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 hap lodiploid organisms, for a female biased sex ratio. Populations of the two Drosophila parasitoids Leptopilina boulardi and L. heterotoma could have su ch a structure because although males and females develop from different ho sts, 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). T his raised the question of whether populations of these two species experie nce any level of LMC and inbreeding. To address this question, we studied m ale and female spatio-temporal patterns of emergence, dispel sal, and male attraction to females. We found that within days, Emergence was synchronize d, with males starting to emerge slightly before females. However, when eme rgence was analyzed day-by-day for individuals laid during the same oviposi tion 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 d ispersed from their natal sites soon after emergence, at a rate similar to that of conspecific females. With laboratory and field experiments. we foun d that dispersing males were attracted to virgin Females via in-flight orie ntation mediated by a volatile sex pheromone. These data suggest that the m ating structures of L. boulardi and L, heterotoma differ from that assumed by classic LMC models. Because males disperse and search for females from o ther parches, local mate competition and inbreeding will be reduced to an e xtent depending on male mating success after dispersal. Inbreeding could al so be reduced because synchronous emergence of males and females mainly res ults from asynchronous oviposition bouts, so that on-patch matings should c oncern unrelated individuals. Such a mating structure explains the absence of a strong sex ratio bias toward females in these two species. More genera lly, through a review of the published literature on sex pheromones, we sug gest that mating structures with a non-negligible fraction of off-patch mat ings could be widespread among parasitoids.