Introduction

Soil-insecticide use at planting is still a widespread prophylactic practice (Veres et al. 2020) despite causing a major impact on biodiversity worldwide (Pisa et al. 2017). The prophylactic approach is unjustified since populations exceeding thresholds do not often occur (Furlan et al. 2020; Labrie et al. 2020) and it goes against the principles of Integrated Pest Management (IPM), as described by Barzman et al. 2015.

Tools such as pheromone traps can be very important for affordable risk assessment of soil pest damage so that farmers and IPM advisors can apply control solutions, including insecticides, only when and where they are needed, thus ensuring that the objectives of Directive 128/2009/EC of the European Parliament are met (Furlan et al. 2023).

Pheromone lures have been developed and optimized for all of the important click beetles of genus Agriotes (Coleoptera, Elateridae) in Europe (Furlan et al. 2001; Tóth et al. 2003), and suitable sex pheromone traps for monitoring them have become available (Furlan et al. 2001, 2020). Known as YATLORf (Yf) traps, they have proven to be an effective, low-cost tool for monitoring adults of the Agriotes spp., Europe’s most important soil pests, as well as essential for IPM of the main arable crops (Furlan et al. 2001, 2020). Yf traps are designed to catch both mainly crawling and mainly flying click beetles, thus encompassing all Agriotes species with agricultural importance in Europe (Furlan et al. 2001); some of these species are important pests in North America, as well (Vernon et al. 2014a).

It became evident during the early phases of our long-term research that trap design and lure position were critical factors for catching different species of click beetle. We noted that in the early swarming period, beetles of a mainly crawling species (A. brevis) were caught only when lures were placed close to the soil in traps unsuitable for ground-level catches (Tóth et al. 2002b). This insight steered trap design toward a model that comprised different ways of catching the beetles and various slots for lure placement. Therefore, a vast number of field trials were conducted to ascertain the most effective trap-management solutions, including lure position in the trap and the effect of vegetation on capture potential (Furlan et al. 2023). The findings of this work led to reliable guidelines for trap use.

It has been found that combinations of lures in the same trap cannot generally be used to capture moths, as the pheromone component(s) of a given species often act as inhibitors, interfering with the pheromonal response of another closely (or not-so-closely) related species. One of the earliest examples was (Z)-8-dodecenyl acetate, the main pheromone component of the Oriental fruit moth (Grapholita molesta Busck, Lepidoptera: Tortricidae) (Arn et al. 1974), used in traps baited with (E,E)-8,10-dodecadien-1-ol, the main pheromone component of the codling moth (Cydia pomonella L, Lepidoptera: Tortricidae), leading to a slump in catches of the latter (Arn et al. 1974).

Consequently, this present study conducted extensive field trials to ascertain the most effective trap-management solutions, including a high number of combinations of pheromone lures for several important genus Agriotes click beetles. Trials exploited information on Yf trap management, particularly on the most suitable lure positions for catching a range of species. We aimed to determine which lure combinations did not have significant detrimental effects on any of the target species, meaning that multi-species trapping became feasible. Herein, we summarize the above trials, which were conducted in a number of European countries from 2001 to 2007.

Materials and methods

YATLORf (Yf) traps (Furlan and Gnes 2003) (Fig. 1) baited with pheromone lures are commonly employed to catch adult click beetles in Europe (Furlan et al. 2007). Our trials used pheromone lures from the CSALOMON® trap family (Plant Protection Institute, CAR, Budapest, Hungary). See earlier reports in the literature for the compositions of single lures (Table 1).

Fig. 1
figure 1

Representation of lure positions in YATLORf traps. Figure reproduced from Furlan et al. 2023 under STM Permissions agreement

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Table 1 Composition of click beetle pheromone lures used in the trials
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A range of sites in Italy, France, Germany, and Hungary were set up for field-trapping trials between 2001 and 2007 (Table 2) using agreed-upon trapping procedures (Roelofs and Cardé 1977; Furlan et al. 2023). The Yf trap’s white bottom was placed face-down so that its brown edge was buried 1–2 cm beneath the surface. A standard seasonal schedule governed by each species’ life cycle and behavior determined how the baits were managed. By way of example, A. ustulatus click beetles in northeast Italy swarm from early June to August, with A. sordidus beetles swarming from April to August, peaking in May, meaning that they swarm for much longer than A. ustulatus. A. brevis and A. sordidus share various swarming characteristics, but A. brevis swarms for longer, starting a little before A. sordidus (Furlan et al. 2007). All lures were replaced every 30 to 40 days when trial duration exceeded 1 month. The only exception was A. brevis, which was never replaced. Each time the traps were inspected, soil and any residue were dusted off the bottom. The insects were removed from the trap as follows: the trap was taken out of the ground and placed inside a large plastic bag; the trap was opened, dropping the insects into the bag; the trap was withdrawn and the bag was sealed immediately; the trap was then restored to its original position. All of the beetles were stored in cool conditions (5–8°C) so that their taxonomy could be recorded (Platia 1994).

Table 2 Main characteristics of trials for catching Agriotes click beetles with different lure combinations
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The traps were set up in blocks. Each block had one trap per lure combination (treatment). Three to eight blocks were used in each trial. The traps were sited 8–10 m apart inside each block; blocks were located a minimum of 30 m apart. We inspected the traps at intervals of several days, mainly twice a week. On inspection captured insects were removed and recorded. The position of the trap was rotated clockwise within the block. The lures were put into three positions:

  • Low: the vial was placed in the bottom slot (i.e., inside narrowest part of funnel) sealed and upside down (Fig. 1);

  • Medium: the lure vial was placed in the middle slot between the white lids sealed and upside down (Fig. 1);

  • High: the lure vial was placed in the top slot between the white lids sealed and upside down (Fig. 1).

For a more detailed description of lure positions, see Furlan et al. 2023.

A summary of the trials is reported in Table 2. Most of the possible lure combinations (treatments) in the same trap were compared with traps baited with a single lure. The combinations are summarized in Table 3. The aforementioned experimental procedure was implemented, with lure positions in the trap being chosen (e.g., the A. brevis lure in the low position) as per the results of Furlan et al. 2023.

Table 3 Lure combinations: influence of a second lure, added to the target lure, on target species catches in field-trapping trials
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Statistical analysis

All of the trials in this publication were set up as factorial trials to compare the number of beetles caught in Yf traps. Since the data available did not have Gaussian distributions (verified with the Shapiro-Wilk test), the following procedure was performed: the data were analyzed with ANOVA after values had been transformed into ranks (Conover and Iman 1981; Noguchi et al. 2020). The separation of rank means was performed with the Tukey HSD test (p < 0.05). Data-processing was performed with STATGRAPHICS 19®. The tables display all of the data as medians of the sampled values (no. of beetles caught by the trap).

Results

Each trial was analyzed independently, with each table referring to the trial code in Table 2, which details the trial characteristics. Tables 4, 5, 6 report results for the main interactions between A. obscurus, A. lineatus, and A. sputator, three species that can be found at the same site (Furlan et al. 2007). The presence of an A. sputator lure did not influence the capture potential of Yf traps single-baited with an A. lineatus lure, but it did significantly reduce A. obscurus catches when compared to single-baited traps. Catches of A. sputator beetles were almost completely inhibited at all trial sites when A. lineatus and A. obscurus lures were used in the same trap.

Table 4 Trial 1 Effect of adding A. obscurus and A. lineatus lures to an A. sputator lure on click beetle catches in Yf traps
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Table 5 Trial 2 Effect of adding A. obscurus and A. lineatus lures to an A. sputator lure on click beetle catches in Yf traps
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Table 6 Trial 3 Effect of adding A. obscurus and A. lineatus lures to an A. sputator lure on click beetle catches in Yf traps
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Table 7 reports the effect of A. ustulatus lures in Yf traps baited with both A. obscurus and A. lineatus lures. The presence of A. ustulatus lures did not influence A. lineatus catches but it did deplete A. obscurus catches. Table 8 reports the effect on A. obscurus catches when an A. litigiosus lure was added; no impact on A. obscurus beetle catches was observed. Likewise, an additional A. brevis lure had no impact on A. obscurus beetle catches. Catches of A. brevis, however, were significantly reduced by the presence of an A. obscurus lure (Table 9). Catches of A. sordidus were not influenced by the presence of an A. lineatus lure in the same trap (Table 10). A. brevis and A. sordidus can be found with A. litigiosus and/or A. ustulatus at the same site (Furlan et al. 2007).

Table 7 Trial 4 Effect of adding an A. ustulatus lure to a Yf trap baited with A. obscurus and A. lineatus lures on click beetle catches in Yf traps
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Table 8 Trial 5 Effect of combining A. obscurus and A. litigiosus lures on click beetle catches in Yf traps
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Table 9 Trial 6 Effect of adding an A. brevis lure to a trap baited with an A. obscurus lure on click beetle catches in Yf traps
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Table 10 Trial 7 Effect of adding an A. sordidus lure to an A. lineatus lure on click beetle catches in Yf traps
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Tables 11 and 12 show that there was no impact on beetle catches when A. brevis and A. sordidus lures were combined in the same trap; likewise, a combination of A. sordidus and A. litigiosus lures showed no interference (Table 13).

Table 11 Trial 8 Effect of combining A. brevis and A. sordidus lures on click beetle catches in Yf traps
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Table 12 Trial 9 Effect of combining A. brevis and A. sordidus lures on click beetle catches in Yf traps
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Table 13 Trial 10 Effect of adding an A. litigiosus lure to a Yf trap baited with A. sordidus lures on catches of A. sordidus and A. litigiosus click beetles
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The presence of A. ustulatus and A. litigiosus lures, single or combined, alongside A. brevis and A. sordidus lures already in the same Yf trap, had no impact on catches of A. ustulatus (Tables 14 and 15) or A. litigiosus beetles (Table 16), meaning that A. ustulatus and/or A. litigiosus catches were not impacted by A. brevis and A. sordidus lures. Table 17 (high A. ustulatus population) and Table 18 (high A. litigiosus population) confirm that A. ustulatus and A. litigiosus lures did not interact negatively.

Table 14 Trial 11 Effect of adding an A. ustulatus lure to a Yf trap baited with A. brevis and A. sordidus lures on catches of A. ustulatus click beetles in Yf
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Table 15 Trial 12 Effect of adding an A. ustulatus lure to a Yf trap baited with A. brevis and A. sordidus lures on catches of A. ustulatus click beetles in Yf
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Table 16 Trial 13 Effect of adding A. ustulatus and A. litigiosus lures to a Yf trap baited with A. brevis and A. sordidus lures on catches of A. litigiosus and A. ustulatus click beetles
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Table 17 Trial 14 Effect of combining A. litigiosus and A. ustulatus lures on click beetle catches in Yf traps
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Table 18 Trial 15 Effect of combining A. litigiosus and A. ustulatus lures on click beetle catches in Yf traps
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Table 19 shows the results of monitoring A. lineatus when lures were placed in the Yf trap to monitor other target species. The A. brevis and A. obscurus lures, which contain the same active ingredients as the A. lineatus lure (geranyl butanoate and geranyl octanoate respectively) caught as many A. lineatus beetles as the specific A. lineatus lure. The presence of an A. ustulatus lure had no impact. The trial also confirmed that catches of A. sputator in the same Yf trap were inhibited when the other lures contained geranyl octanoate. Table 20 shows that catches of A. rufipalpis were also inhibited when lures containing geranyl butanoate (contained for example in A. lineatus lures) were used in the same Yf trap.

Table 19 Trial 16 Evaluation of whether A. lineatus and A. obscurus can be monitored when their specific lures are replaced with ones for other species containing an active ingredient in A. lineatus lures
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Table 20 Trial 17 Effect of adding geranyl butanoate (in A. lineatus and A. obscurus lures) to geranyl hexanoate (only compound in A. rufipalpis/A. sordidus lures) on catches of A. rufipalpis click beetles in Yf traps
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Discussion

Table 21 shows results of pairwise comparisons when a second lure was added to the same trap alongside the target species lure. In most cases, there was no significant influence, but when the A. lineatus or A. obscurus lures were added to the A. sputator lure, catches of A. sputator plummeted. Likewise, when the A. lineatus lure was added to traps containing the A. rufipalpis lure, catches of A. rufipalpis also decreased significantly. It was also demonstrated that A. litigiosus and A. ustulatus lures could be added as third and fourth lures to traps already baited with A. brevis and A. sordidus lures without any negative impact on the number of catches by single lures (Table 16).

Table 21 Influence of a second lure, added to the target lure, on target species catches in field-trapping trials
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The addition of A. litigiosus and/or A. ustulatus lures to traps baited with A. sordidus and/or A. brevis lures did not reduce the catches of any of the species, provided that each lure was placed into its correct trap position: A. brevis in the low position; A. sordidus in the medium position; A. litigiosus and A. ustulatus in the medium and/or high positions (Furlan et al. 2023). This way it was possible to bait the same trap with up to four different lures, with optimal results for each of the target species.

It is clear from the results that lures for many Agriotes species can be combined without loss of performance against most species. The only clear exceptions were A. sputator and A. rufipalpis (both sensitive to the addition of other lures containing geranyl octanoate); and A. brevis (sensitive to the addition of the A. obscurus lure, which also contains geranyl butanoate). In bare soil or sparse vegetation, A. brevis and A. lineatus require the lure to be in the same trap position (i.e., low), meaning that in practical terms it would be better to use two separate traps when both species are important. Individual traps are also advisable when the target species (e.g., A. brevis and A. sputator) share a pheromone component (Table 1 and 21). This prevents any interference between lures and the need to separate similar beetles when counting the total for each species.

The inhibitory effect on A. sputator catches can probably be attributed to geranyl octanoate, which is used in both the A. lineatus and A. obscurus lures. No other lures contained this compound. Likewise, as geranyl octanoate is contained in lures for other species, catches of A. rufipalpis decreased significantly when lures containing it were added to traps.

In North America-based studies, the presence of geranyl hexanoate (pheromone component of A. mancus Say) also inhibited A. sputator catches (Singleton et al. 2023), suggesting that A. sputator is more sensitive to the components of additional lures than other Agriotes spp.

Using A. sputator and A. obscurus lures in the same trap enables A. lineatus to be caught as well, since A. obscurus lures contain geranyl octanoate and A. sputator lures contain geranyl butanoate. Both act as A. lineatus attractants, and the second main component in A. obscurus lures, geranyl hexanoate, does not inhibit A. lineatus.

No data have been presented on the addition of a second lure when A. proximus was the target species. High numbers of A. proximus were only caught in Portugal, with lower numbers being caught in Bulgaria at one site in earlier Europe-wide trappings (Furlan et al. 2007). Furthermore, we were unable to conduct trials specifically targeting A. proximus over the course of the present trials. As A. proximus has proven to be similar to A. lineatus (Tóth et al. 2008) in all other aspects of its pheromone composition, we have reason to believe that the two species are also similar with regard to the effect of lure position and additional lures in the same trap.

In light of the interaction between lures and the effect of lure position, the most effective lure combinations in Yf traps are summarized in Table 21. In practical terms, some combinations proved to be useless, as the two species were not usually important at one site and in the same period of the season simultaneously.

Also note that the findings of this study obtained on European populations should not be automatically applied to populations on other continents. For example, Canada-based studies showed that the addition of the A. lineatus lure to traps with the A. obscurus lure vastly reduced catches of A. obscurus when it was the target species (Vernon et al. 2014b; van Herk et al. 2022). The authors state that A. lineatus do not enter an A. obscurus trap if they have a choice. This discrepancy between the Canada-based studies and the present paper may have been caused by the former using a trap with a vastly different design to the YATLORf traps. Alternatively, the two species, which were introduced to North America from Europe, might have acquired new evolutionary traits in their newly invaded geographical area. In the present study, the combination of A. obscurus and A. lineatus lures were tested in Germany, Hungary and Italy, with no interference being found in any country. No trials were conducted in the UK, where it is highly probable that the Canada click beetles came from. Therefore, we conclude that Yf traps can be multi-baited with A. obscurus and A. lineatus lures on mainland Europe.

The pheromone lures for Agriotes spp. discussed in the present study are remarkably different from the pheromone lures for moths. It is most rare that lures for two moth species of the same genus can be combined in the same trap without inhibitory effects. However, the present paper demonstrates that this can be done without any issues for most of the click beetles studied. One possible explanation is that competition for a species-specific pheromonal communication channel is more intense in moths, where a compound can be found in the pheromone of dozens of species (or more) (Roelofs and Cardé 1977), thus making the role of secondary pheromone compounds more important. The results of our study suggest that competition for a selective communication channel might not be so intense in Agriotes click beetles. This notion is also indirectly supported by the relatively few “side catches” by the lures used in earlier trials conducted at numerous sites across Europe (Furlan et al. 2007).

A recent North America paper reports that the pheromones of some Agriotes spp. (non-native to North America but introduced by human activities) can be combined with pheromones of Limonius spp., which is indigenous to North America (Lemke et al. 2023). This may support indirect evidence that multi-baiting pheromone traps could work when the target species did not evolve in that geographical area, opening up the opportunity of adding pheromones of heterogenic click beetles to the same trap in a bid to monitor multiple elaterid pests. However, as Lemke et al. (2023) correctly comment, such mixed lures should be specifically evaluated for each species combination.

Conclusions

Sex pheromone traps have become an important tool for monitoring wireworms belonging to genus Agriotes (Furlan et al. 2020), as well as for monitoring other genera, e.g., Limonius (Lemke et al. 2023) and Melanotus (Schoeppner et al. 2023). However, their use must be optimized if they are to make a tangible contribution to IPM. In our research, multi-baiting the same trap, i.e., using several lures, enabled various species to be caught in one trap. It was possible to multi-bait a trap with up to four different lures (A. brevis, A. sordidus, A. litigiosus, and A. ustulatus) with good results, thus demonstrating for the first time that important soil pest species belonging to the same genus can be monitored with multi-baited sex pheromone traps.

Brief guidelines for optimum use of Yf traps (compiles the results of Furlan et al. 2023 and those included in this paper):

  1. 1.

    The most reliable way to assess beetle population pressure when the target field is covered with dense vegetation is to position the trap just outside the target field, or in a nearby field, i.e., within about 200 m, with bare soil or sparse vegetation. Target fields with bare soil or sparse vegetation have no restrictions, thus the best position is inside the target field. The only exception is A. brevis traps, which must always be placed in the target field (Furlan et al. 2023);

  2. 2.

    Traps may be multi-baited in accordance with conditions in the monitored area. Possible combinations are listed in Table 21. The addition of A. litigiosus and/or A. ustulatus lures to traps initially baited, or still baited with A. sordidus and/or A. brevis lures did not reduce the catches of any species provided that each specific lure was placed into its correct trap position: A. brevis in the low position; A. sordidus in the medium position; A. litigiosus in the high position; and A. ustulatus in the medium position (Fig. 1);

  3. 3.

    In both single and multi-baited traps, A. brevis and A. lineatus lures need to be placed in the low position when the field has bare soil or sparse vegetation cover. The high position is less suitable for A. brevis, A. obscurus, and A. lineatus when the field has bare soil or sparse vegetation cover. There are no restrictions for the other species, i.e., any position is suitable.

Multi-baited traps can significantly reduce monitoring costs in terms of materials, as one trap baited with four lures saves the cost of three Yf traps. In addition, major savings are made in terms of the time and travel needed for trap inspections. One downside is that they require more time for beetle identification and counting, as the captured species need to be separated. The results of this study suggest that competition for a selective communication channel might not be very intense in Agriotes click beetles.

This research enables Yf traps to be exploited to their fullest potential, resulting in easier, more affordable Integrated Pest Management (IPM) of soil pests, such as wireworms. Consequently, farmers can establish which areas and fields have pest populations below the economic threshold (Furlan et al. 2020) and implement IPM in accordance with Directive 128/2009/EC.