Sex Allocation and Sexual Asymmetries in Intra-Brood Competition in the Parasitic Wasp Bracon hebetor

Abstract

1. Bracon hebetor is a gregarious parasitoid with a female-biased sex ratio. While female-biased sex ratios are often indicative of local mate competition (LMC), B. hebetor outbreeds, making LMC an unlikely explanation for biased sex ratios in this species. In this study we examined an alternative hypothesis that may explain female-biased sex ratios: competitive asymmetries between larval sons and daughters. 2. We measured two types of competitive asymmetries: asymmetric density responses (differential responses to clutch size), and asymmetric composition responses (responses to change in clutch sex ratio). We measured competitive asymmetries in terms of their effects on body size, development time, survivorship, longevity, and daily and lifetime fecundity or mating ability. 3. Males and females emerged as smaller adults, developed more quickly and experienced higher rates of larval mortality when they developed in larger clutches. The effect of clutch size on body size and survivorship was similar for males and females. Increasing clutch size decreased male larval development time more than female development time, although the effect was marginal. 4. Male and female body sizes increased as within-clutch sex ratios became more male-biased, indicating that females, not males, are stronger competitors. This should lead to male-biased sex ratios, rather than the female-biased sex ratios that exist in B. hebetor. Sex ratio had no influence on developmental time or survivorship of either sex. 5. Larger males and females lived longer than smaller individuals, whether they fed on hosts or honey. Larger females had higher daily and lifetime fecundities than smaller females. Larger males copulated with more females and sired more daughters both per day and per lifetime than did smaller males. These body size effects were only detectable when hosts were abundant. At low host densities, differences between large and small wasps were absent. 6. Although we detected asymmetric composition responses in terms of body size, the effects were slight. Therefore, we conclude that competitive asymmetries are unlikely to have important effects on male and female fitness or on the sex allocation decisions of B. hebetor.