The effect of direct and indirect defenses in two wild brassicaceous plant species on a specialist herbivore and its gregarious endoparasitoid

Most studies on plant defenses against insect herbivores investigate direct and indirect plant defenses independently. However, these defenses are not necessarily mutually exclusive. Plant metabolites can be transmitted through the food chain and can also affect the herbivore's natural enemies. A conflict may arise when a natural enemy is attracted to a plant that is suboptimal in terms of its own fitness. In addition, plant defenses are often studied in cultivated plant species in which artificial selection may have resulted in reduced resistance against insect herbivores. In this study, we investigated both direct and indirect plant defenses in two closely related wild brassicaceous plant species, Brassica nigra L. and Sinapis arvensis L. The herbivore Pieris brassicae L. (Lepidoptera: Pieridae), which is specialized on brassicaceous plant species, developed faster and attained higher pupal mass when reared on B. nigra than on S. arvensis. In contrast, Cotesia glomerata L. (Hymenoptera: Braconidae), which is a gregarious endoparasitoid of P. brassicae caterpillars, developed equally well on P. brassicae irrespective of the food plant on which its host had been reared. The feeding strategy of the parasitoid larvae, that is, selectively feeding on hemolymph and fat body, is likely to allow for a much wider host-size range without affecting the size or development time of the emerging parasitoids. In flight chamber experiments, C. glomerata, which had an oviposition experience in a host that fed on Brussels sprout, exhibited significant preference for host-damaged B. nigra over host-damaged S. arvensis plants. Headspace analysis revealed quantitative and qualitative differences in volatile emissions between the two plant species. This parasitoid species may use a range of cues associated with the host and the host's food plant in order to recognize the different plant species on which the host can feed. These results show that there is no conflict between direct and indirect plant defenses for this plant-host-parasitoid complex.