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

Abstract

Our study investigates the host exploitation strategies of Eurytoma serratulae and E. robusta (Chalcidoidea, Eurytomidae), two parasitoid species that co-occur in gall populations of the tephritid Urophora cardui on Cirsium spp. The endoparasitoid E. 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 E. 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 E. serratulae is the superior exploiter of host populations with high gall densities. Within single galls E. robusta is an important hyperparasitoid of E. serratulae, but E. serratulae has no adverse influence on E. robusta. Parasitism by E. 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 E. robusta often leads to smaller galls, as cases of unsuccessful parasitization result in empty gall cells.

Predatory and Parasitic Insects Associated with Urophora cardui L. (Diptera: Tephritidae) Galls on Canada Thistle, Cirsium arvense L. (Asterales, Asteraceae) in North Dakota

“…In the Palearctic region, Eurytoma robusta and E. serratulae are the most common hymenopteran parasitoids of U. cardui larvae, and the two species can co-occur within a single gall [ 3 , 21 ]. In contrast, the ectoparasitoid P. elevatus was the dominant parasitoid species in our study.…”

“…In southern Finland, P. elevatus was present at 25 out of 75 sites [ 58 ], and in northern Germany and Denmark, Johannesen and Seitz [ 18 ] found P. elevatus parasitoids at 13 of the 20 sites they sampled, and its rate of parasitizing U. cardui ranged up to 11%. However, the general conclusion is that in the Palearctic region, P. elevatus does not have a major impact on U. cardui populations [ 9 , 17 , 21 ].…”

“…Endophagous insects that spend part of their life cycle inside plant tissues are often parasitized by hymenopterans [ 19 ]. Several species of wasps, primarily in the Eurytomidae and Pteromalidae families, parasitize Urophora within galls [ 10 , 17 , 20 , 21 ], and parasitism is a key cause of mortality for U. cardui immatures [ 9 ]. In Europe, U. cardui larvae within Canada thistle galls are commonly parasitized by the koinobiont endoparasitoid Eurytoma serratulae Latreille (Hymenoptera: Eurytomidae) and the idiobiont ectoparasitoid Eurytoma robusta Mayr (Hymenoptera: Eurytomidae) [ 3 , 18 , 21 ], with the two species often co-occurring within a single gall [ 17 ].…”

Torn Between Nature and Lab: A Dying Breed of Plant Scientists?

“…Several doctoral theses which were supervised together with my colleague Helmut Zw€ olfer, centered around galls on the stems of the Canada thistle (Cirsium arvense), induced by the gall-fly Urophora cardui by oviposition into the young stem just below the developing flower head. Such galls are micro-ecosystems as parasitoids of the gall fly can establish higher trophic levels (Zw€ olfer et al 2007(Zw€ olfer et al , 2015. The 3 major questions were: How is gall growth initiated, which cues lead the insects to the thistle, which signals lead the female parasitoids (Eurytoma serratulae, Eu.…”