CONSTRAINED OVIPOSITION AND FEMALE-BIASED SEX ALLOCATION IN A PARASITIC WASP

In haplodiploid organisms such as parasitic wasps, substantial oviposi tion by females without sperm is predicted to cause mated females to b ias their offspring sex ratios towards daughters. The effect of the pr oduction of sons by unmated and sperm-depleted (constrained) females o n sex allocation by mated females was studied in two populations of th e parasitic wasp Bracon hebetor over 3 years. B. hebetor females who d epleted their sperm reserves from prior matings rarely remated and bec ame constrained to produce only sons. Constrained females readily ovip osited and produced clutches similar in size to those produced by mate d females. Although the fraction of constrained females in the populat ion varied considerably between sites and sampling dates, it was usual ly high enough to favor the production of female-biased sex ratios by mated females. Mated females consistently produced female-biased sex r atios. However, we found no evidence that the sex ratios produced by m ated females from the field shifted in relation to the proportion of c onstrained females in the population. Females held with males or held in isolation also produced female-biased sex ratios. These findings su ggest that, in B. hebetor, mated females produce sex ratios that refle ct the average fraction of constrained females over evolutionary time.