SEX-RATIO MANIPULATION BY THE PARASITOID WASP SPALANGIA-CAMERONI IN RESPONSE TO HOST AGE - A TEST OF THE HOST-SIZE MODEL

A sex ratio response to host resources as measured by external host dimensions has been demonstrated in many parasitoid wasps, including Spalangia cameroni. The responses generally are in the direction predicted by sex ratio theory, specifically the host-size models. Here I show that female S. cameroni also respond to differences in resource availability not associated with changes in external host dimensions, and this response is in the direction predicted by host-size models. When given old and young hosts simultaneously, female S. cameroni oviposit a greater proportion of sons in old than in young host pupae, at least for 0-day old versus 3-day old hosts. Old hosts weigh less than young hosts but are not significantly different in external width. Thus it appears that the offspring sex ratio response may result from mothers detecting physical or chemical changes within the host which are associated with host age. No evidence is found that the manipulation in response to host age has been selected for via an effect of host age on wasp size; there was no significant effect of host age on either male of female wasp size. A second prediction of the host-size models is also supported by this study: when each female is presented with only a single host age, rather than two host ages simultaneously, host age has no effect on offspring sex ratio. © 1990 Chapman and Hall Ltd.