No Intersexual Differences in Host Size and Species Usage in Spalangia Endius (hymenoptera: Pteromalidae)

Spalangia endius were collected from fly pupae, primarily house fly and stable fly, from a poultry house in Indiana. Male and female wasps did not differ within and across host species in host size usage. Also, despite stable fly pupae being significantly smaller than house fly pupae, the proportion of male wasps emerging from the two host species was similar. _____________________________________________ Early in the 1900's, entomologists observed that in some species of parasitoid wasps, males tended to emerge from smaller hosts than did females, resulting in a negative relationship between host size and parasitoid sex ratio (proportion males) (reviewed in Flanders 1939, 1946). Since then, a group of sex ratio models, the host quality models have been developed to explain this pattern (Charnov 1979, Charnov et al. 1981, Werren 1984). These models were designed for solitary species (species in which one offspring completes development per host). In these models, the prediction that male parasitoids should emerge from smaller hosts than females is based on the assumption that developing on a small host will be more detrimental to a female than to a male in terms of future ability to reproduce. The rationale of the assumption was that wasps will be smaller when developing on smaller hosts; and even small males may be able to mate successfully, whereas small females probably lay fewer eggs than large females (Charnov et al. 1981). There is some support for this idea in a few species of parasitoid wasps (Charnov et al. 1981; Jones 1982; van den Assem et al. 1989), but not in all species (King 1988). In those species or populations in which the assumption is valid, natural selection is expected to favor females that oviposit a greater proportion of males in small than in large hosts (Charnov et al. 1981). Female wasps can potentially control the sex of their offspring by controlling fertilization because they have haplodiploid sex determination. Under haplodiploid sex determination, males develop from unfertilized eggs and females from fertilized eggs. Here the host quality models' prediction of a greater proportion of sons from small than from large hosts is tested using field collections of the solitary parasitoid wasp Spalangia endius Walker (Hymenoptera: Pteromalidae). The wasps emerged from house fly pupae (Musca domestica Linnaeus) and stable fly pupae (Stomoxys calcitrans (Linnaeus)) (Diptera: Muscidae) collected from a poultry house in northern Indiana.