Male orientation to trail sex pheromones in parasitoid wasps: does the spatial distribution of virgin females matter?

We studied male locomotory response to trails and patches of sex pheromone (left respectively by free-ranging females and females constrained to stay on a small area) in the two parasitoids Aphelinus asychis (Hymenoptera: Aphelinidae) and Trichogramma brassicae (Hymenoptera: Trichogrammatidae). Under the hypothesis that the spatial distribution of virgin females differs between these species (scattered among host plants in A. asychis, gregarious at emergence sites in T. brassicae), we predicted that male locomotory response to their sex pheromones should also differ: A. asychis males should follow pheromone trails on plants in order to encounter the females along these trails, whereas T. brassicae males should stay on pheromone patches, at emergence sites, and mate the females on these patches. Using an improved video-tracking system, we found that males of both species respond to conspecific sex pheromone trails and patches, but that the response does not differ much between species. Males released on marked substrates walked in a more convoluted pattern (i.e. higher path fractal dimension and higher number of crossings within tracks) than males released on unmarked substrates. On pheromone patches, males turned persistently in the same direction when leaving the patch, which explains a higher number of visits on marked patches than on unmarked patches, and possibly, higher track convolution on pheromone trails. Contrary to our hypothesis, male A. asychis did not follow female trails more accurately than male T. brassicae, and male T. brassicae did not stay longer on pheromone patches than male A. asychis. We argue that these discrepancies between our predictions and the observed responses originates from discrepancies between the assumed spatial distribution of virgin females and their actual distribution in the wild.