Eurytoma serratulae and E. robusta (Hymenoptera, Eurytomidae): complementary host exploitation strategies of coexisting parasitoids and their impact on the host Urophora cardui

Our study investigates the host exploitation strategies of Eurytoma serratulae and Eurytoma robusta (Chalcidoidea, Eurytomidae), two parasitoid species that co-occur in gall populations of the tephritid Urophora cardui on Cirsium spp. The endoparasitoid Eurytoma serratulae detects the host larvae before an externally visible gall is formed. It profits from large galls, as its parasitization rate increases with increasing numbers of chambers per gall. Oviposition by the ectoparasitoid Eurytoma robusta does not occur until a distinct gall with chambers has been formed. Its parasitization rate reaches highest values in medium-sized galls. Eurytoma robusta is the dominant parasitoid in host populations with low and moderate gall densities, whereas Eurytoma serratulae is the superior exploiter of host populations with high gall densities. Within single galls Eurytoma robusta is an important hyperparasitoid of Eurytoma serratulae , but Eurytoma serratulae has no adverse influence on Eurytoma robusta . Parasitism by Eurytoma serratulae induces host larvae to promote gall growth, an effect that is profitable to both the parasitoid and the remaining host larvae in the gall. Parasitism by Eurytoma robusta often leads to smaller galls, as cases of unsuccessful parasitization result in empty gall cells.