Sex‐ratio bias in an aphid parasitoid‐hyperparasitoid association: a test of two hypotheses

Abstract

Abstract. We tested the hypothesis that biased sex ratios in the aphidiid parasitoid, Aphidius ervi, are the result of sex‐specific mortality of immatures due to hyperparasitism. The solitary hyperparasitoid, Dendrocerus carpenteri, deposits its eggs on the prepupa or pupa of the primary parasitoid after its pea‐aphid host is mummified. In dichotomous choice tests, females of D.carpenteri accepted immatures of both sexes of A.ervi with equal frequency. Independent of the sex of the primary parasitoid, the probability of a mummified pea aphid being hyperparasitized did not differ between large (adult at the time of death) and small (fourth nymphal instar) mummies. As predicted by‘host quality’models of offspring sex allocation, D.carpenteri produced a higher proportion of female than male offspring in large mummies. These laboratory results were supported by field data. Because hyperparasitism by D.carpenteri is random with regard to host sex, it should have no influence on optimal sex allocation by the primary parasitoid.