Multimodal mechanisms of early mate-detection in the parasitoid wasp Pimpla disparis (Hymenoptera: Ichneumonidae)

Males of the hymenopteran parasitoid Pimpla disparis have been observed to aggregate on gypsy moth, Lymantria dispar, host pupae before the emergence of a female. This led me to test the hypothesis that males respond to chemical cues associated with parasitized pupae. Results of laboratory experiments suggest that females mark the host pupae they have parasitized and that males discern between such pupae and those not parasitized. Males continue to recognize parasitized pupae throughout the development of the parasitoid. To investigate potential acoustic and vibratory cues that males may exploit to detect the presence and track the progress of a developing parasitoid (DePa; future mate) inside a host pupa, I analyzed DePa-derived cues by airborne sound and laser Doppler vibrometer recordings. Parameters (e.g., amplitude) of sound and vibratory cues change significantly over time and thus could ‘inform’ a visiting adult male about the stage of DePa’s development. To test the hypothesis that male P. disparis memorize and revisit the location(s) of parasitized host pupae as a strategy to attain mates, we color-coded P. disparis males in a field survey and recorded their behaviour. We learned that they revisit parasitized moth pupae on consecutive days, and arrest on those pupae with a near-emergence parasitoid. These results are supported by laboratory experiments, revealing that males memorize both the macro- and micro-locations of parasitized host pupae. DePa’s quiescence a few days before emergence could be a cue for a visiting male that the emergence of a mate will soon take place but it would not help the male to precisely predict the time of emergence. In contrast, oral fluid produced by emerging adult parasitoids may be indicative of the emergence process. I tested the hypothesis that semiochemicals associated with DePa’s emergence arrest males on a parasitized host pupa. I found that these semiochemicals emanate from oral fluid secreted by parasitoids while chewing their way out of a host. Attraction of males to oral fluid semiochemicals from males and females indicates that mate-seeking males co-opt chemicals involved in the eclosion process as a mate finding cue, taking a 50% chance that the prospective mate is a female.