Females of the parasitoid wasp, Dendrocerus carpenteri (Hymenoptera: Megaspilidae), adjust offspring sex allocation when competing for hosts

Abstract

Parasitoid females may adjust offspring sex allocation according to the number and quality of hosts available. Because in solitary species only one offspring survives per host, already parasitized hosts are of low quality and generally rejected. Superparasitism (i.e., sequential oviposition by the same or different females) results in aggressive interactions and competition for nutritional resources among larvae. We examined variations in the offspring sex ratio of Dendrocerus carpenteri (Curtis) (Hymenoptera: Megaspilidae), a solitary ectoparasitoid developing as a hyperparasitoid on the prepupae and pupae of primary aphid parasitoids inside mummified aphids. Mated females produced a female-biased sex ratio of 0.433 (proportion of sons) when caged singly and provided with 12 mummies for 2 h; they parasitized an average of four mummies/h and rarely superparasitized. Superparasitism increased when two females were caged together and provided with 12 mummies, from 1.18 to 1.24 and 1.38 eggs/host parasitized in 1, 2 and 3 h, respectively. The offspring sex ratio became increasingly more female-biased with increase in superparasitism; however, sex ratio variations were not correlated with cohort size. One mated and one unmated female provided with 12 mummies and caged together for 1 h produced a mean cohort sex ratio of 0.645, which differed from the one predicted (0.717) by an algebraic model incorporating the assumptions that both females contribute equal numbers of offspring and that the mated female does not change her offspring-sex allocation strategy. The observed shift in the cohort sex ratio to an increased female-bias indicates that mated females of D. carpenteri change their behaviour when encountering parasitized mummies or a conspecific competitor in the same patch. By depositing fertilized rather than unfertilized eggs, a female can increase the proportion of her daughters among parasitoids competing for a diminishing host supply.