Only Females Oviposit: Chemical Discrimination of Adult Stink Bug Sex by the Egg Parasitoid Trissolcus japonicus

Abstract

Egg parasitoids foraging for suitable hosts scattered in the environment rely mainly on chemical cues. Elucidating the chemical ecology of natural enemies is important in the development of effective and successful strategies for conservation biological control. In this context, the host cuticular hydrocarbons, which are exploited by several species of egg parasitoids as contact kairomones, could be used to retain them by providing information about the presence and the sex of adults of the target species: sex is important because only females of the host species lay the eggs that can be subsequently utilized for parasitoid reproduction. However, the chemical basis of host sex discrimination in egg parasitoids is not well understood. We carried out behavioral and chemical bioassays to investigate the role played by contact chemical cues left by adults of the brown marmorated stink bug, Halyomorpha halys Stål, in host egg searching behavior and adult host sex discrimination by the egg parasitoid Trissolcus japonicus (Ashmead). A first set of bioassays showed that parasitoids spent more time exploring patches contaminated with chemicals associated with adult H. halys females compared with adult males. Similar responses were displayed by T. japonicus when hexane extracts of H. halys were tested suggesting that non-polar chemical compounds are involved in host sex discrimination. GC-MS analysis of hexane extracts revealed quantitative differences in the cuticular compounds of the two sexes, with 1-hexadecene (more abundant in males) being the most important component in determining these differences. Hexane extracts of H. halys females blended with synthetic 1-hexadecene significantly reduced the wasps’ arrestment responses compared to crude extracts.

No adverse effects of symbiotic control on the parasitism of Halyomorpha halys by egg parasitoids

“…Several substances (e.g. host cuticular hydrocarbons and plant volatiles) have been shown to attract T. japonicus (Akotsen-Mensah et al 2021;Arif et al 2021). We cannot exclude that some of them may share components with those included in the odour blend resulting after our treatment, even though the volatile compounds associated with egg masses treated with Dentamet ® are currently unknown.…”

Does the fundamental host range of Trissolcus japonicus match its realized host range in Europe?

“…One reason for the observed difference could be that 'oviposition sites' chosen for the exposure of sentinel eggs may not have matched with those that would have been chosen naturally by the stink bugs. Since sentinel egg masses were exposed on plastic tags, lower parasitism may have resulted from a lack of important chemical cues used for host location by T. japonicus (Arif et al 2021;Malek et al 2021). On the contrary, if these cues were absolutely essential, one would not have expected high levels of parasitism of sentinel eggs as observed in Grugliasco in 2020 or China (Zhang et al 2017).…”