Effects of an invasive plant on the performance of two parasitoids with different host exploitation strategies

Abstract

In their new range, exotic plants create the possibility for novel interactions to occur with native consumers. Whereas there is evidence that these novel interactions can be negative for native insects, alien plants that are closely related to native species may in fact act as important food sources for native insects during the growing season. Thus far, studies with invasive plants have mostly focused on plant–herbivore interactions. However, to better understand how top-down and bottom-up processes may affect the success of potential invaders we also need to consider the effects of invasive plants on higher trophic levels. We examine multitrophic interactions on an exotic invasive crucifer, Bunias orientalis, and a native crucifer, Brassica nigra. The performance of a specialist herbivore, Pieris brassicae, and two of its gregarious endoparasitoids, the koinobiont Cotesia glomerata and the idiobiont Pteromalus puparum, was investigated. Emphasis was laid on parasitoid host-resource use strategies and how these may be differently affected by the quality of the exotic food plant. P. brassicae larvae performed poorly on the exotic plant, with lower survival, longer development time and a lower pupal mass, than on the native plant. The exotic plant affected the performance of the two parasitoid species in different ways. C. glomerata survival was strongly co-ordinated with the survival of its larval host, showing also high mortality. Adult wasps that survived on Bu. orientalis had an extended development time and small body size. By contrast, Pt. puparum survival was similar on pupal hosts reared on both plant species. Our results show that constraints imposed by differing plant quality of native and exotic plants on trophic interactions can depend on resource use strategies of the species involved, suggesting that effects of exotic species should be elucidated on a case-by-case basis.