Abundance of antennal chemosensilla in two parasitoid wasps with different degree of host specificity may explain sexual and species differences in their response to host-related volatiles.

The endoparasitic wasps (Hymenoptera: Braconidae), Microplitis croceipes (specialist), and Cotesia marginiventris (generalist) are parasitoids of lepidopteran larvae that differ in their degree of host specificity. Previous studies by our group have reported key differences in the response of both parasitoids to host-related volatiles. To possibly understand the basis for our results, we compared the abundance of antennal sensilla in both sexes of both parasitoid species using scanning electron microscopy and silver-staining techniques. Males of both species had relatively longer antennae than conspecific females. Five major sensilla types were recorded in both species: sensilla chaetica (nonporous), s. trichodea (nonporous), s. placodea (multiporous), s. basiconica (two types, type 1 with terminal opening and type 2 with wall pores), and s. coeloconica (nonporous). Silver staining confirmed multiple pores on the walls of s. placodea and s. basiconica type 2 and terminal openings on s. basiconica type 1, suggesting chemosensory function. In general, both putative chemosensilla types, s. placodea and s. basiconica, were more abundant in M. croceipes (specialist) than in C. marginiventris (generalist), and this was true for both sexes. Comparing the sexes, s. placodea and s. trichodea were significantly more abundant in M. croceipes males compared to females. In contrast, s. placodea was relatively more abundant in female C. marginiventris than in males. These results may explain the reported differences in the responses of both parasitoid species to host-related volatiles and are discussed in relation to the possible roles of the sensilla types in the behavioral ecology of the parasitoids.