Parasitoid mating structures when hosts are patchily distributed:: field and laboratory experiments with Leptopilina boulardi and L-heterotoma

Populations spatially structured at the time of mating may experience local mate competition (LMC) and inbreeding, two factors known to select, in haplodiploid organisms, for a female biased sex ratio. Populations of the two Drosophila parasitoids Leptopilina boulardi and L. heterotoma could have such a structure because although males and females develop from different hosts, many hosts are clumped within fruits decaying on the ground. However, contrary to theoretical expectations, we found field sex ratios to be only slightly female biased (L. heterotoma) or even male biased (L. boulardi). This raised the question of whether populations of these two species experience any level of LMC and inbreeding. To address this question, we studied male and female spatio-temporal patterns of emergence, dispel sal, and male attraction to females. We found that within days, Emergence was synchronized, with males starting to emerge slightly before females. However, when emergence was analyzed day-by-day for individuals laid during the same oviposition bout, males and females emerged on different days. A similar analysis for fruits collected in the field showed that about 20% of males and 20% of females emerged in the absence of any potential mate. Furthermore, males dispersed from their natal sites soon after emergence, at a rate similar to that of conspecific females. With laboratory and field experiments. we found that dispersing males were attracted to virgin Females via in-flight orientation mediated by a volatile sex pheromone. These data suggest that the mating structures of L. boulardi and L, heterotoma differ from that assumed by classic LMC models. Because males disperse and search for females from other parches, local mate competition and inbreeding will be reduced to an extent depending on male mating success after dispersal. Inbreeding could also be reduced because synchronous emergence of males and females mainly results from asynchronous oviposition bouts, so that on-patch matings should concern unrelated individuals. Such a mating structure explains the absence of a strong sex ratio bias toward females in these two species. More generally, through a review of the published literature on sex pheromones, we suggest that mating structures with a non-negligible fraction of off-patch matings could be widespread among parasitoids.