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.