Despite being model taxa for theoretical research into sex ratio manipulation, there is a paucity of field studies that assess sex ratios of hymenopteran parasitoids. Here, the field variation in the sex ratio, brood size, and larval mortality of the parasitoid A. fuscicollis is quantified and related to variation among host species and brood types. There was a significant relationship between brood size and host volume. Brood size was significantly higher in the larger host species (Yponomeuta cagnagellus and Y. rorellus) than the smaller host species (Y. evonymellus and Y. padellus), but after controlling for host volume, brood size was significantly higher in Y. padellus and significantly lower in Y. evonymellus than the other host species. Brood size was also significantly higher in mixed broods than female or male broods. Larval mortality differed significantly among hosts and was lowest in Y. padellus. The proportion of mixed broods varied from 4 to 44% per host population. Within mixed broods, the sex ratio was highly variable (>90% females to >90% males). The median sex ratio within mixed broods differed significantly among host species and was least female biased in Y. padellus. There was no significant relationship between the percentage of females, female mortality, or male mortality, and host volume in mixed broods. Although highly variable among populations, the global sex ratio was slightly male biased in the smaller host species and female biased in the larger host species.
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