Performance of secondary parasitoids on chemically defended and undefended hosts

Defensive chemicals produced by plants can travel up the food chain by being sequestered by herbivores, and then in turn being sequestered by their parasitoids. Insect species with wide host ranges are predicted to perform poorly in the face of specific chemical defence. However, a species at a high trophic level is expected to have a wide host range. This creates a conflict for hyperparasitoids, many of which depend on specialized hosts. We studied the performance of two hyperparasitoids, Lysibia nana and Gelis agilis, both of which have wide host ranges, on two host species, one chemically defended and the other not. We predicted that both hyperparasitoids would perform better using the undefended host Cotesia glomerata than the defended host C. melitaearum, which sequesters terpenoid allelochemicals (iridoid glycosides). Furthermore, we expected that the progeny of G. agilis collected from an area where hosts defended by iridoid glycosides are absent (the Netherlands) would perform poorly using C. melitaearum in comparison with G. agilis collected from an area where C. melitaearum is a common host (Åland, Finland). In a series of laboratory experiments we found that, contrary to prediction, both hyperparasitoids performed well on both hosts, reaching a larger size on C. glomerata, but having a higher conversion efficiency and developing more quickly on the chemically defended C. melitaearum. Lysibia nana metabolized the plant derived iridoid glycosides, which are chemicals that it does not normally encounter. Gelis agilis retained some of the iridoid glycosides. But whereas Finnish G. agilis retained both aucubin and catalpol, Dutch G. agilis mainly retained aucubin, illustrating that though generalists, local populations still cope differently with toxic allelochemicals. Von Pflanzen synthetisierte chemische Abwehrstoffe können sich in der Nahrungskette anreichern, indem sie zunächst von Herbivoren und dann von deren Parasitoiden akkumuliert werden. Es wird angenommen, dass Insektenarten mit einem breiten Wirtsspektrum schlecht mit spezifischen chemischen Abwehrstoffen zurechtkommen. Arten in oberen Trophieebenen haben in der Regel ein breites Wirtsspektrum. Dadurch entsteht ein Konflikt für Hyperparasitoide, die häufig von einem spezialisierten Wirt abhängig sind. Wir haben zwei Hyperparasitoide mit breitem Wirtsspektrum, Lysibia nana und Gelis agilis, auf zwei Wirten untersucht. Einer der beiden Wirte unterliegt einer pflanzlichen chemischen Abwehr.