Background

The two-spotted spider mite, Tetranychus urticae Koch, is one of the major pests of agricultural crops grown worldwide and is found on approximately 1200 host plant species in 70 genera (Do Amaral et al. 2020). Management of T. urticae is difficult and pesticides are often used for this purpose. The ability of T. urticae to develop resistance to a range of chemicals has created critical situations for practical pest control. To overcome this problem a more durable crop protection solution based on integrated pest management (IPM) systems is necessary (Sedaratian et al. 2009).

Phytoseiid mites play a major role in keeping phytophagous mite populations at low densities and decreasing their deleterious impact (Al-Azzazy and Alhewairini 2020). Their high consumption rate of prey individuals, high reproductive rate, and rapid developmental rate make them important biological control agents (Hoy 2011). Flexibility in feeding behavior, such as feeding on alternative prey or feeding on non-prey foods, including fungal spores, nectar, and pollen has generally been observed in generalist phytoseiid species (Overmeer 1985). Most generalist phytoseiid mites use alternative food sources, promoting their persistence in the field when prey is absent or scarce, and facilitating their mass-rearing for augmentative biological control purposes (McMurty et al. 2013; Yazdanpanah et al. 2021).

Neoseiulus cucumeris (Oudemans), a generalist predatory phytoseiid mite, is known to feed on the immature stages of various pests. The broad prey range and ability to survive on plant pollen categorized it as a type III lifestyle (subtype III-e) predator (McMurtry et al. 2013). Mass rearing aims to cost-effectively produce a large number of efficient predators in a short time available commercially for augmentative release (Nordlund 1998). Cost-effective rearing of N. cucumeris on more economic food sources such as factitious prey would accelerate the mass production of this predator.

Generalist predatory mites can feed and develop on many different non-prey foods, including eggs of the Mediterranean flour moth, Ephestia kuehniella Zeller (Calvo et al. 2015), and the stored product mite, Tyrophagus putrescentiae (Schrank) (Astigmatidae), which has been commonly used as a factitious prey for the commercial production of different phytoseiids, including N. cucumeris (Britto et al. 2012; Pirayeshfar et al. 2020).

The effects of long-term rearing and feeding on an alternative diet on a predator’s performance need to be evaluated if such a diet is to be used for mass rearing (Bellutti 2011; Sørensen et al. 2012). In the present study, the predation capacity and life table parameters of N. cucumeris were determined after long-term feeding on T. urticae as one of its natural prey for the first time. In addition, we assessed the effects of long-time rearing of the predator on T. putrescentiae, and eggs of E. kuehniella as alternative prey, and then compared the population growth parameters and potential of this predator when reared long-term on its main prey and alternative foods.

Methods

Stock culture of two-spotted spider mite, T. urticae

An initial population of T. urticae was obtained from weeds growing around the Faculty of Agriculture, Tarbiat Modares University, Tehran, Iran. Some female and male specimens of the spider mite were mounted in Hoyer's medium on microscope slides and identified using the identification keys. The colony of T. urticae was established by releasing them onto cultivated beans (Phaseolus vulgaris L.) at the five-leaf stage. New bean plants were planted and added to the colony weekly. The T. urticae colony was kept under greenhouse conditions.

Eggs of the Mediterranean flour moth, E. kuehniella

Eggs of E. kuehniella were obtained from a colony that had been established at the laboratory of the Faculty of Agriculture, Tarbiat Modares University, Tehran, Iran. Before the experiments.

Fresh eggs (less than 24 h old) were kept in a freezer (− 18℃ for 48 h) and refrigerated (4℃) for up to two weeks.

Stock culture of stored product mite, T. putrescentiae

The individuals of T. putrescentiae were originally collected from infested Petri dishes containing the fungus Alternaria sp., and then reared on wheat bran at 27 ± 1 °C, 60% ± 5 RH, and a photoperiod of 16L:8D h in a Plexiglas container (10 × 7 × 4 cm).

Stock culture of the phytoseiid predator, N. cucumeris

The stock culture of N. cucumeris was obtained from Bio-Planet, Italy. Individuals were transferred to a green plastic sheet (16 × 11 × 0.1 cm) sitting atop a water-soaked sponge in a Plexiglas container (30 × 15 × 12 cm). All edges of the plastic sheet were covered by moist tissue paper which prevented mites from escaping (Walzer and Schausberger 1999). Water was put in the containers daily to prevent the strips from drying out. Some cotton fibers were added to the plastic sheet to provide a substrate for oviposition. The stock cultures of N. cucumeris were kept in a growth chamber at 25 ± 1 °C, 60 ± 5% RH, and a photoperiod of 16L:8D h. A mixture of pre-adult stages of T. urticae (about 1000–2000) for the rearing of N. cucumeris on T. urticae and a mixture of immature stages of T. putrescentiae (2000–3000) for rearing this predator on T. putrescentiae was offered as food two times a week.

Experimental setup

The experimental units were quite similar to the stock culture rearing units but smaller. The 3-cm-diameter units consisted of green plastic sheets (3 × 3 × 0.1 cm), plastic trays (7 × 5 × 4 cm), and wet sponges. About 40 pairs of N. cucumeris were selected randomly from the stock culture and kept in a new experimental unit for less than 24 h so as to have the same-aged eggs as the predator. The newly laid eggs were transferred to the experimental units individually. After larval emergence, one of the three diets (T. putrescentiae, T. urticae, and eggs of E. kuehniella) was offered. For the experiments, about 30–40 and 50–60 nymphs of T. urticae were provided daily for the nymphs and adults of the predator, respectively. About 3–5 eggs of E. kuehniella were offered to the experimental units as a diet every day, and about 50–70 pre-adult stages of T. putrescentiae were used at two-day intervals. After adult emergence, females were coupled with the males of the same treatment. All replicates of each treatment were monitored daily to record development, fecundity, longevity, and survival rate. Since most N. cucumeris immatures could not complete their development and reach adulthood on the eggs of E. kuehniella this diet treatment was discontinued but the mentioned procedure was repeated for generations of 10, 20, and 30 of N. cucumeris colonies that were fed on T. putrescentiae and T. urticae. All experiments were carried out in the laboratory at standard conditions (25 ± 1 °C, 60 ± 5% RH, and a photoperiod of 16L:8D h).

The predation capacity of N. cucumeris was determined on T. urticae as prey for predators maintained on T. urticae for 30 generations. First, an experiment was done to estimate the predation rate by adults of each sex. Same-aged mated adults (10 females and 10 males) were transferred singly to the experimental units (20 units total). About 50 immature T. urticae were allocated to each unit as food, and the number of mites consumed was recorded daily. The mean number of prey consumed was calculated for each sex. Finally, the ratio of the feeding rates of females to males was obtained per day to guide the main experiment. To calculate the predation rate of N. cucumeris, the number of consumed T. urticae was recorded daily during the life table experiments until the death of all individuals.

Data analysis

The two-sex life table procedure (Chi and Liu 1985; Chi 1988) using the TWOSEX-MSChart software (Chi 2021) was used to calculate the life table parameters and analyze data of N. cucumeris at different generations on each diet tested. The variances and standard errors of all parameters, including the gross reproductive rate (GRR), net reproductive rate (R0), intrinsic rate of increase (r), finite rate of increase (λ), and mean generation time (T) were estimated using the bootstrap procedure with 100,000 samples. Multiple comparisons for different generations as well as diets were carried out using the paired bootstrap test in TWOSEX-MSChart software.

According to Chi and Yang (2003), the parameters of net predation rate (C0) (mean number of T. urticae consumed by an individual predator during its life span), transformation rate (Qp) (the number of T. urticae needed for the production of an offspring from a predator), stable predation rate (ψ) (the total predation capacity of a stable population), and finite predation rate (ω) (the predation potential of N. cucumeris population by combining its finite rate of increase [λ], age-stage predation rate [cxj], and stable age-stage structure [axj]) were calculated using CONSUME-MSChart software (Chi 2021). The variances and standard errors of predation parameters were estimated using the bootstrap resampling method. The statistics of C0, Qp, ψ, and ω were compared based on the paired bootstrap test (with 100,000 resampling), using the TWOSEX-MS Chart program (Chi 2021).

Results

Life table parameters of N. cucumeris fed on E. kuehniella

The predator, N. cucumeris, did not feed consistently on the eggs of E. kuehniella, and about 7% of individuals reached adulthood. Only two females developed and lived less than 8 days without oviposition; therefore, no long-term rearing of N. cucumeris occurred on this diet. The mean duration of protonymph and deutonymph was 6.8 and 7.5 d, respectively. The age-stage-specific survival rates (sxj) indicate the initiation and termination of all immature and adult life stages while survivorship (lx) shows the probability that a newborn individual will survive to age x and is calculated by pooling all individuals of both sexes and clearly indicates the inadequacy of this diet (Fig. 1). Since a few individuals of N. cucumeris reached the adult stage and there was no fecundity, analyses of the life table data for this treatment were not possible.

Fig. 1
figure 1

Age-stage-specific survival rate (sxj), and age-specific survivorship (lx) of Neoseiulus cucumeris fed on Ephestia kuehniella eggs

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Life table parameters of N. cucumeris fed on T. putrescentiae

Significant differences in the duration of immature stages were observed among different tested generations (Table 1). The larval period was the shortest immature stage in all generations tested. Both protonymphal and deutonymphal durations were longer in G1 compared with the other generations. The longest developmental time was in G1 (10.7 d). Female and male longevity and the total lifespan (from birth to death) of N. cucumeris were not significantly different in G1, G10, and G30 (Table 1). The longest TPOP (total pre-oviposition period) was observed in G1. The oviposition period and fecundity of N. cucumeris in G10 and G30 were significantly higher than in the other tested generations (Table 1).

Table 1 Long-term effects of feeding Neoseiulus cucumeris on Tyrophagus putrescentiae, Tetranychus urticae, and Ephestia kuehniella on the duration of different life stages (days), fecundity (eggs/female) and immature survival (%) (Mean ± SE)
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The adult stage started at 7, 5, 6, and 6 d in G1, G10, G20, and G30, respectively. Females of the first generation lived more days, while both sexes in G20 had the shortest lifetime (Fig. 2). Based on the fecundity curves, the highest daily fecundity was observed in G10 (2.69 eggs) at the age of 11 days, while the lowest was at the age of 12 days in G20 (1.5 eggs) (Fig. 3).

Fig. 2
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Age-stage survival rate (sxj) of sequential generations (G1–G30) of Neoseiulus cucumeris reared on Tyrophagus putrescentiae

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Fig. 3
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Age-specific survivorship (lx), age-specific fecundity (mx), and age-stage-specific fecundity (fxj) of sequential generations (G1–G30) of Neoseiulus cucumeris reared on Tyrophagus putrescentiae

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Analysis of life table parameters of N. cucumeris indicated differences among the tested generations (Table 2). There was no significant difference between generations in terms of GRR. The lowest value of R0 was observed in G20, but was not significantly different compared with G1. The first generation had the lowest values of r and λ, as well as it had the longest T value (28.2 days) (Table 2).

Table 2 Generation-dependent life table parameters (mean ± SE) of Neoseiulus cucumeris reared on Tyrophagus putrescentiae and Tetranychus urticae
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Life table parameters of N. cucumeris fed on T. urticae

There were no significant differences in the duration of the egg, larva, total pre-adult, APOP, TPOP, female longevity, male longevity, total lifespan, oviposition, and immature survival (Table 1). G1, followed by G30 and G20, had the highest value of fecundity (Table 1). The start of the adult stage differed little among the generations; 6, 6, 5, and 6 d in G1, G10, G20, and G30, respectively (Fig. 4). Females of the first generation lived longer (54 d) (Fig. 4). The life table parameters, including lx, mx, and fxj of N. cucumeris during 30 generations of feeding on T. urticae (Fig. 5) indicated the highest fecundity was observed in G20 (2.25 eggs) at the age of 14 d. The analysis of the life table data of N. cucumeris showed significant differences among the tested generations (Table 2). Females in G1 had a higher GRR than the other generations. The parameters R0, r, and λ in G1 did not differ significantly from G20 and G30.

Fig. 4
figure 4

Age-stage survival rate (sxj) of sequential generations (G1–G30) of Neoseiulus cucumeris reared on Tetranychus urticae

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Fig. 5
figure 5

Age-specific survivorship (lx), age-specific fecundity (mx), and age-stage-specific fecundity (fxj) of sequential generations (G1–G30) of Neoseiulus cucumeris reared on Tetranychus urticae

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Life table parameters of N. cucumeris fed on T. putrescentiae versus T. urticae

In G1, the total development time of N. cucumeris was significantly shorter when the predator fed on T. urticae than on T. putrescentiae. The total lifespan of N. cucumeris was longer in G1, G10, and G30 when this predator fed on T. putrescentiae. The oviposition period and fecundity were significantly shorter in G10 when T. urticae was the food source. In addition, GRR, R0, r, and λ were substantially lower in G10 when feeding on T. urticae compared with feeding on T. putrescentiae in G10.

Predation capacity of N. cucumeris after long-term feeding on T. urticae

No predation activity was observed by larvae in all treatments (Fig. 6). Age-stage-specific consumption rates increased with increasing predator stage; deutonymphs consumed more prey than protonymphs, and the same trend was observed for adults compared with deutonymphs. The predation rate of females during the oviposition period was higher than in pre- and post-oviposition periods (Fig. 6). The number of prey consumed by females was 36, 21, 18, and 21, whereas it was 22, 13, 11, and 11 for males in G1, G10, G20, and G30, respectively. The age-specific predation rate (kx) is the mean number of T. urticae consumed by N. cucumeris at age x, and the age-specific net predation rate (qx) can be determined by considering the survivorship (Fig. 7).

Fig. 6
figure 6

Age-stage-specific predation rate (cxj) of sequential generations (G1–G30) of Neoseiulus cucumeris reared on Tetranychus urticae

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Fig. 7
figure 7

Age-specific predation rates (kx) and age-specific net predation rates (qx) of sequential generations (G1–G30) of Neoseiulus cucumeris reared on Tetranychus urticae

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The highest value of the net predation rate (C0) was 467 nymphs in G1, decreasing to 260, 166, and 238 prey in G10, G20, and G30, respectively (Table 3). According to the values of Qp, N. cucumeris required a maximum of 49 prey (observed in G10) to produce an egg. The highest and lowest values of ψ and ω of N. cucumeris were observed in G1 and G20, respectively (Table 3).

Table 3 Generation-dependent predation rates (mean ± SE) of Neoseiulus cucumeris fed on Tetranychus urticae
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Discussion

The effects of feeding on factitious and main prey on the performance of N. cucumeris have been studied but only after one generation (e.g., Sarwar 2009; Delisle et al. 2015; Li and Zhang 2016; Patel and Zhang 2017; Al-Shemmary 2018; Li et al. 2021) and long-term rearing on such diets on the effectiveness of N. cucumeris remains an open question. We evaluated the performance of N. cucumeris when offered either T. urticae, T. putrescentiae, or eggs of E. kuehniella. Furthermore, the predation capacity of this predator on nymphs of T. urticae was evaluated during long-term rearing.

Although N. cucumeris was able to successfully develop from egg to adult stage on all tested diets, feeding on the eggs of E. kuehniella led to high immature mortality. Whereas 83% and 77% of eggs reached adulthood when T. urticae and T. putrescentiae were offered as food, respectively, only 7% did so on E. kuehniella. We found that juvenile stages of N. cucumeris had difficulty feeding on intact E. kuehniella eggs. The ability of phytoseiid mites to feed on different food sources depends on the morphology of their mouthparts (Flechtmann and McMurtry 1992). Delisle et al. (2015) reported that eggs with a flaw on their outer membrane resulted in more feeding activity. The unfavorable effects of E. kuehniella can be related to the resistance of the egg’s chorion, antifeedant factor on the eggs, and the imbalance in the specific nutrients needed for immature development (Riahi et al. 2018). We found E. kuehniella eggs are not a suitable food source for N. cucumeris due to short adult longevity, long pre-adult duration, high immature mortality, and lack of fecundity. Similarly, E. kuehniella eggs did not support the development and survival of Typhlodromus bagdasarjani Wainstein and Arutunjan immatures, as none developed beyond the protonymphal stage (Riahi et al. 2018). Conversely, a high level of survival (90% and 88%, respectively) was observed for N. cucumeris and Neoseiulus barkeri (Hughes) on eggs of E. kuehniella (Al-Shemmary 2018). Furthermore, the performance of N. barkeri was higher than Neoseiulus zaheri Yousef and El-Borolossy by feeding on the eggs of E. kuehniella (Momen and El-Laithy 2007). Momen et al. (2020) evaluated the development and reproduction of N. barkeri on E. kuehniella and Mycetoglyphus fungivorus Oudemans in the first, third, and sixth generations of rearing and concluded that the fecundity and oviposition rate of the predator were higher on E. kuehniella than on M. fungivorus. Similarly, not only did N. cucumeris (Al-Shemmary 2018) and Amblyseius swirskii Athias-Henriot (Nemati et al. 2019) oviposit when fed on E. kuehniella eggs, but they also showed the highest fecundity on this diet compared to pollen grains and eggs of other insects. In addition, the fecundity of Neoseiulus californicus McGregor (Khanamani et al. 2017a) and A. swirskii (Riahi et al. 2017a) was high when they were fed on an artificial diet enriched by eggs of E. kuehniella.

The shorter development time in N. cucumeris reared on T. putrescentiae and non-significant differences of population growth parameters in later generations in the current study indicated that long-term rearing on T. putrescentiae led to enhanced performance of the predator. In contrast to our findings on N. cucumeris, T. putrescentiae was the less favorable factitious prey for A. swirskii and T. bagdasarjani (Riahi et al. 2017b, 2018). Enrichment of the basic diet provided with this factitious prey with some protein sources resulted in increasing the fecundity of N. barkeri (Huang et al. 2013). However, Pirayeshfar et al. (2020) showed that the reproduction of A. swirskii was influenced by both the stage and the food substrate used for the rearing of T. putrescentiae and they recommended this mite as an excellent alternative food for mass-rearing programs. These inconsistencies may be due to the predator species and different diets used for rearing the acarid mite.

Although most companies in their brochures recommend N. cucumeris to control thrips, long-term feeding on T. urticae caused no decline in performance and its predation potential remained high. We suggest T. urticae is prey for this predator. The predation rate of females on T. urticae was higher than that of males and other stages, especially during the oviposition period. In the current study, the value of C0 of N. cucumeris was 467 prey in G1. However, the value of this parameter was 201 when the predator fed on T. atlanticus (Popov and Kondryakov 2008). Furthermore, for the same tetranychid species, this value was 528 prey for N. californicus (Khanamani et al. 2017b) and 464 prey for A. swirskii (Riahi et al. 2017c). N. cucumeris requires a maximum of 49 prey to produce an egg. In contrast, there were 27 and 28 prey for N. californicus (Khanamani et al. 2017b) and A. swirskii, respectively (Riahi et al. 2017c). Although N. cucumeris performs poorly in the dense webbing of spider mites, it showed higher predation on the eggs of T. urticae compared with other more specialist predators such as N. californicus and P. persimilis (Li and Zhang 2016). In conclusion, N. cucumeris can be a useful predator against T. urticae, especially when the spider mite density is low. Consistent with our findings, Luo et al. (2014) reported successful biocontrol of T. urticae by N. cucumeris.

The total development time of N. cucumeris was significantly shorter on T. urticae compared with T. putrescentiae in G1. However, there was no significant difference between the two diets at higher generations (G20 and G30). Similarly, the growth rate of N. cucumeris was higher when the predator fed on T. urticae than on T. putrescentiae in G1 but continuing the rearing process resulted in no significant change in reproductive rates between the two diets. This may be due to the lower mobility of T. putrescentiae immatures, making them more available to the predator, which conserves energy for the rest of the predator's life. In addition, differences in the nutritional level of the prey and feeding on immotile or motile prey may require different abilities to decode infochemicals and hunt (Dicke et al. 1998). Sarwar et al. (2009) indicated that N. cucumeris preferred T. putrescentiae followed by T. urticae. They concluded that the predator adapted to prey that had been the routine diet for a long time. The non-significant difference in r in the later generations of the current study indicated that N. cucumeris accepted natural and factitious prey as diets for long-term rearing. Although it would be better to rear the phytoseiid mites on the most common natural prey (Goleva and Zebitz 2013; Massaro and de Moraes 2016), it gives rise to high labor costs due to the need for fresh plants for raising prey. Therefore, using T. putrescentiae has more practical advantages than using T. urticae and could alleviate the problems associated with providing prey mites continuously and facilitate the cost-effective rearing process of N. cucumeris.

Conclusions

Our results indicated that E. kuehniella eggs were not accepted as food by different life stages of N. cucumeris and could not sustain their development and reproduction for at least one generation. On the other hand, the acarid mites, T. putrescentiae and T. urticae supported the development and reproduction of N. cucumeris for more than one generation without any losses in the fitness of this predator after 30 generations. Therefore, T. putrescentiae proved to be a suitable cost-effective diet for mass-rearing of N. cucumeris because of the high performance of the predator during long-term feeding on this factitious prey.