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Wasps in the genus Trissolcus (Hymenoptera: Scelionidae) are parasitoids of stink bugs and other insects in the Pentatomoidea superfamily (Order Heteroptera) and typically undergo sib-mating behavior where males emerge first from parasitized host insect eggs and remain near the natal site to mate with sib-females as they emerge. Although common in certain groups of parasitoid wasps, sib-mating often leads to inbreeding and subsequent reduced genetic diversity and fitness. During field surveys for native and exotic natural enemies of the invasive brown marmorated stink bug (BMSB, Halyomorpha halys Stål) in northern Utah, we discovered a male Trissolcus euschisti guarding a green stink bug (Chinavia hilaris Say) egg mass that was determined post-observation be a non-sib male based on the timing of its presence to subsequently emerging T. euschisti males and females. This finding suggests alternative mechanisms for avoiding inbreeding depression in a sib-mating species, and that outbreeding may be more prevalent than once thought in this sib-mating system.
Wasps in the genus Trissolcus (Order Hymenoptera: Family Scelionidae) are obligate egg parasitoids of insects in the superfamily Pentatomoidea (Order Heteroptera), particularly stink bugs (Heteroptera: Pentatomidae). Trissolcus wasps have been the focus of biological control efforts for the economically important invasive brown marmorated stink bug (BMSB, Halyomorpha halys Stål) in North America and Europe. Research has shown that chemical control programs lack effectiveness and sustainability; therefore, there is a strong interest in identifying effective biological control agents.
Host eggs utilized by parasitoids are vulnerable to competitors (such as egg predators and other parasitoids) when naturally laid; therefore, female parasitoids, including those in the genus Trissolcus, will often stay with their brood to guard them against challengers. In studied representatives of Trissolcus, once the development of progeny is nearly complete, the parent female will depart. Males will emerge from eggs in near unison and then competitively mate with sibling females when they emerge up to 3 days later.
Sib-mating is commonplace in the parasitoid family Scelionidae, which leads to inbreeding and may consequently result in fitness decline. Numerous animal taxa have evolved behavioral strategies for avoiding inbreeding risks. In vertebrates, due to selective breeding and mate-choice, sib-mating rarely occurs except in instances of accident or error, leading to more prevalent outbreeding. However, sib-mating species (primarily plants and insects) have developed other strategies to prevent inbreeding depression. Loch and Walter postulate that outbreeding in Trissolcus may occur if males and/or females can locate and/or attract the opposite sex after leaving the natal site. In addition, females may be able to locate parasitized host eggs and mate with unrelated males as they emerge from host eggs. Finally, males may be able to locate host eggs parasitized by unrelated females and compete with emerging males for access to the later-emerging females.
Evidence for mating after leaving the natal site is provided by Loch and Walter, where the outbreeding potential of Trissolcus basilis (Wollaston), an egg parasitoid of southern green stink bug (Nezara viridula Linnaeus) was explored. They found that a single, newly emerged male-dominated the natal site by guarding the eggs and chasing and behaving aggressively toward subordinate males. As many as 18% of females, however, dispersed from their natal egg mass to mate with subordinate males outside of the guarded arena. In addition, 25% of females were mated more than once by dominant and/or subordinate males near or away from their natal egg mass, and 13% of females departed the natal site and surrounding area without mating at all, leaving as virgins. Female dispersal from their natal site and polyandry are documented mechanisms for outbreeding in a typically inbreeding species. Non-sib mating at the natal site, another mechanism for outbreeding, has been suggested but has not heretofore been documented for parasitoid wasps in the genus Trissolcus.
We herein describe what we believe to be the first documented observation of non-sib male guarding behavior in a Trissolcus wasp at the natal site. Researchers routinely document guarding behavior by females and sib males at natal sites but never guarding by non-sib males. We explain our reasoning behind the conclusion that the male is a non-sib and consider other possible explanations for this observation.
Of the 13 eggs in the egg mass, a single male parasitoid emerged 6 days after collection. During the two observation periods that included the single emerged male, the originally marked male was guarding the eggs, aggressively chasing away the emerged male. 9 female wasps emerged between the last hourly check on Day 6 and the first hourly check on Day 10. The 3 remaining un-emerged eggs had the chorion chewed open with fully developed female wasps nearing emergence. The marked male was aggressively guarding the un-emerged wasps against the previously emerged wasps, regardless of the approaching wasp’s sex (Supp. Video). Active guarding of the un-emerged wasps by the marked male occurred over the entire 8 h observation period, indicating that the marked male was both dominant at the natal site and likely waiting to mate with the un-emerged females. We suspect that the marked male had already mated with many or all of the emerged females in this manner (mated upon emergence); however, mating would have occurred between the last observation on Day 6 and first observation on Day 10; hence no mating was observed between the marked male and any female. The 3 un-emerged wasps were unable to complete emergence and expired, at which time the observations were terminated. All wasps (i.e., the marked male, emerged male and females, and un-emerged females) were later identified as T. euschisti.
The observation of a non-sib male guarding a stink bug egg mass from emerging males and competing females has been suggested as a potential method for outbreeding, but remained undocumented. Given that males emerge from eggs within a mass in near unison and the first male emergence occurred 6 days after egg collection, it is likely that the male present at collection sought out the egg mass as a non-sib T. euschisti. This observation is additionally supported by the darkening of the eggs that occurred after collection, which typically occurs around 7 days after parasitism. Egg darkening post-collection indicates that parasitoids were still developing in the host eggs while the marked male was already present.
There is evolutionary pressure to avoid inbreeding depression in many faunae. For example, in larger animals, there is evidence for sex-biased dispersal, in which one sex disperses a greater distance than the other. Kin-detection can also prevent inbreeding, in which individuals recognize their kin using different visual, olfactory, or auditory cues to prevent sib-mating. In primarily sib-mating systems, other mechanisms to avoid inbreeding depression must be in place. In the case of Trissolcus wasps, while females can mate with unrelated males after departing the natal site, outbreeding may also occur via egg guarding by non-sib males as we propose here. To confirm, this mechanism requires support from other studies and could give a new light to inbreeding depression avoidance in sib-mating systems.
The documentation of the single observation herein was discovered during field surveys for a separate study. While we did observe guarding behavior of the non-sib-male at the natal site for a short period, the wasps and egg mass were collected to prevent the original wasps from departing. Further, although mating was not directly observed, it is presumed to have occurred shortly after female emergence based on the normal mating behavior of other males in the genus Trissolcus. This observation should lead to further studies to more thoroughly evaluate this potential mechanism.
While the entire host apple tree was surveyed for egg masses, it is possible that the guarding male emerged from a nearby, unseen egg mass parasitized by the same parent and/or sought to mate with the female that originally parasitized the egg mass or other passing females. However, these alternative explanations neither describe nor detract from the uniqueness of the guarding behavior of the male observed in this study.
Additional field and lab observations are needed. Specifically, closer attention should be paid to the area surrounding field-collected egg masses to maximize the likelihood of repeated documentation and to better understand and document the frequency of this behavior. Collaborators throughout the world working on biological control of brown marmorated stink bug are often equipped with colonies of Trissolcus spp. that would support such field and lab experimentation.
Weekly from late May to mid–October 2018, we conducted surveys for naturally-laid stink bug egg masses on the underside of leaves in diverse cropping systems in northern Utah. On June 19, a green stink bug (Chinavia hilaris Say) egg mass containing 13 eggs were discovered on an apple tree leaf in Salt Lake City, Utah (N 40° 45’ 8.297” W 111° 52’ 25.281”). At the time of detection, a male parasitoid wasp was guarding the egg mass (defined as walking or standing on top of host eggs and using aggressive behaviors to chase away competitors), and 2 female parasitoid wasps were also present. The guarding male appeared to be preventing female access to the eggs via aggression. After observing the guarding behavior for 1 min, the egg mass and 3 adult wasps were collected into a 47 mm clear, capped petri dish (Fisherbrand, Pittsburgh PA) and transported to the lab for observation and analysis. The decision to collect the egg mass after 1 min was based on a concern that prolonged observation could have resulted in further disturbance of the natal site and departure of parasitoids.
In the lab and on the day of collection, the petri dish was chilled for 10 min at 4.4°C to slow wasp movement, and the dorsal thorax of the guarding male was marked with a dot of yellow acrylic paint to differentiate it from other males emerging from the egg mass (Fig. 1, Supp. Video). The female parasitoids were vouchered in ethanol and identified. The marked male was left in place for observation. The egg mass was reared at 27°C, 16:8 h light/dark cycle, and checked hourly from 8:00–16:00 for 9 days to document parasitoid emergence.
With the initial male remaining on the egg mass, darkening egg coloration was first noted on Day 3, indicating the likelihood of previous parasitism, and new adult wasps emerged on the 6th and 10th day after collection (June 25 and 29, 2018, respectively). The petri dish was placed under a microscope camera (Leica EZ4D, Wetzlar, Germany) to observe wasp behavior: two 10 min observations at 10:00 and 14:00 on Day 6 (1 new wasp emerged) and nine 10 min observations on Day 10 conducted each hour from 8:00–16:00 (9 new wasps emerged). Observations were captured on video using Leica Acquire Software. All parasitoid wasps were vouchered after observations and identified using the key to Nearctic Trissolcus.
Funding was provided by the National Institute of Food and Agriculture, U.S. Department of Agriculture, Specialty Crop Research Initiative under award number 2016-51181-25409; USDA Specialty Crop Block Grant; USDA APHIS PPQ under cooperative agreement AP18PPQFO000C099, and Utah State University Extension. This research was supported by the Utah Agricultural Experiment Station, Utah State University, and approved as journal paper number 9213.
We thank Lily Bourett, Hanna Kirkland, Kate Morgan, Ryan West, and Ben Steadman for field and lab assistance, Elijah Talamas for species confirmations, and Wasatch Gardens for access to the study site.