Brood Size and Sex Ratio Regulation in the Parasitic Wasp Nasonia Vitripennis (Walker) (Hymenoptera: Pteromalidae)

Abstract

The parasitic wasp Nasonia vitripennis varies its brood size and sex ratio dependent upon whether it is the first or second wasp to parasitize a fleshly (Sarcophaga bullata) pupa. Some of the proximate causes of this variability were investigated. In particular, research was conducted to investigate (1) effect of time between parasitization upon the probability of superparasitism, brood size and sex ratio of the superparasite, (2) whether the sex and number of larvae in host affects the oviposition behavior of the and (3) whether there is a temporal sequence in laying sons versus daughters during parasitization. It was found that the probability of superparasitism decreased, average brood size of the decreased, and proportion of sons increased with increasing time since the previous parasitization. A closer examination revealed that with a 3-24 hour delay between attacks, there is no significant relationship between brood size and sex ratio of the suprparasite or between brood size and sex ratio of the or between brood size of the first wasp and brood size of the superparasite. However, by a 48 hour delay, there was a significant negative relationship between brood size and proportion sons of the superparasite. In addition, a significant negative relationship between the brood size of the first and second wasp developed at a 48 hour delay, suggesting that the is able to detect large differences in first wasp brood size. The presence of feeding larvae in a host, at a 48 hour delay, significantly decreases the probability of superparasitism, but does not reduce the brood size of those wasps which do superparasitize. In N. vitripennis, a first wasp to parasitize a host does not lay sons first in the oviposition sequence. This contrasts to the observed in certain other species. Data on the are less clear, but suggest that sons may be laid early in the bout. Based on the data, it is hypothesized that N. vitripennis has two sex ratio patterns. In the primary parasite pattern a strongly daughter biased sex ratio is produced and sex ratio is independent of brood size. In the superparasite pattern sex ratio is strongly influenced by brood size. Typically 100% sons are produced when few eggs are laid, but the proportion of daughters increases with increasing number of eggs laid into the host. Results are discussed in relation to ecological theory and in particular to models of sex allocation under superparasitism in parasitic wasps.