Enhancement of Samurai Wasp [Trissolcus japonicus (Ashmead)] for Biocontrol of Invasive Brown Marmorated Stink Bug [Halyomorpha halys (Stål)] in Utah

Objective 1:

Results

Trissolcus japonicus Detections

In 2019, T. japonicus was detected in two of five counties surveyed (24 total sites; Salt Lake and Weber) throughout the sampling period. Seasonal detections occurred from 171 to 1043 DD_12℃. In 2020, detections occurred in three of five counties surveyed (31 total sites; Davis, Salt Lake, and Utah) and from 262 to 1149 DD_12℃. Detections in 2021 were present in all six counties surveyed (36 total sites; Box Elder, Cache, Davis, Salt Lake, Utah, and Weber) and occurred throughout the entire sampling period (210- 1367 DD_12℃). By the end of September 2021, T. japonicus was detected 85 km both north and south of the original 2019 detection site in Salt Lake City (Fig. 1). In all years, detections began increasing around 600 DD_12℃, peaking from 700 to 1,000 DD_12℃ in 2019 and 2020, and from 1,000 to 1,200 DD_12℃ in 2021(Fig. 2). Mean T. japonicus captures per YSC increased each year with 1.61 (±0.58), 2.23 (±1.34), and 3.51 (±0.84) in 2019, 2020, and 2021, respectively.

Landscape Effects on Trissolcus japonicus Abundance

In the first two years of surveys, T. japonicus only occurred at sites with highly urban surroundings (Fig. 3A). In 2021, 8% of detections were in low urban sites, 13% in medium urban sites, and 79% of captures occurred in high urban sites. Urbanness was a significant effect each year (2019: χ2 = 7.65, df = 2, p = 0.022; 2020: χ2 = 7.38, df = 2, p = 0.025; 2021: χ2 = 46.0, df = 2, p < 0.001). Pairwise comparisons were not significant in 2019 and 2020. In 2021, T. japonicus was more abundant at high than low urban sites (p < 0.001). Across years, there were no significant differences in T. japonicus abundance in low urbanness sites (χ2 = 3.41, df = 2, p = 0.182). However, in medium and high sites the effect was significant (medium: χ2 = 12.1, df = 2, p = 0.002; high: χ2 = 40.2, df = 2, p < 0.001) and there were higher captures in 2021 than in 2019 (medium: p = 0.023; high: p < 0.001) and 2020 (medium: p = 0.006; high: p < 0.001) (Fig. 3A).

Groundcover type significantly affected T. japonicus abundance in the 24 orchard sites in 2021 (χ2 = 6.96, df = 2, p = 0.031); there were significantly more captures in bare ground orchards than turfgrass orchards (p = 0.029). Other pairwise comparisons were not significant. A total of 64 T. japonicus were captured; 31 (48%) in bare ground orchards, 9 (14%) in turfgrass, and 24 (38%) in floral (Table 1). Trissolcus japonicus experienced a similar peak (1000-1300 DD_12℃) across all groundcover types in detections (Fig. 4B) and peak season analysis did not demonstrate any significant differences in abundance across groundcover types (χ2 = 3.03, df = 2, p = 0.22).

Mean captures of H. halys were low with 0.72 (±0.15), 0.23 (±0.08), and 0.06 (±0.03) adults per biweekly for bare, turfgrass, and floral, respectively (Table 1). Halyomorpha halys demonstrated similar patterns of peak abundance to T. japonicus (Fig. 4A) and mean T. japonicus captures per YSC in commercial orchards with H. halys presence were significantly higher compared to mean detections at sites with H. halys absence (χ2 = 11.5, df = 1, p < 0.001).

Landscape Effects on Native Trissolcus spp. Abundance

Of native Trissolcus spp. detected, T. euschisti was by far the most abundant (70%), followed by T. utahensis (Ashmead) (9%), T. hullensis (Harrington) (4%), T. ruidus (Johnson) (<.1%), T. strabus (Johnson) (<.1%), and T. parma (Johnson) (<0.1%) (Table 2). The exotic T. japonicus made up only 3-7% of Trissolcus spp. detected in northern Utah each year. An average of 13% of Trissolcus captures were unidentifiable beyond genus due to damaged specimens.

Urbanness was a significant effect in captures of native Trissolcus species in 2019 (χ2 = 8.42, df = 2, p = 0.015), but not in 2020 (χ2 = 0.567, df = 2, p = 0.753) or 2021 (χ2 = 4.68, df = 2, p = 0.096). Only pairwise comparisons between low and high urbanness in 2019 were significant (p = 0.015) and no clear trends across years were present (Fig. 3B). Across years there were no significant differences in Trissolcus abundance in low urbanness sites (χ2 = 3.91, df = 2, p = 0.142). However, in medium urbanness sites the effect was significant (χ2 = 14.6, df = 2, p < 0.001) with higher captures in 2021 than 2020 (p < 0.001).  In high urbanness sites there was also a significant effect of year (χ2 = 31.3, df = 2, p < 0.001) with lower captures in 2020 than in either 2019 (p < 0.001) or 2021 (p = 0.002) (Fig. 3B).

In 2021, orchard groundcover type did not significantly influence captures of native Trissolcus spp. combined throughout the season (χ2 = 2.07, df = 2, p = 0.356); however, response of native Trissolcus only started to diverge after 900 DD_12℃ and significant effects occurred during peak abundance, from 900 DD_12℃ through the end of sampling (χ2 = 10.3, df = 2, p = 0.006).  A total of 1,688 Trissolcus were captured in the 24 orchard sites: 394 (23%) in bare ground, 386 (23%) in turfgrass, and 908 (54%) in floral (Table 1). Pairwise comparison demonstrated that peak season native Trissolcus spp. abundance was significantly higher in floral orchards than bare (p = 0.007) and higher with borderline significance in floral than turfgrass (p = 0.0518). Trissolcus spp. abundance was similar between bare and turfgrass groundcovers and showed no significance in pairwise comparisons. Trissolcus spp. proportions were similar across mean biweekly captures for each groundcover type (Fig 5).

In contrast to T. japonicus, peak total Trissolcus spp. captures were misaligned with H. halys detections (Fig. 4A-C), and its presence or absence did not have a significant effect on captures (χ2 = 0.779, df = 1, p = 0.377).

Discussion

               Despite the unique geographic challenges of extreme climate and high elevation, Trissolcus japonicus has successfully established in northern Utah since its initial detection in 2019 (Holthouse et al. 2020). Northern Utah consists of dry, semi-arid, and desert climates with average low temperatures below the estimated  minimum threshold for T. japonicus development (12.2° C; Qiu et al. 2007) for up to eight months of the year (Utah Climate Center 2022).  As adventive T. japonicus populations continue to persist in this harsh climate, it is crucial to understand its phenology in order to support conservation efforts in the Intermountain West.

The peak season of T. japonicus detections observed in this study falls in the months of July and August and is similar to the results Quinn et al. (2021) observed in the eastern U.S. In 2021, the peak season of T. japonicus was later than in previous years, possibly due to extreme drought conditions in Utah. The 18 months from January 2020 to June 2021 were the driest 18 month period on record for northern Utah (NCEI 2022). Some studies have shown that precipitation may play a greater role than temperature in some insect taxa occurrence in North America (Gutiérrez Illán et al. 2020), including the occurrence of T. japonicus’ primary host H. halys (Illán et al. 2022). Future research on climatic variables other than degree days will help further elucidate the potential range and seasonality of the exotic T. japonicus in the Intermountain West. Regardless, the capacity to survive and expand its distribution in areas previously considered marginal or unsuitable in geographic models (Avila and Charles 2018) has implications for range expansion of the exotic species in North America.

Halyomorpha halys has demonstrated a notable ability to disperse and thrive outside of predicted ranges (Kriticos et al. 2017, Illán et al. 2022); it follows, therefore, that T. japonicus may demonstrate similar capabilities. Recent analysis of H. halys populations found that proximity to urban areas was one of the most important factors mediating H. halys occurrence and likely reflects human-assisted transport, the use of human structures for overwintering sites, and increased ornamental plant diversity (Illán et al. 2022). Our analysis demonstrates that T. japonicus presence is also highly affected by proximity to urban areas; this may be due to its direct reliance on its host and/or its sustainment from urban area resources.

 This study focused on the resources provided by the habitat in a 1 km buffer around each site, and groundcover composition in 2021 surveys of commercial orchard sites. The significant influence of urban habitat in contrast to the nonsignificant factor of groundcover type during peak season may be evidence that T. japonicus populations are utilizing resources from a greater dispersion area than previously hypothesized. Further research utilizing increased buffer sizes around sites may demonstrate an even greater extent of the wasps’ mobility to access floral and other resources. In contrast, this may indicate that urban ornamentals provide resources more appropriate to the exotic T. japonicus as compared to those provided by strip-cropped flowering plants, weeds, or wildflowers naturally occurring in orchard settings.

Overall, orchards with bare soil had comparable captures of T. japonicus to floral sites, while turfgrass sites had few T. japonicus. The interval between cultivation events at bare soil sites may have given rise to forbs that were beneficial to parasitoids. In addition, frequent disruptions of turfgrass from mowing and other management practices may have been a detriment to T. japonicus populations (Horton et al. 2003).  Though insecticide applications were assumed to be similar across orchards and groundcover types, application pattern variation may have contributed to biased results as T. japonicus is highly sensitive to certain insecticides and is unlikely to survive in settings with broad-spectrum insecticide applications (Lowenstein et al. 2019).

A large underlying factor in the presence and abundance of T. japonicus is the presence and preferences of its host; the benefits floral understory provide are irrelevant without the insect host to support parasitoid populations. In 2021, we attempted to measure the effect of host presence on parasitoid abundance alongside groundcover by trapping for H. halys; however, the abundance of H. halys was low and these data were condensed to presence absence information. The presence of H. halys adults was a significant factor of T. japonicus abundance and the demonstrated alignment of seasonality between the host and natural parasitoid provides strong support for the continued reliance of T. japonicus on its natural host rather than pentatomids native to North America.

While we did not measure the effect of crop type on T. japonicus abundance, of the orchard crops surveyed in this study (apple, peach, and tart cherry), peach is known to be a host of particular significance to H. halys nymphal development and survivorship (Acebes-Doria et al. 2016). In this study bare groundcover sites more often had peach crop than apple or tart cherry and this may have had a significant effect on H. halys abundance. Regardless of the cause, on average, sites with bare groundcover had higher abundance of H. halys which could have significantly skewed the comparative proportion of T. japonicus at those sites and biased the apparent effect of bare groundcover. Had there been equal abundance of H. halys between all groundcover types, the benefits of floral resources for T. japonicus may have been more distinct.

Trissolcus japonicus presence was significantly affected by groundcover over the course of the season, but captures were not significantly different among groundcover types during the peak season of abundance and our prediction of increases due to floral resources was not supported. In addition to possible host bias, this may be due to overall low populations of T. japonicus in Utah as it is still in the establishment phase (Holthouse et al 2020). Research in Frederick County, VA  demonstrated relatively few T. japonicus captures in the first few years of surveys after its initial detection in 2015 followed by subsequent increases in relative abundance in following years (Dyer et al. 2022). Additionally, the suboptimal climatic conditions of the Intermountain West may keep T. japonicus at relatively low densities as compared to locations with more suitable climate and availability of its host, H. halys.  Surveys in this study represent T. japonicus still in its early establishment phase, and further investigation of this exotic outside its native range may yield more accurate assessments of its response to varying groundcovers.

Native Trissolcus species made up the majority of Trissolcus detected in agricultural and urban site surveys and provided a much larger sample size to detect true differences due to groundcover. Native species increased in response to floral resources supporting our original hypothesis of parasitoid preference for floral groundcover, though this was only apparent during the peak season of abundance, late in the growing season (>900_12℃). Other studies have demonstrated the importance of floral resources for Trissolcus, particularly on  fecundity (Hajirajabi et al. 2016, Foti et al. 2017), and foraging may be of increased priority to parasitoids as opportunities to reproduce increase alongside stink bug host abundance.

The results from this study continue to provide essential information on the presence and abundance of Trissolcus parasitoids native to northern Utah. Trissolcus euschisti was the most abundant Trissolcus spp. in northern Utah making up over two thirds of Trissolcus spp. detections on YSC, in line with previous YSC surveys in Utah (Holthouse et al. 2021b). While this wasp does have the ability to attack and successfully parasitize H. halys, rates of wasp emergence are low and have not shown sufficient evidence of effectively suppressing H. halys populations in Utah or elsewhere (Cornelius et al. 2016, Dieckhoff et al. 2017, Schumm et al. 2019, Costi et al. 2020, Holthouse et al. 2020, Konopka et al. 2020). In addition, our results suggest a seasonal misalignment of native Trissolcus abundance with that of H. halys.  Conversely, T. japonicus demonstrated similar seasonality to its natural host in our study and has already shown the ability to suppress H. halys populations despite their low abundances (Holthouse et al. 2020).

The data presented have significant implications for T. japonicus as a biological control agent of H. halys in the Intermountain West and other regions. We present evidence of the importance of urban habitat to the presence and establishment of T. japonicus and suggest that the parasitoid is taking advantage of resources outside of resident orchard sites. In addition, the seasonality implications of the research include improved timing precision for mass releases of wasps with optimal ability for successful establishment, avoidance of harmful pesticides, and for provision of beneficial floral resources.  This study supports strategies for further research on an effective biological control agent of H. halys and advances our understanding of the establishment of T. japonicus and native Trissolcus spp. in northern Utah.

Objective 2:

Results

Preliminary results for this experiment were disappointing. When placed in the y-tube apparatus, T. japonicus individuals did not select an odor preference in most experimental runs. Many individuals remained within the insertion tube or in the main stem for the duration of the 5 min trial period without making a choice. After adjusting air flow, the trial wasps would choose to move around the y-tube back and forth between the two arms with no distinct searching behaviors. Results for preliminary trials (Fig. 6) show there was little difference between the control and floral odor arm residence times. ANOVA tests run on the pairs did not show statistical differences. As more trials were run, the average residence time became increasingly similar as seen in the buckwheat trials (n=46).

As part of the wasp longevity and fecundity trials we only ran a few trials to test experimental set up. The few preliminary trials that were run suggest that buckwheat, alyssum, alfalfa, and red clover may be life sustaining, while hairy vetch and birdsfoot trefoil show much less promise as wasps died within a 48 hr period.

Discussion

After reviewing preliminary results, numerous adjustments were made to the experimental equipment and design, including air flow, floral load, and pre-experiment feeding status of wasps. Eventually, the decision was made to not continue this study. Our experimental set-up and/or equipment did not seem to be an appropriate means to measure T. japonicus attraction to floral resources. Research produced after the abandonment of this experiment determined that T. japonicus reaches peak activity 6-7 days post emergence (Paul et al. 2022) and it is possibly that the 24-48 hr old wasps were too young and inactive to produce accurate results.

The results for the y-tube experiment were to determine which flowers should be tested as part of the wasp longevity and fecundity trials. Without the initial results to verify which floral resources may be attractive, we chose to cancel the wasp longevity and reproduction trials. Resources that are life sustaining, but not attractive to T. japonicus, serve little benefit in an agricultural setting without attractive volatiles to guide the wasps to floral resources.  

At this point, the study of kairomones as part of objective 3 was showing great promise, but the results were inconclusive due to low parasitism rates. We decided to shift our research focus away from this objective 2 study in favor of being able to perform a more controlled kairomone experiment in a mesocosm arena as part of objective 3.

Objective 3:

Results

Field Trials

Of 92 total egg masses containing 2,472 eggs deployed in the field, only 13 masses had evidence of parasitism (14.1%; guarding female present or eclosed parasitoids) of which 6 masses supported successful emergence of adult parasitoids (6.5%). The lure treatments in order of highest proportion of eggs with successfully emerged parasitoids were 100% n-tridecane (10.6%), hexane control (5.2%), 80% n-tridecane (2.9%), and 90% n-tridecane (0.0%) (Fig. 7). Though the 100% lure resulted in double the parasitism of the control and more than three times that of the 90% and 80%, the analysis of all egg fate categories did not support statistical differences (p > 0.05; Table 3)

T. japonicus attacked 5.4% of deployed egg masses. This included one egg mass in each of the control and 80% attractant treatments and three in the 100% attractant treatment. Of the 129 H. halys eggs T. japonicus parasitized, there was a high rate of successful emergence (78.3%). Trissolcus euschisti attacked 6.5% of egg masses deployed with two egg masses in each of the control, 100%, and 90% attractant treatments and none in the 80% attractant treatment. Successful emergence of T. eushisti only occurred in one of the control treatment egg masses and the rate of overall emergence from attacked egg masses (136 eggs) was 1.8%. Two egg masses in the control treatment were parasitized by an unknown Trissolcus sp(p) and demonstrated no successful emergence (Table 4). In one instance, a T. euschisti female was found guarding an egg mass which later produced 26 T. japonicus individuals (92.9% emergence rate) and 2 inviable eggs.

Undeveloped parasitoid rates ranged from 1.6-5.9% across treatments, were highest in the 80% attractant treatment, and lowest in the 100% attractant treatment. The rate of unhatched H. halys was also variable across treatments making up the largest percentage of egg fate for the control treatment (26.5%) and was noticeably lower in the 100% attractant treatment compared to others. There was less than 2% difference in the predation between lure treatments. Sunken rates were relatively high (17.7-26.0%), and the combination of missing and empty eggs on average across lure treatments made up <10% of egg fate. Rates of hatched H. halys nymphs were similar across lure treatments containing stink bug kairomones (36.4-41.0%) but were unexpectedly low for the control lure making up approximately half of that seen in non-control lures.

Mesocosm Trials

In the mesocosm system, T. japonicus females showed significant preference for the kairomone blend over the control for two of the four treatments tested (Fig. 8). The 10 mg-80% n-tridecane showed the most significant response (p = 0.003) with 73% of individuals choosing the kairomone lure. There was also a significant difference between the control and 10 mg-100% n-tridecane treatment lure (p = 0.04) with 60% of wasps choosing the kairomone-treated side. For the 10 mg-90% n-tridecane treatment, 55% of wasps chose the kairomone lure; however, there was no significant deviation from the expected response (p = 0.55). The choice between the low load rate treatment of 5 mg-100% n-tridecane and the control also showed no significance (p = 1); an even proportion of wasps chose each side. The proportion of no choice wasps per treatment varied from 20%-32% of wasps tested and was not associated with treatments.

Discussion

This study is the first to assess and provide proof-of-concept for kairomone-infused rubber septa lures to attract T. japonicus and other potential H. halys parasitoids in field and mesocosm settings. Though the kairomone chemicals tested herein have been previously evaluated in a small-scale lab setting (Y-tube olfactometer) (Zhong et al. 2017) implementation in the field may provide different results due to the influence of external factors on the shape, concentration, longevity, and spatial extent of the kairomone odor plume. In addition, interaction of the lure plume with nearby plant surfaces and mixing with other volatiles may give rise to plume masking or plume amplification, of which little is known in a parasitoid searching context (Dicke et al. 2003).

Our field results contrast those of Malek et al. (Malek et al. 2021) where the combination of the attractant and repellent was preferred over the attractant alone. In our study, not only did the 100% n-tridecane attractant lure have a numerically (not statistically) higher parasitism rate than the control, the lures containing the previously identified repellent (E)-2-decenal had less parasitism than those without the repellent, including the hexane control. These results may suggest that under field conditions a combination of the attractant and repellent is less effective at increasing parasitism by Trissolcus spp., at least with the specific load rate tested and placement in near proximity to the target host egg masses.

The lures used in the field study contained a load rate of 10 mg per lure, high in comparison to standard pheromone lures (1 mg), in the hopes of ensuring the attraction of parasitoids (Danielle Kirkpatrick Trécé, Inc., Adair, OK); however, this relatively high load rate may have been counterproductive to our objective. In studies of pheromone lure load rates, it has been found that attraction often plateaus and can even become repellent to the insect at high release rates (Knight and Light 2005, Stelinski et al. 2005, Charles et al. 2015). In addition, our lures were placed directly adjacent to the H. halys egg masses (Fig. 5- Methods section), which may deter parasitoid attraction as the host female stink bug does not typically remain near the eggs following oviposition.

Effect of lure load rate on parasitoid attraction was explored by examining both a 5 and 10 mg treatment of only the attractant n-tridecane in the mesocosm trials. The results of these trials demonstrated that the higher load rate of 10 mg was a more viable option for attracting T. japonicus and did not have a repellent effect. In contrast, the 5 mg treatment demonstrated no difference in attraction as compared with the hexane control lure. Based on these findings, the load rate may not have been detrimentally high in the field trials; however, we did not assess if a load rate greater than 10 mg could increase parasitoid attraction in a field setting.

Given the low number of egg masses tested in field trials and lack of statistically significant results, the parameters of this field study may have been unsuitable to discern differences in attractiveness of the different kairomone lure treatments. We observed low parasitism rates across all egg mass deployment periods in this study. In previous surveys, the site selected for the field study had the highest abundance of T. japonicus observed in northern Utah with concomitant high parasitism rates of H. halys (Holthouse et al. 2020). However, compared to T. japonicus populations in its native geographic regions and other adventive populations in the U.S., the abundance of T. japonicus in Utah is relatively low (Talamas, Herlihy, et al. 2015, Quinn et al. 2021). In addition, Utah suffered from significant drought over the time frame of this study (summer 2021) with 99.94% of the state in “extreme” or “exceptional” drought categories (Utah Climate Center) which may have had a negative effect on host abundance, parasitoid wasp populations, and H. halys egg parasitism rates (Zhu et al. 2012).

Other researchers have observed that lab-reared egg masses perform inferiorly to wild egg masses in terms of their attractiveness to parasitoids (Jones et al. 2014, Abram et al. 2017, Holthouse et al. 2020). The lures tested here were an initial attempt to solve this issue and increase the accuracy of parasitism rates detected in deployed egg mass surveys. Parasitism was observed in wild H. halys egg masses near deployed, unparasitized egg masses during the field study. Without an in-depth analysis of the kairomone plume release from lures, it is unknown if the plumes for each lure treatment may have overlapped within the tree block (lures were separated by approximately 3 m of tree canopy), and potentially interfered with one another. In addition, this may suggest the lure dispersed kairomones had unknown interactions with the surrounding environment or may have lacked the necessary kairomone load rate and/or ratio necessary to match the high attractiveness of wild egg masses with their natural kairomones intact.

Interestingly, the results of the mesocosm environment did not fully align with the field trial results, suggesting lures containing (E)-2-decenel may be repellent, but were better aligned with those of Malek et al. (Malek et al. 2021) where the 80% n-tridecane ratio was more attractive than n-tridecane alone. The result of wasp attraction to the 100% and 80% lures and unexpected lack of attraction to the 90% treatment occurred both in the field and in the mesocosm trials. Research has suggested that the variable secretions by different sexes and life stages of H. halys may be linked to their specific functions at said life stages and physiological states (Harris et al. 2015, Nixon et al. 2018, Malek et al. 2021). Perhaps the 100% and 80% n-tridecane treatments in this study are more closely associated with the gravid/ovipositing adult female or egg life stages that can be exploited by T. japonicus, while the 90% treatment is associated with other life stages. Further investigation is necessary to understand these counterintuitive results.

The mesocosm trials presented the opportunity to test the response of a much larger sample size and demonstrate significant differences between the control and treatments verifying the validity of these lures. While these trials supported the attractiveness of certain lures in a controlled environment, they also didn’t include many of the external factors at play in an authentic environment such as plant volatiles and competing parasitoids and predators. In addition, our mesocosm trials did not include the presence of host eggs which may have altered T. japonicus response.

In contrast, the field trials provide essential information about the interaction of host and parasitoid in a novel geographic and climatic environment. The results further verify the preliminary research in Utah on the effectiveness of the exotic T. japonicus and the common native T. euschisti. In this study, as in Holthouse et al. (Holthouse et al. 2020), T. euschisti demonstrated the ability to parasitize H. halys in a similar proportion to the natural parasitoid of H. halys, T. japonicus. However, T. euschisti seems an inviable option for successful biological control due to very low adult wasp emergence and successful stink bug nymph development likely associated with the failure of T. euschisti eggs to hatch or early death of larvae within H. halys eggs (Konopka et al. 2020). Additionally, this poses an evolutionary trap for T. euschisti and other native Trissolcus species that accept H. halys eggs as ovipositional sites despite their poor reproductive investment (Peri et al. 2021), although there is evidence that the recent arrival of T. japonicus may have implications for success of native Trissolcus on H. halys egg masses.

Research has demonstrated that Trissolcus spp. are able to parasitize host eggs that have previously been parasitized. In the case of T. japonicus and T. mitsukurri (Ashmead), each species outperformed the other when it was the first to oviposit, though T. mitsukurri was more aggressive in chasing off T. japonicus when present concurrently (Costi et al. 2022). Similar timing of competition and outperformance may have been the case in our observation of T. euschisti guarding an egg mass (presumably parasitized by T. japonicus prior to T. euschisti oviposition) that only produced T. japonicus offspring. Conversely, research by Konopka et al. (Konopka et al. 2016) demonstrated that parasitism by the exotic T. japonicus can provide facultative parasitism opportunities for native Trissolcus such as T. cultratus (Mayr) to successfully develop in H. halys eggs when they would otherwise fail.

Regardless of their reproductive success, native Trissolcus species can reduce the developmental success of H. halys embryos providing low levels of control for the pest (Abram et al. 2014). The attraction of T. eushisti and other native Trissolcus species to kairomone lures containing n-tridecane and (E)-2-decenal were not explored in a mesocosm environment in this study. However, the use of these kairomone lures in the field did result in attacks from T. euschisti in addition to T. japonicus. Consequently, further research into the physiological and behavioral interactions between the exotic T. japonicus and North American native Trissolcus requires investigation to fully evaluate the potential efficacy of biological control programs against H. halys.

While much investigation is needed into the complex system of parasitoids using semiochemicals to locate their host, the mesocosm results presented support the validity of using rubber septa as a release device for kairomones of stink bugs, and this research provides not only preliminary results but also a baseline for future work with experimental lures for H. halys parasitoids in a field environment.

Tables and Figures

Table 1. The number of H. halys adults and Trissolcus spp. captured in orchards with bare, turfgrass, and floral understories. Proportional abundance of species captured in each groundcover type is provided in parentheses. Data were collected 19 May through 5 October 2021 at 24 commercial orchard sites in northern Utah.

*Trissolcus wasps that were unidentifiable below the genus level due to damage are presented as Trissolcus spp.

Table 2. The number of Trissolcus spp. identified on yellow sticky cards in northern Utah, 2019-2021. Proportional abundance of each species by year is provided in parentheses.

*Trissolcus wasps that were unidentifiable below the genus level due to damage are presented as Trissolcus spp.

Table 3. Statistical results from Kruskal-Wallis tests to compare each egg fate category differences across treatment lure types.

Table 4. Number of Halyomorpha halys egg masses parasitized (and percent of eggs with wasp emergence) in kairomone lure treatments for observed parasitoid species. Lure treatments are labeled based on percentage of n-tridecane attractant to (E)-2-decenal repellent; the control contained only hexane. Ninety-two H. halys egg masses were deployed containing 2,472 eggs on Catalpa speciosa leaves in Salt Lake City, UT, from 24 June through 27 August 2021.

*A T. euschisti female was found guarding an egg mass that resulted in 0% T. euschisti and 92.9% T. japonicus emergence. It was therefore only counted in the T. japonicus column.

Fig. 1. Trissolcus japonicus captures from late May through early October in northern Utah 2019-2021. Number of sites surveyed per year is shown as n at the top of each map. The map insets by year depict T. japonicus captures at and near the original detection site in Salt Lake City.

Fig. 2. Mean captures of Trissolcus japonicus on yellow sticky cards from late May through early October 2019-2021. Results are plotted by average degree days accumulated (DD12℃) at the end date of each 14 d (+1 d) deployment period.

Fig. 3. Mean biweekly captures of Trissolcus japonicus (A) and native Trissolcus spp. (B) in sites with low, medium, and high proportions of urban land in the surrounding 1 km boundary of survey sites. Lowercase letters represent significant differences among urbanness categories within a year. Uppercase letters represent significant differences among years within urbanness categories (Kruskal-Wallis test and Bonferroni corrected Dunn’s test, p > .05).  

Fig. 4. Mean captures of Halyomorpha halys (A) on clear dual panel sticky strap, and Trissolcus japonicus (B) and Trissolcus spp. (C) on yellow sticky cards by average accumulated degree days of each deployment end date. Counts were averaged by groundcover designations of bare, turfgrass, and floral with eight sites of each designation.

Fig. 5. Mean captures of Trissolcus species per biweekly yellow sticky card deployment from May-September 2021. Cards were deployed at eight sites of each groundcover designation (bare, turfgrass, and floral). “Unidentifiable spp.” represents wasps that were unidentifiable beyond genus due to physical damage during capture.

Fig. 6. Residence time in the treatment vs. control arm of the Y-tube olfactometer out of 5 minutes.

Fig. 7. Fate of field-deployed Halyomorpha halys eggs (n=2,472) in each kairomone lure treatment, shown as percentage of total eggs. Lure treatments are labeled based on percentage of n-tridecane attractant to (E)-2-decenal repellent: hexane (control, n=598), 100% (n=615), 90% (n=617), and 80% (n=597). A total of 6 egg mass deployments were made in Salt Lake City, UT, from 24 June through 27 August 2021.

Fig. 8. Proportional response of Trissolcus japonicus female adults that chose a kairomone lure treatment or control (n = 32–105) within the 30 min experimental period. Chi-squared values are presented and numbers in parentheses represent sample size; * p < 0.05; ** p < 0.01.

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