Sex ratio response to conspecifics in a parasitoid wasp: test of a prediction of local mate competition theory and alternative hypotheses

Maternal manipulation of offspring sex ratio in response to conspecifics is considered in relation to sex ratio theory using the parasitoid wasp Spalangia endius. Females produced a greater proportion of sons in response to mated but not virgin females. This is the first demonstration of a differential sex ratio response to virgin versus mated females and provides support for local mate competition theory. More recent sex ratio models that predict sex ratio responses to conspecifics, specifically constrained, perturbation, and crowding models, were not supported. An increased proportion of sons in response to another mated female occurred on the second day of oviposition but not on the first, and the day effect resulted from experience not age. When females oviposited alone after 2 days' exposure to another female, they still produced a greater proportion of sons than if they had always been alone, but only if the other female was mated, not if she was virgin. Females do not seem to assess the presence of virgin versus mated females indirectly by using a low density of males or a long latency to mate as an indicator for virgin females: neither affected offspring sex ratio. That mated females adjusted their sex ratios in response to other mated females, but not virgin females or males, may be due proximally to mated females not often encountering the latter. Virgin females and males are not located as deep in the oviposition substrate as mated females.