Test of the adaptiveness of sex ratio manipulation in a parasitoid wasp

In behavioral ecology it is generally assumed that behavior is adaptive. This assumption is tested here for sex ratio manipulation in response to host size in the parasitoid wasp Spalangia cameroni. Females produce a greater proportion of daughters on larger hosts. If this behavior is adaptive, it is not through a positive effect of host size on the fitness of daughters, as theory suggests and as found for other species. Females that developed on larger hosts were not more successful at drilling into hosts, were not more successful at interspecific competition for hosts, and did not have greater dispersal ability as measured by wing loading (weight/area of wing and thorax). The possibility that S. cameroni's sex ratio manipulation may be adaptive through a negative effect of host size on the fitness of sons cannot be ruled out. Relative to males from larger hosts, males from smaller hosts had lower wing loading and thus potentially greater dispersal ability. The actual effect of wing loading on fitness remains to be tested.