Offspring sex ratio in response to host size in the parasitoid wasp Spalangia endius
Napoleon, Melanie E.
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The host-size model prediction that a greater proportion of daughters will be produced from large hosts than from small hosts was tested in the wasp Spalangia endius, a parasitoid of house fly pupae (Musca domestica). Also tested were the model's assumptions that increasing host size increases wasp size and increases reproductive success in females more than in males. One strain of Spalangia endius tested behaves according to the hostsize model. Daughters generally emerged from larger hosts than did sons for one strain of S. endius, and this did not appear to result from differential mortality. However, another strain did not manipulate sex ratio. As the model assumes, host size increased wasp size for both sexes in both strains. I examined the effect of host size on male and female reproductive success in the strain that manipulated sex ratio. The original model assumes that host size has no effect on male reproductive success, and indeed this was the case for longevity, development time, mating success, and wing loading. For females, developing on a large host results in greater longevity and more offspring except perhaps at the largest host sizes. Greater longevity should increase fitness; however, developing on a large host also increases development time, which may decrease fitness. Due to the longevity results and the difference in behavior between the two strains, support for the hostsize model is mixed.