Sex ratios, virginity and local resource enhancement in a quasisocial parasitoid

Sclerodermus harmandi is an economically beneficial species of parasitoid wasp that also has an unusual level of sociality: groups of female foundresses reproduce on a single host and exhibit cooperative post-ovipositional brood care. The beneficial effects females have on each other’s reproductive success provides, via the theory of local resource enhancement (LRE), an explanation for their female biased progeny sex ratios which is part of the same framework for understanding sex ratio evolution as the more often invoked theory of local mate competition (LMC). Here we show that S. harmandi sex ratios are over-dispersed, with high variance largely attributable to the common occurrence (60%) of developmental mortality. Developmental mortality is also positively associated with the proportion of broods which contain only females at emergence (virgin broods). Virginity is also more common when broods are produced by smaller numbers of foundresses. Virginity is expected to be disadvantageous under LRE, as it is under LMC, but theory for LRE is less extensively developed. We suggest approaches for the development of LRE theory, in particular using models of “population elasticity” in which the intensity of kin competition is reduced because extra resources are available to local populations that are more cooperative. For S. harmandi, such extra resources may include large hosts that can only be successfully utilized when multiple foundresses cooperate.