The failure to discriminate: superparasitism ofTrichoplusia niHübner by a generalist tachinid parasitoid

Parasitoids are dependent on their hosts, which provide all of the resources for larval development. Parasitoid fitness, therefore, is directly related to the host quality, as determined by host size, age and health (e.g. parasitisation status); and this can only be assessed by the female parasitoid during host selection. Most studies of parasitoid-host interactions have focused on hymenopteran parasitoids rather than dipterans that are believed to be less discriminating during host selection. We assessed the impact of host quality and superparasitism on parasitoid fitness inCompsilura concinnataMeigen, a gregarious tachinid dipteran parasitoid, and its lepidopteran host the cabbage looper,Trichoplusia niHübner.FemaleC. concinnataparasitised all host stages ofT. nithat were presented to them, but emergence rates were higher from older hosts. Females readily superparasitised hosts. The number of flies emerging was higher from hosts parasitised at later instars, and flies emerged earlier from heavily superparasitised hosts. Superparasitism decreased parasitoid pupal weight and development time, indicating intra-host competition between parasitoid larvae and skewed the parasitoid sex ratio in favour of males.Host discrimination does not seem to be well developed inC. concinnata. Hosts are superparasitised despite the effects of superparasitism on offspring and sex ratio. This could be due to the wide host range ofC. concinnata; avoiding high superparasitism could occur naturally due to host switching and, therefore, developing host discrimination mechanisms for one host species may not be crucial.