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 phero mone (left respectively by free-ranging females and females constraine d to stay on a small area) in the two parasitoids Aphelinus asychis (H ymenoptera: Aphelinidae) and Trichogramma brassicae (Hymenoptera: Tric hogrammatidae). Under the hypothesis that the spatial distribution of virgin females differs between these species (scattered among host pla nts 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 plant s in order to encounter the females along these trails, whereas T. bra ssicae males should stay on pheromone patches, at emergence sites, and mate the females on these patches. Using an improved video-tracking s ystem, we found that males of both species respond to conspecific sex pheromone trails and patches, but that the response does not differ mu ch between species. Males released on marked substrates walked in a mo re convoluted pattern (i.e. higher path fractal dimension and higher n umber of crossings within tracks) than males released on unmarked subs trates, On pheromone patches, males turned persistently in the same di rection when leaving the patch, which explains a higher number of visi ts on marked patches than on unmarked patches, and possibly, higher tr ack convolution on pheromone trails. Contrary to our hypothesis, male A. asychis did not follow female trails more accurately than male T. b rassicae, and male T. brassicae did not stay longer on pheromone patch es 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 a ctual distribution in the wild. (C) 1998 Elsevier Science Ltd. All rig hts reserved.