Prior experiences of endoparasitoids affect their ability to discriminate NPV-infected from non-infested caterpillars

Nucleopolyhedrovirus (NPV) and parasitic wasps are two important biological control agents of lepidoptera caterpillars, and the compatibility of parasitism and NPV infection remains a complicated process. In this study, we examined the effects of different learning experiences by the endoparasitoid Microplitis pallidipes on their ability to discriminate NPV-infected from healthy caterpillars (larvae of beet armyworm, Spodoptera exigua). Compared to the parasitoids with no prior host contact (control), parasitoids with oviposition experience with healthy caterpillars had shorter searching time, shorter attacking time, a greater number of attacks, higher parasitism rate and greater comprehensive discrimination ability to both non-infected and NPV-infected caterpillars. They also had a higher percentage of first attacks on non-infected caterpillars than NPV-infected caterpillars, and these differences were more obvious with increasing time post-virus-inoculation. However, the values of these six host-discrimination indicators on NPV-infected caterpillars were not significant between the control parasitoids and parasitoids with oviposition experience on NPV-infected caterpillars from hours 24 to 96 after virus inoculation. Compared to parasitoids with oviposition experience on healthy caterpillars, parasitoids with oviposition experience on NPV-infected caterpillars generally had longer searching time, longer attacking time, fewer attacks, lower parasitism rate, and lower comprehensive discrimination ability to both non-infected and NPV-infected caterpillars, and a lower percentage of first attacks on non-infected vs. NPV-infected caterpillars. These results suggest that experience with healthy hosts helps parasitoids distinguish NPV-infected caterpillars from healthy caterpillars, but that experience with NPV-infected caterpillars did not convey the same discrimination ability.