful in producing varieties that control pest insects with chewing mouthparts, but they are not effective against sucking insects (Lu et al. 2024). Furthermore, reduced feeding pressure by herbivores on Bt cotton has reduced inducible non-volatile toxic terpenoids, resulting in better performance of Lygus bugs on current Bt cotton varieties (Eisenring et al. 2019). However, recent progress in research with cotton breeding lines having a new Cry51Aa2.834-16 Bt protein has shown efficiency against thrips and Lygus spp. (George et al. 2021). A whole-plant caged field experiment showed high efficiency against young Lygus spp. nymphs by reducing cotton palatability and leading to population reduction up to 30-fold (Gowda et al. 2016), and in field monitoring experiments, immature thrips were reduced by 40–60% (Yates-Stewart et al. 2023).

Alfalfa, which is the second most important crop for Lygus bugs, suffers from damage to buds, flowers, and pods and has a reduced seed yield (Schull et al. 1934). There are reports of some minor differences between varieties in Lygus performance and induced damage (Gordon and Teuber 1993; Acharya et al. 2008), and Lygus species-specific differences in alfalfa preferences (Carcamo et al. 2003), but comprehensive analyses of the traits that are related to alfalfa resistance against Lygus are not available.

In strawberry, a high yielding, early season cultivar may contribute to reducing the incidence of damage by L. lineolaris, because females lay relatively more eggs per receptacle ("fruit”) on plants with few receptacles (Rhainds and English-Loeb 2003). Easterbrook and Simpson (2000) reported that L. rugulipennis did not show any oviposition preference or difference in nymphal density amongst four strawberry cultivars. However, damage to cv. Bolero was lower, suggesting that cv. Bolero has no resistance to L. rugulipennis, but has a greater tolerance to feeding by this species. A three-year monitoring of 20 strawberry cultivars in Poland indicated that only three cultivars, ‘Malling Pandora’, ‘Pegasus’ and ‘Senga Sengana’ had consistently less than 5% of strawberry fruits damaged by L. rugulipennis.

In carrots, improved resistance against L. hesperus and L. elisus nymphs was found in S3 generation hybrids derived from the open-pollinated cultivar Imperida (Scott 1977). Kainulainen et al. (2002) find that L. rugulipennis laid the lowest number of eggs on cultivar Parano out of seven tested carrot cultivars, but the variation of essential oils in these cultivars did not explain this response. Alvarado-Rodriquez et al (1986) reported that in some common bean cultivars, at least two general mechanisms function to provide resistance: low preference in oviposition by L. hesperus females and reduced nymphal growth and survival. Twenty-two Vigna unguiculata (cowpea) origins of the tested 86 origins showed resistance against L. hesperus (Moshy et al. 1983). L. lineolaris adult densities were the lowest on snap bean cultivars ‘Greencrop’ and ‘PV-857′ out of six cultivars tested (Li et al. 2023).

In the small seedling stage, several vegetable and forage plants are susceptible to overwintering L. hesperus adult feeding damage in areas where the hibernation period is short (Fye 1984). The species include onion, radish, turnip, rape, clover, alfalfa, lentil, pea, bean, dill, and carrots. Crucifers, forage, and umbellifer seedlings were the most susceptible, whilst bean, pea, and lentil seedlings suffered less damage and recovered faster (Fye 1984).

Prospective Lygus resistance factors in future cultivars

Polyphagous insect herbivores are basically well adapted to detoxify a wide range of plant defence compounds, which is already indicated by the high diversity of food plants. As we have shown in previous paragraphs, the increase of carbon-based secondary metabolites in a crop plant presumably does not improve Lygus resistance enough. In research on Lygus bug control, Bt technology has recently shown some potential for sufficient efficiency against Lygus, as reported in the experiments using the new Cry51Aa2.834-16 Bt protein (Cervantes et al. 2019; George et al. 2021).

RNA interference (RNAi)-mediated gene silencing technology is a developing new biotechnological method that has high potential for sustainable control of herbivorous insects (Upadhyay et al. 2011; Ortola and Daros 2024). RNAi technology in pest insect control involves exploiting the natural biological process of RNA interference to target and disrupt or silence specific genes essential for the survival of pests (e.g., development, metabolism). Typically, double-stranded RNA molecules that are designed to match the sequences of target genes in the pest insect are employed. When these RNA molecules are introduced into the insect exogenously (e.g., by spraying on crops) or endogenously (via a transgenic host plant), they trigger a cellular response that degrades the corresponding messenger RNA in herbivorous insects, thus preventing the expression of the targeted gene (Ortola and Daros 2024). The method has been efficient against some coleopteran and lepidopteran pests and against leaf sap-sucking whiteflies (Bemisia tabaci Genn.) (Upadhyay et al. 2011).

Other transgenic technologies that could have an important role in controlling hemipteran pests on crop plants in the future were reviewed by Chougule and Bonning (2012). They concluded that, in addition to Bt cry, certain lectins and plant protease inhibitors may have great potential to act as natural insecticides in crop plants. Lectins are natural carbohydrate-binding proteins that are widely distributed in animals, plants, and microorganisms. For example, a mannose-binding lectin isolated from Colocasia esculenta (L.) Schott leaves, when supplemented with artificial food, was shown to have deleterious effects on the red cotton bug, Dysdercus cingulatus F. (Hemiptera: Pyrrhocoridae), by affecting the growth and fecundity and even influencing the second generation of the bugs that have been reared on an artificial diet with a sublethal dose of the lectin (Roy et al. 2002). Recently, Indian mustard (Brassica juncea L.) plants expressing C. esculenta tuber agglutinin (CEA), a non-allergenic lectin, enhanced mortality and reduced fecundity of mustard aphid (Lipaphis erysimi Kalt.) (Das et al. 2018). Future transgenic crop plants may have fusion proteins where, e.g., cry and lectin proteins are stacked to have wider resistance against herbivorous insects (Boddupally et al. 2018; Kumari et al. 2022).

Plant protease inhibitors (PPI) are naturally occurring molecules that inhibit the function of protease enzymes and act as a part of the plant's natural defence system against herbivores (Chougule and Bonning 2012) and fungal pathogens (Chiu et al. 2022). Transgenic PI plants have had success against weevils, moths, and thrips, but no reports against Lygus bugs are available so far (Kumari et al. 2022). Concerns about the detrimental effects of PPI-transgenic plants on non-target beneficial arthropods have been presented (Divekar et al. 2023).

Habitat properties

Habitat properties and distance from the hibernation site or winter host habitat may affect the probability of the landing of an herbivorous insect on the host plant crop (Nissinen et al. 2022). In the boreal zone, requirements for suitable overwintering habitat and breeding period habitat for Lygus species are partly opposite. The winter hibernation of L. rugulipennis at the adult stage is in the forest litter and in the branches of conifer trees close to the ground (Varis 1972). This requires sufficient snow cover during the coldest periods, whilst there should not be flooding during snow melt in the spring before the end of the hibernation so that the hibernating individuals do not drown. Therefore, dry forest sites with sandy soil that do not have a high water-holding capacity are optimal overwintering sites for Lygus adults (Holopainen and Rikala 1990).

High temperatures and low humidity levels are growth-limiting factors for Lygus nymphs (Boneß 1963; Brent and Spurgeon 2019). Studies with L. pratensis nymphs (Liu et al. 2015) showed that the mean optimal developmental temperature is 33.6 °C and the lethal temperature is 40.9 °C. Low relative humidity (RH 45%) reduced the nymphal growth rate both at 25 and 35 °C. These results supported earlier observations that showed that high humidity during rainy periods might induce Lygus outbreaks on cotton (Yang et al. 2004). In turn, Brent and Spurgeon (2019) reported that cotton drought stress had only minor effects on L. hesperus adults, and the authors suggested that short-term increases in crop canopy temperatures caused by moderate drought stress are unlikely to impact L. hesperus population growth substantially. Crop irrigation (Zhang et al. 2017) and timing of irrigation period (Wood et al. 2019) are known to increase Lygus population on crop plants. For example, more L. hesperus were found in cotton plots irrigated weekly than biweekly in a two-year study (Flint et al. 1996), and the population density of L. hesperus was higher at higher irrigation levels (Leigh et al. 1970) or unaffected (Asiimwe et al. 2014). In some crops, such as chia, irrigation reduced Lygus abundance compared with non-irrigated plots, but the effect can be cultivar dependent (Oeller et al. 2021). Irrigation and the availability of water have been found to determine L. pratensis distribution on cotton in China (Zhang et al. 2017). Current evidence suggests that irrigation could be a crucial factor in supporting the increasing density of Lygus bugs on crops. Therefore, irrigation should also be considered in the planning of trap crop strategies so that trap crops have enough moisture.

Lygus spp. adaptation on new crops and development of pest status

There are a few examples where, if some abiotic or biotic environmental conditions have changed in the agroecosystem or a novel crop plant is taken to cultivation, Lygus population may develop more strongly and intensify the damage to the crop to the point of reaching the economic threshold.

Transgenic cotton

An example of the effects of change in the biotic condition is Lygus problems on cotton after the transgenic Bt cotton was used to control defoliating moths such as Helicoverpa spp. and Heliothis spp. and to reduce the use of insecticides (Lu et al. 2008). When Bt toxin killed most of the moths and insecticide use was reduced, Lygus bugs and other Mirids were no longer controlled by these same insecticide sprayings (Lu et al. 2024). Lygus bugs are also oligophagous predators that, in all nymphal stages and as adults, can efficiently predate on eggs and from the first to the third instar of Heliothis spp. larvae (Cleveland 1987). However, when the moth-specific Bt-toxin on the cotton cultivar killed most of the moth larvae in the field, Lygus bugs that occupied the plants did not have enough animal food, and they started to feed and lay eggs on the most protein-rich parts of the cotton plants, including the apical terminal, floral buds (squares), and developing fruit (bolls). Visible damage to cotton plants is first necrotic spots as a result of stylet piercing injuries and small holes caused by female ovipositor when eggs are partly drilled under the plant surface (Cooper and Spurgeon 2011).

Bioenergy and green chemistry crops

Novel crops grown for bioenergy and green chemistry are examples of new plant species taken in agricultural production that became attacked by Lygus species. Hagler et al. (2016) reported that three desert-adapted crops, vernonia (Centrapalus pauciflorus), lesquerella (Physaria fendleri), and camelina (Camelina sativa), were readily taken as hosts by Lygus hesperus, which fed and deposited eggs on each plant species, with the greatest amount of egg deposition on vernonia and the least on camelina.

Wordwide Lygus problems in conifer nurseries

Various growth disturbances and “bushy-top” and multiple-leader symptoms were reported in conifer seedling production in forest nurseries in Europe, Northern America, and New Zealand (South 1991). During the 1970s and early 1980s, the problems intensified, and several hypotheses were suggested as the causal agents for the growth disturbance, including the use of herbicides, nutrient imbalance (Raitio 1985), frost injury (Hofstra et al. 1988), plant viruses (Soikkeli 1985), and feeding by springtails (Bevan 1964) or thrips. Damage intensity in Finnish nurseries reached in some Scots pine seedbeds up to 90% rejected seedlings, and therefore more effort was put into finding the causal agent (Raitio 1985; Poteri et al. 1987). Soon, the entomological studies demonstrated that most of the hypotheses were misdiagnosed, and feeding by Lygus bugs was the causal agent for the damage (South 1991).

In fact, as early as 1918, it was reported for the first time that hardwood and conifer seedlings can be damaged by Lygus bugs in nursery environments in Missouri, USA (Haseman 1918). In Finland, Varis (1972) observed that in May, L. rugulipennis can be found on conifer saplings near winter rye fields. Also in Finland, in spring 1984, nursery stock monitoring by sweep netting was started in mid-May on third-year bare-rooted Scots pine (P. sylvestris L.) in the field, and in a plastic greenhouse seedbed in Suonenjoki METLA research station (Holopainen 1986; Holopainen and Rikala 1990). The first L. rugulipennis adults were observed on May 22, the first nymphs on June 5 on bare-rooted Scots pine, and seedbeds on June 5 and July 3, respectively. Laboratory feeding and oviposition tests with L. rugulipennis adults collected from nursery stock (Holopainen 1986; Poteri et al. 1987) quite rapidly indicated that Lygus adults feed, females lay eggs, and nymphs can develop on pine seedlings, leading to the described multiple-leader symptoms. Simultaneous nursery studies in Canada (Shrimpton 1985) and the USA (South 1986) confirmed the causal agent role of L. lineolaris in the multiple-leadering of pine seedlings. Later studies (Schowalter 1987; Kytö 1992; South 1991; Kohmann 2006) confirmed that Lygus bugs have become widely recognised as a causal agent for this problem in conifer production.

Intriguing questions are why and how a polyphagous heteropteran species became a significant nursery pest on conifers that have been less preferred host plants in laboratory experiments (Varis 1972; Holopainen 1989). This development took place in a decade from the late 1970s to the early 1980s, although conifer tree nursery production has existed for several decades, e.g., from the late 1800s in Finland (Tasanen 2010) and from the early 1900s in the USA (Burch 2005). South (1991) considered that the main reason for Lygus injury problems in this period was a major change in the way weeds are controlled in conifer nurseries. In the 1950s, weeds in nurseries in Canada and the USA were controlled with mineral spirits that are toxic to germinating and young herbaceous plants, but conifers are resistant (Duryea and Landis 1984). Mineral spirits also have insecticidal and repellent properties. South (1991) suggested that in the 1970s, the price of crude oil greatly increased, and the use of oil-based mineral spirits declined, particularly after 1975, and that reduced the repellent effects on Lygus bugs; their numbers started to increase in nursery environments, reaching an early economic injury level in the early 1980s.

In Finland, the 1970s were a period of intensification of forest-tree seedling production in nurseries with improvements in irrigation and fertilisation, and most nurseries were established in sandy soils where weed density was lower and mechanical control was possible (Rikala 1978; Tasanen 2010). Insecticide use on conifers was very occasional because small seedlings were more harmed by fungal diseases than insects. During the boom of pine production in the late 1970s, mechanical weed control became too laborious (Tasanen 2010), and at the same time, herbicides suitable for conifers, such as atrazine, intensified herbicide use in nurseries (Jaakkonen and Sorvari 2006). The rapid change from mechanical weed control to using efficient herbicides probably substantially reduced the numbers and diversity of weeds in nursery beds. Host plant selection experiments showed that the most common weed species, Senecio vulgaris L. (Fig. 1a), was more attractive for L. rugulipennis feeding and oviposition host than conifer seedlings (Holopainen 1989). In the analysis of the putative direct effects of herbicide spraying on Scots pine seedlings with the herbicide atrazine, no evidence of changes in chemical quality of seedlings or susceptibility of seedlings to L. rugulipennis was found (Holopainen et al. 1992). Hence, indirect effects of herbicides through the loss of alternative host plants could be a more straightforward explanation for the Lygus shift to feed on conifers.

Fig. 1
figure 1

Lygus bug-attracting plant species that are suitable for trap cropping. a Groundsel (Senecio vulgaris L.) (Asteracea), b Red clover (Trifolium pratensis L.) (Fabaceae), c Alfalfa (Medicago sativa L.) (Fabaceae), d Buckwheat (Fagopyrum esculentum Moench) (Polygonaceae), e Annual fleabane (Erigeron annuus (L.) Pers.) (Asteraceae). Photographs a, b, and d by J.K. Holopainen, c by Orest Lyzhechka, and e by H.M.T. Hokkanen

Full size image

Another, perhaps more plausible, reason for the increased Lygus attack on conifer seedlings could be the increased irrigation of nursery stocks. As a result, the L. rugulipennis population that was already adapted to a well-irrigated nursery environment possibly switched the host plant from the weeds to conifers and started to breed and feed on pine seedlings, causing damage that resulted in multiple leader shoots. Irrigation is used, particularly in the early growing season of May–July (Rikala 1978), to stimulate seedling growth. Irrigation, together with improved fertilisation, increases the availability of free amino acids from seedlings to Lygus nymphs and adults (Holopainen et al. 1995). The improved conifer seedling growth and development in nurseries compared to the natural forest environment, probably made it possible for better survival of nymphs on Lygus damaged conifer seedlings due to faster development of lateral buds and meristems in multiple leaders, which provided high quality food for nymphs (Holopainen 1986).

Moreover, during hot and dry weather periods, nursery fields are irrigated several times of the day, which might result in evaporative cooling (oasis effect) (Ruehr et al. 2020) and high humidity in the nursery stock. This might be an attractive habitat for the Lygus bugs that migrate from the nearby overwintering sites in the forest to more open breeding habitats. Particularly, the early instar nymphs are sensitive to high temperatures and host plant water stress (Boneß 1963; Brent and Spurgeon 2019). Furthermore, the volatiles released from conifer seedling resin ducts should not be repellent to Lygus bugs because adults are already adapted to conifer needle monoterpenes during their hibernation in degrading needle litter in forest floors (Kainulainen and Holopainen 2002).

A third, potentially plausible explanation for Lygus pressure on conifer nurseries could be simply the location of forest nurseries. They were often established in dry sandy soils (Tasanen 2010) and surrounded by dry forest types that are suitable for the overwintering of Lygus adults in forests amongst conifer needle litter under the snow (Varis 1972). Distance from the hibernation site can be an important factor in determining insect pressure and damage to crop plants, especially young seedlings. Ferguson et al. (1997) reported that Lygus bugs and the damage to linseed (Linum usitatissimum L.) were more abundant close to field boundaries (3–25 m from the field margin) with woody plants than more central areas of the field (50–100 m from the field margin). Forest nurseries surrounded by conifer forests might be under pressure from migrating Lygus bugs from all directions.

Lygus control methods

Lygus bugs have traditionally been secondary pests, which will be often controlled when control measures are taken against more important and visible pests on crop plants. In this section, we only briefly summarise chemical and non-chemical control methods, and in the following section, we describe the current experiences of TC methods in Lygus control.

Cultural, mechanical and traditional Lygus control

In smaller-scale crop production, such as vegetable production, many traditional control measures against herbivorous insects are taken as a part of integrated pest and pollinator management (IPPM) (Egan et al. 2020), and typically they are used in organic farming. Methods can be classified as: (1) Crop rotation and diversification; for example, in cotton, the crop rotation should be organised so that cotton should be planted in large blocks away from more preferred corn and soybeans to minimise the Lygus bug movement to cotton blocks (George et al. 2021). Diversification is related to trap crops, but also maintenance of natural enemies of Lygus, e.g. on strawberry crops (Hagler et al. 2021). (2) Management of weed hosts of Lygus; weeds are often considered major Lygus sources for crop plants. Barlow et al. (1999) investigated L. hesperus performance on alfalfa weeds and found that survival was lowest on monocot weeds and better than on alfalfa on some dicot weeds such as Capsella bursa-pastoris (L.) Medikus and Senecio vulgaris. S. vulgaris has earlier been found attractive for Lygus spp. in forest nurseries (Schowalter 1987; Holopainen 1989) and may have potential to be used as a TC plant, (3) Netting and floating row covers improve microclimate and protect vegetable seedlings against many herbivorous insect species, including Lygus bugs (Rekika et al. 2008; Bui et al. 2024), and in strawberries, early growth before flowering (Hernández-Martínez et al. 2023). (4) Mechanical removal; tractor mounted vacuums for Lygus spp. management have been used on organic strawberries (Wells et al. 2020) but also on strawberry trap crops (Swezey et al. 2013).

Biological control

Natural biological control of Lygus bugs relies on predators and parasitoids that actively search for and respond to Lygus bug populations growth on the crop. Particularly flowering plants in off-crop habitats maintain a rich population of natural enemies (Ruberson 1998; Nieto et al. 2023), and predators and parasitoids can be important components of Lygus, reducing the effects of trap crops. Vertebrate predators such as insectivorous birds have been reported only occasionally (Kullenberg1944). Generalist invertebrate predators include spiders, predatory beetles, aphid-attending ants, and predatory bugs (Hagler et al. 2020). These feed on all developmental stages of Miridae plant bugs, including Lygus spp. Parasitic wasps can be specialised on eggs and nymphs of Lygus species, whilst tachnid fly parasitoid larvae can be found in Lygus adults (Wheeler 2001).

Recent biological control methods using biotic organisms in Lygus management USA include the use of European Peristenus spp. (Hymenoptera: Braconidae), originally endoparasitoids of L. rugulipennis nymphs (George et al. 2021). Some success has been in controlling L. lineolaris in alfalfa (Day 1996) and in strawberry (Tilmon and Hoffmann 2003). In California, mass rearing and releases of the native egg parasitoid Anaphes iole Girault (Hymenoptera: Mymaridae) and nymphal parasitoid Leiophron uniformis (Gahan) (Hymenoptera: Braconidae) have been evaluated for management of Lygus spp. in strawberries and shown increased parasitism rates (Lahiri et al. 2022). It has been suggested (George et al. 2021) that microbial biopesticides would be particularly appropriate for sucking insect population management, at least in combination with chemical insecticides, because the fungal entomopathogens Beauveria bassiana and Entomophthora sp. have shown efficiency against L. lineolaris (Steinkraus and Tugwell 1997).

Chemical control

Synthetic insecticides have been used intensively in Lygus bug control, particularly in cotton, and this has led to insecticide resistance in L. lineolaris against several pesticide groups, including organophosphates, pyrethroids, carbamates, and neonicotinoids (George et al. 2021; Du et al. 2024; Zhu et al. 2024). Insect growth regulator benzophenyl urea (Novaluron) is a chitin inhibitor and still effective against L. lineolaris nymphal stages (George et al. 2021), although some populations have already indicated increased tolerance against Novaluron (Parys et al. 2016). Insecticide resistance should be considered in TC strategies aimed at L. lineolaris removal from the selected trap crop.

On strawberry plantations in the UK, the most effective insecticide treatment against L. rugulipennis was the pyrethroid bifenthrin, followed by the neonicotenoids thiacloprid and acetamiprid, whilst the neonicotinoid flonicamid and organophosphate indoxacarb were not as effective (Fitzgerald and Jay 2011). In China, pyrethroid lambda-cyhalothrin controls L. pratensis by killing the insects, e.g., on Bt cotton, after exposure to lethal doses, but also by suppressing their population growth when individuals are exposed to sublethal doses of the chemical (Tan et al. 2021).

Trap cropping experiences with Lygus

A number of field studies have shown success in trap cropping of Lygus spp., resulting in lowering of Lygus numbers or damage in the main crop (Table 3). The crop plants in these studies have involved cotton, strawberry, lettuce, cowpea and tomato.

Table 3 Field experiments where trap crop plants reduced Lygus spp. performance or damage on the crop plant
Full size table

Cotton

The Lygus species targeted in the trap cropping research with cotton have usually been L. pratensis and L. hesperum. The trap cropping field studies have been carried out in the Southwestern USA (California, Arizona), and in China. Alfalfa, sunflower, ironweed Vernonia galamensis Cass. Less. (Asteraceae), and safflower have been used as trap crop plants in these studies.

Lygus control by trap cropping was for the first time demonstrated in California. At the end of the 1960s, Lygus bugs, a key pest of cotton in California, were shown to prefer lushly growing alfalfa over cotton, and strips of this crop interspersed in cotton fields virtually eliminated the need to spray the main crop for lygus control (Stern et al. 1969). In the study of Stern et al. (1969), strips of alfalfa about six metres wide were interplanted in cotton fields at 100 to 150 m intervals, in a 65 ha cotton field. Most alfalfa was sprayed four to six times with insecticides, and some fields were harvested for seed. Lygus numbers were about 100-fold in alfalfa compared to their numbers in cotton, whilst Lygus in cotton did not need additional control (Stern et al. 1969).

Recently in Arizona, Hagler et al. (2021) showed by using ironweed V. galamensis as a trap crop for L. hesperus that the pest numbers were fourfold higher in the trap crop compared to cotton. No chemical control on cotton was necessary. Vernonia also attracted predatory arthropods, and pests were retained on the trap crop throughout the season. Their study involved planting of four rows of Vernonia embedded within a 96-row cotton field (Hagler et al. 2021). Throughout different stages of the cotton-growing season, Hagler et al. (2021) assessed the abundance of true bug pests, their predators, and spiders using whole-plant and sweep net procedures. Their findings showed a significant attraction of arthropods towards Vernonia trap crop across all phases of cotton growth. The movement of arthropods was tracked from the trap crop to cotton fields utilizing a protein immunomarking technique. The results demonstrated minimal capture of marked specimens beyond the Vernonia trap crop 1, 3, and 6 days after marking. The strong affinity of arthropods towards the Vernonia plants demonstrates its potential as both a trap crop for cotton pests and a sanctuary for natural enemies (Hagler et al. 2021).

Two further studies from China demonstrate the utility of two trap crop species from the sunflower family as trap crops for L. pratensis in cotton. Zhang et al. (2020a, b) investigated the attractiveness of sunflower (Helianthus annuus) to L. pratensis adults within cotton plots and evaluated the efficacy of inter-planted sunflower strips in controlling L. pratensis populations in the field. Field trials assessed six combinations of two sunflower varieties and three planting dates (early, middle, and late). Results indicated that the abundance of L. pratensis adults was highest on the early planted variety, reaching 3.7–5.8 times higher levels compared to nearby cotton plots. In commercial cotton fields, the combined strategy of deploying sunflower strips along field edges and periodic insecticide applications on these strips led to significant reductions: (1) 41.9–44.0% decrease in mean abundance of L. pratensis on cotton, (2) 27.3–30.6% reduction in cotton leaf damage and 44.8–46.0% decrease in boll damage, and (3) 7.5%-8.0% increase in mature boll count. The study highlights sunflower's potential as an effective trap crop for L. pratensis and underscores the role of sunflower strips in environmentally friendly management practices for cotton crops.

Wang et al. (2021) evaluated the potential of safflower (Carthamus tinctorius) as a trap plant for managing L. pratensis through both laboratory and field experiments. Olfactometer assays revealed a strong attraction of L. pratensis to volatiles emitted by safflower. In field trials, safflower plots harboured higher populations of L. pratensis (both adults and nymphs) compared to adjacent cotton plots. Early sown safflower exhibited higher L. pratensis abundance than mid-sown or late-sown safflower, providing more favourable conditions for the settlement and reproduction of L. pratensis. The density of L. pratensis on safflower trap crops, regardless of sowing patterns, was notably higher than on neighbouring cotton fields. Intercropping safflower trap crops proved more effective in reducing L. pratensis densities on cotton than employing safflower as 'spot' trap crops or peripheral trap crops, although this efficacy might also be influenced by the total area covered by safflower trap crops. Moreover, regular chemical control of L. pratensis on safflower trap crops resulted in a 10% increase in cotton boll count and a 33% reduction in boll damage compared to cotton without safflower trap crops and insecticide treatments. These findings suggest that safflower holds promise as an effective trap crop for managing L. pratensis and could contribute significantly to its control in cotton cultivation.

Strawberry

Two groups from the USA have reported field studies concerning the trap cropping of Lygus spp. on strawberry cultivations. During the growing season, they all have used alfalfa as the trap crop species, whilst the off-season study used mullein (Verbascum thapsus) plants to lower Lygus populations for the following season (Dumont and Provost 2022). Both L. hesperus and L. lineolaris have been targeted in these studies.

Swezey et al. (2007) conducted a study in an organic strawberry field in California, employing trap cropping experiments with a completely randomised design. They found that L. hesperus adults and nymphs were more abundant in alfalfa trap crops compared to adjacent strawberry rows. Over three experimental years, employing a tractor-mounted vacuuming device for twice-weekly summer vacuuming of alfalfa trap crops led to a reduction in adult and nymph abundance by 72 and 90%, respectively, within the trap crops. This vacuuming practice also resulted in a significant decrease in damage caused by L. hesperus in nearby unvacuumed organic strawberry plants, compared to either untreated controls or the organic strawberry grower’s standard whole-field vacuuming approach. Additionally, vacuuming the alfalfa trap crops reduced costs for organic growers (including tractor, fuel, and labour) by 78% compared to the expenses associated with whole-field vacuuming practices.

In their 2013 publication, Swezey et al. discuss their investigation into the abundance and distribution of Lygus spp. nymphs across two alfalfa trap crops separated by 50 rows of strawberries. They found that nymphs were clustered in the alfalfa trap crops compared to the interior strawberry rows, where nymphs were less abundant. The majority of nymphs were concentrated within the trap crops, and nymphal densities in the interior strawberry rows remained well below economic thresholds. To track the movement of Lygus spp. from marked alfalfa trap crops into adjacent strawberry rows or trap crops, Swezey et al (2013) employed an enzyme-linked immunosorbent assay mark-capture technique. They observed that the majority of marked-captured L. hesperus adults and Lygus spp. nymphs remained within the alfalfa trap crops rather than dispersing into the strawberry rows. This limitation on Lygus spp. movement within alfalfa associated with organic strawberries was identified as a crucial aspect of effective trap cropping.

Reporting on further studies from the California group, Nieto et al. (2023) investigated the movement patterns of Lygus spp. and their associated predators from weed habitats to strawberry fields with alfalfa trap crops, utilizing a protein mark–capture technique. They collected insects and spiders from weeds, strawberry plants, and alfalfa trap crops at intervals of 1 day, 2 days, and 2 weeks following the application of albumin protein marks to weeds bordering the strawberry fields. Results showed that the majority (79%) of marked Lygus spp. adults that migrated from weeds were found in the alfalfa trap crops, whilst all nymphs migrated to the strawberry plants. Predators marked with protein were mostly observed immigrating to the strawberry plants rather than the trap crops, resulting in a marked predator-to-Lygus spp. ratio of 5:1. The utilization of trap cropping effectively diminished the colonization of Lygus adults to the strawberry fields. Additionally, the researchers suggested that transitioning weedy areas into habitats with native perennial plantings could further reduce the risk of pest migration whilst concurrently preserving beneficial insect populations.

In the eastern and central regions of North America, L. lineolaris poses a significant threat to strawberry crops. Hetherington (2023) and Hetherington et al. (2014) investigated the efficacy of alfalfa perimeter strips in mitigating L. lineolaris populations within June-bearing strawberry fields. Over a three-year period, they monitored L. lineolaris densities and the abundance of beneficial arthropods on three commercial strawberry farms where alfalfa was planted as a trap crop near strawberry plots. The alfalfa perimeter strips were observed to attract and concentrate L. lineolaris populations, resulting in a 36% reduction in L. lineolaris densities in adjacent strawberry plots compared to control areas. Using a protein immunomark-capture experiment to assess movement between the alfalfa strips and neighbouring strawberry plots, Hetherington et al. (2024) found that approximately three times as many L. lineolaris migrated from strawberries to alfalfa compared to the reverse direction. Furthermore, a disproportionate number of adult females were observed immigrating to alfalfa, indicating a potential preference for oviposition in alfalfa by L. lineolaris. Whilst the presence of alfalfa perimeter strips increased the abundance and diversity of beneficial arthropods in the experimental plots overall, these effects were primarily localized to the alfalfa strips, with minimal spillover into adjacent strawberry plots. These findings suggest that the selective attraction of L. lineolaris to alfalfa drives the observed reductions in population, highlighting the role of alfalfa perimeter strips as effective trap crops in June-bearing strawberry cultivation (Hetherington 2023; Hetherington et al. 2024).

Lettuce

Two European field studies report successful trap cropping of L. rugulipennis for the protection of the lettuce crop. Rämert et al. (2001) evaluated the efficacy of trap crops in mitigating Lygus spp. populations within a lettuce agroecosystem through a series of field experiments conducted at two locations in Sweden. The effectiveness of trap crops was determined by comparing Lygus densities in lettuce plots with those in adjacent plots containing various cover crops, including Melilotus officinalis, Vicia sativa, Trifolium pratense (Fig. 1b), and Medicago sativa (Fig. 1c), as well as the weed Artemisia vulgaris. All trap crops tested exhibited greater attractiveness to Lygus compared to lettuce, with adult Lygus populations ranging between 5 and 30 times higher on cover crops and more than 100 times higher on A. vulgaris. Lygus rugulipennis was identified as the dominant species across all plant types. These findings suggest that a diverse array of trap crops, including nitrogen-fixing cover crops, holds potential for effectively reducing Lygus spp. populations in lettuce cropping systems.

Accinelli et al. (2005) conducted field trials in Northern Italy to explore an agroecological approach for controlling L. rugulipennis infestations on lettuce using trap crops. The study compared two treatments: lettuce planted alongside trap crops, represented by plots neighbouring alfalfa strips, and lettuce grown without trap crops, consisting of four plots surrounded by bare soil. Two lettuce cultivars were assessed in the trials. Results indicated the overall effectiveness of alfalfa as a trap crop for managing L. rugulipennis. However, in some instances, alfalfa alone did not sufficiently prevent damage to lettuce, necessitating localized chemical interventions on alfalfa. The research highlighted the potential use of alfalfa as a trap crop for lettuce during periods when pest abundance is low, such as outside the peak infestation months of August and September. It also cautioned against relying solely on trap crops for managing Lygus bugs, particularly when lettuce varieties are highly susceptible to bug damage (Accinelli et al. 2005).

Cowpea and tomato

Two further field studies assessed the feasibility of trap crops in controlling Lygus spp. on cowpea and on tomato. On cowpea, Bensen and Temple (2008) used Sesbania sp., Crotalaria sp., and Cajanus sp. as trap crops in a split–split plot field experiment. They found that trap crop species mixture rows hosted more L. hesperus than did cowpea plots in most of the sweep net samples. However, damage was not reduced in plots with trap crop rows.

On tomato, Balzan and Moonen (2014) tested flowering field margin strips compared with natural vegetation. Flowering plant mixture strips containing Fagopyrum (Fig. 1d), Phaseolus, Vicia, Coriandrum, Foeniculum, and Borago were sown in 3 m × 25 m plots in eight 6 ha organic tomato fields. The study showed that the flowering strips acted as a trap crop for Nezara viridula and Lygus spp. and reduced bug densities on tomato. Crop injury from these pests was significantly lower in proximity to flowering strips (Balzan and Moonen 2014).

Trap crop plants for Lygus spp.

The field studies reported in Table 3 used, in the majority of cases, as trap crop plant alfalfa, Medicago sativa (in 7 out of the 13 studies included). The remaining studies involved a number of different species, raising the question of what would be the best trap crop species for Lygus spp. Host plant preference studies invariably indicate that the most preferred plant families for Lygus are Asteraceae, Brassicaceae, and Fabaceae (Young 1986; Holopainen and Varis 1991). Host plant finding in Lygus is facilitated by olfactory and visual cues, and it would be necessary to optimize the attraction to the trap crop using both sets of cues. Alfalfa is no doubt an attractive plant to Lygus in many circumstances, but it may not be the best. Using legume crops such as alfalfa, or crucifer crops such as mustard or rapeseed, may be attractive to the farmer from a practical point of view (ease of obtaining seed, familiarity with sowing, etc.), whilst possibly better trap crops are less familiar to the farmer.

Many studies indicate that plants from the family Asteraceae are highly effective as trap crops for Lygus bugs (e.g., Halloran 2012; Halloran et al. 2013; Zhang et al. 2020a, b; Hagler et al. 2021; Wang et al. 2021). We speculate that it is likely that many of the plants in the sunflower family combine chemical and visual cues to efficiently attract Lygus bugs.

Halloran (2012) and Halloran et al. (2013) show that Erigeron annuus (Fig. 1e), the annual fleabane, releases a complex blend rich in green leaf volatiles, terpenes, and other compounds, and that this blend is highly attractive to female Lygus bugs. These attractive cues function over long distances and result in both attraction and arrestment of L. lineolaris. Lygus bugs have been shown to consistently prefer the annual fleabane over other weeds and many target crops, and when E. annuus was available, Lygus will leave cotton and migrate onto E. annuus (Fleischer et al. 1988). Halloran (2012) and Halloran et al. (2013) showed that E. annuus releases significantly more total volatiles than cotton, both in the presence and absence of different types of Lygus damage. In addition to an increase in the number of compounds (from 93 to 114), many compounds that are released constitutively in low amounts, are up-regulated when attacked by Lygus bugs, and that volatile emissions from E. annuus are significantly higher (sometimes up to tenfold) than emissions from the co-occurring cotton crop plant (Halloran 2012, Halloran et al. 2013).

E. annuus is a herbaceous, annual or biennial flowering plant in the family Asteraceae, native to North and Central America. It can grow to between 30 and 150 cm in height. It produces up to 50 small daisy-like flowers in an open, branching cluster at the top of the plant. Flowers are about 2 cm across, with 80 to 125 narrow white petals and a yellow centre disk. As the season progresses, lower lateral buds also expand into heads, creating a panicled cloud of the small white flowers (https://www.minnesotawildflowers.info/flower/annual-fleabane). These showy, white and yellow flowers and flower clusters may act as effective visual cues to Lygus bugs, in addition to the attractive volatile emissions, and may explain the overall attractiveness of this plant to Lygus.

Other effective trap crop plants for Lygus spp. from the family Asteraceae include safflower (Carthamus tinctorius) (Wang et al. 2021), sunflower (Helianthus annuus) (Zhang et al. 2020a, b), and ironweed (Vernonia galamensis) (Hagler et al. 2021). Safflower is a highly branched, herbaceous, thistle-like annual plant, native to South Asia. Plants are 30–150 cm tall with globular flower heads having yellow, orange, or red flowers. Sunflower is a familiar plant with a showy yellow flower, native to the Mexico and Southern-USA. Ironweed is native to East Africa and is the main source for vernonia oil. Depending on environmental conditions, the plant may become extremely tall (Thompson 1994). These plants are all tall with showy flowers, which we suspect enhances their attractiveness to Lygus along with the volatile blends.

Conclusions and future prospects

The aim of this review was to collect information on factors that affect the host plant and habitat selection by Lygus spp. and evaluate the current knowledge of the trap cropping of Lygus bugs to improve the sustainable management of Lygus bugs in future agriculture and horticulture. An impressive list of known host plant species of dominating Lygus species indicates that these insects have a high adaptation capacity to new host plants. This is probably a result of the capacity of Lygus bugs to use their stylet to feed on fast-growing young meristematic cells that have not yet accumulated host-plant-specific defence compounds. The ability of Lygus bugs to avoid the plant chemical defence opens the possibility of feeding on nearly any crop or trap plant species that are in the active growth stage, although some plant species are clearly preferred more than others.

Challenges for trap cropping of Lygus bugs stem from their diversity, adaptability, and enormously broad host range. Several features common to all pestiferous Lygus bugs allow generalisations and adopting information generated for one species to apply for another Lygus species. Such features include characteristics of their lifecycle, feeding behaviour, and the type of damage they cause to their host plants. Further common traits are, for example, the ease of adaptation to new host plants but also host plant resistance factors.

Species-specific fine-tuning in designing TC for Lygus is needed based on the geographical location of where TC will be developed and implemented. Latitude, for example, causes variation in terms of overwintering/hibernation and migration to crops at the beginning of the season. This is related to the timing of the dispersal flight—nocturnal or during the daytime—with potential implications on TC colour, shape, allelochemical production, or placement.

Lygus spp. differ in their responses to plant emitted volatiles, but it is not clear how comparable the published data are, as the physiological condition of the studied insects is usually not given. Lygus spp. share the same basic components for their sex pheromones, but with great variation in their respective shares. Some successful experiments have been conducted to utilise the accrued information about the chemical ecology of Lygus, by testing semiochemical push–pull arrangements (e.g., Fountain et al. 2021).

More broadly, an ideal trap crop for Lygus and other pests would:

  • Share the same agronomic practices with the economically important crop. The cotton/alfalfa system is a good example. Cotton is an annual, and alfalfa is a perennial. The grower must maintain the alfalfa (water, harvest, etc.) throughout the year.

  • Also, the alfalfa must be frequently cut and bailed, which requires extra equipment and labour. When this is done, the lygus will move into the nearby cotton (Sivakoff et al. 2012).

  • Alfalfa requires a lot of irrigation. An ideal trap crop should not require too much agronomic input (water, fertilizer, etc.). This holds especially true with climate change issues.

  • The grower can harvest, sell, or use the trap crop. Alfalfa is a good (positive) example here. Growers can sell the hay or use it to feed their own livestock.

As knowledge about the most attractive TC plants for Lygus bugs accumulates, a major question remains: what happens to the bugs successfully attracted to the TC? Theoretical and modelling studies emphasise the importance of retention time to the effectiveness of the TC strategy (e.g., Hannunen and Ekbom 2001; Hannunen 2005; Holden et al. 2012). An unsustainable and laborious technique is to kill the bugs with chemical pesticides in the trap crop before they disperse further. Similarly, mechanical removal by vacuuming the TC plants periodically can sometimes be used. Both of these methods are non-selective and likely not sustainable as a long-term solution.

One possibility is to take advantage of ‘dead-end trap crops’, if available (Shelton and Badenes-Perez 2006). We encourage, in most cases, to develop TC arrangements where a highly attractive TC not only attracts the Lygus bugs but also their natural enemies, such as mirid-specific parasitoids and polyphagous predators. If the retention time of Lygus in these TC is long and possibly allows reproduction and establishment of long-term pest/natural enemy dynamics (at least over the season), there might not be any spill-over of the pest to the main crop even in the absence of any other active pest management. This may require planting a mixture of TC plant species containing highly attractive food plants and suitable species for oviposition. We encourage for future studies on Lygus trap cropping to include promising plant species from different plant families, with focus on Asteraceae, Brassicaceae, and Fabaceae, and to include attractive visual cues along with effective volatile blends. Furthermore, incorporating repellent elements (e.g., Bui et al. 2024) into a push–pull system may further improve the success of Lygus trap cropping.