Sex-Ratio Determination in Three Wasp Species Ectoparasitic on Bean Weevil Larvae

Abstract

We investigated the control of the sex ratio in three ectoparasitic solitary wasp species, Dinarmus basalis (Rondani), Anisopteromalus calandrae (Howard), and Heterospilus prosopidis Vier., parasitizing azuki bean weevil (Callosobruchus chinensis (L)(Coleoptera: Bruchidae)) larvae. Using host age as a measure of host quality, we ask the question whether these wasps can adjust their sex ratio to different sized hosts.

By either artificially transferring wasp eggs onto larger hosts, or letting eggs develop on the hosts on which these were laid, we measured the primary and emergence sex ratios of offspring wasps on different aged hosts. In all species, sex ratios from older hosts were more female biased. However, there was significant differential mortality between sex during the larval stage, especially on younger hosts.

Further experiments showed that wasps did not lay more female eggs on larger hosts of the same age, suggesting that offspring sex was not controlled by the individual size of the host on which a female oviposited.

There was a strong positive correlation between the size of host on which a male developed and his mating ability and longevity. A similar positive correlation was seen between the size of host on which a female developed and her fecundity and longevity.

We demonstrate 1) that female discrimination of host quality does influence sex allocation and 2) that the allocation of offspring sex by host size significantly increases a female contributions to the next generation (her fitness).