Variation in plant morphology and chemistry can directly influence the performance of insect herbivores. A growing number of studies indicate that plants can also influence the performance of the natural enemies of the herbivores. Plant species in the Brassicaceae produce secondary compounds known as glucosinolates (GLS) whose hydrolysis products are potentially toxic to many herbivores. Some specialist insects are known to sequester the GLS, but up to date, there is no data on impact of GLS sequestration by insect herbivores on the performance of their parasitoids. We asked whether the performance of the GLS-sequestering aphid Brevicoryne brassicae and its parasitoid Diaeretiella rapae were affected by the host plants on which they feed. We selected four host plant species, one cultivar of Brassica oleracea and B. napus, and two wild species, B. nigra and Sinapis arvensis. Among other traits, these species differ in GLS content and composition as well as in some morphological characteristics. Population growth rate of Br brassicae was not affected by host plant species. However, adult aphids were the smallest, but also lived the longest when they had developed on the B. oleracea cultivar. Parasitoids were larger when their aphid hosts fed on the wild species. Unexpectedly, parasitism rate was lower on B. oleracea. Thus, variation in host plant characteristics had an impact on the fitness of the parasitoid through its aphid host. In contrast to previous studies, which have shown that sequestration is a good defence mechanism against predators, we did not observe that Br brassicae benefits from the sequestration of GLS to limit attacks by its main parasitoid.
