- Female-biased predation is rare in nature; however, sphecid wasps often take more female than male prey, including Isodontia mexicana, which hunt Oecanthus tree crickets.
- This study tests the hypothesis that wasps prefer females because they are larger than males. This predicts a female sex bias only for sexually size-dimorphic prey.
- Prey from artificial I. mexicana nest holes in Central Ontario were compared with surviving crickets sampled from the hunted population. Sex ratios of prey and survivors were examined and compared with the occurrence of female-biased sexual size dimorphism. Logistic regression was used to determine whether body size, sex, species, and life stage of crickets predicted capture by wasps.
- As predicted, wasps took a disproportionate number of adult females only of sexually size-dimorphic prey Oecanthus nigricornis. No sex bias was found in adult prey of Oecanthus quadripunctatus or in nymphal prey of either species. However, female-biased sexual size dimorphism did not necessitate female-biased predation: even though female O. nigricornis nymphs were larger than males, female nymphs were not hunted more often. Body size was a significant predictor of predation, but this relationship was non-linear. There was also evidence of an interaction among sex, life stage, and body size of prey in relation to predation risk.
- These results support the size-preference hypothesis, but do not rule out alternative hypotheses. For example, sex differences in behaviour or life-history traits that develop in adulthood may also contribute to differences in predation risk. Predation that consistently targets large adult females of a population may result in evolutionary changes in the behaviour or life history of the prey species.