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Testing the host specificity of Celatoria compressa (Diptera: Tachinidae) as a potential biological control agents for WCR (Western Corn Rootworm) in Europe

The results predict that C. compressa would have a narrow host range in Europe, being restricted to few genera among the Galerucinae. This is consistent with the host records of C. compressa in Mexico, which was only found from Diabrotica sp. and Acalymma sp. There are several lines of evidences that host specificity in Dipteran parasitoids, which attack adult insects, are more often determined by the events leading up to oviposition, rather than by the events occurring after oviposition. The specifically modified ovipositors and specialized attack behaviour of these parasitoids further reinforces its host specificity. It is likely that the more host specific nature of Celatoria species when compared to many other tachinids, is due to the elaborately modified piercing ovipositor of the females, which would be directly related to the host acceptance of adult Coleoptera by Celatoria sp. In addition, the larviposition behaviour has been thoroughly investigated in C. compressa, C. diabroticae and C. setosa.

The host-selection of Diptera, especially Tachinidae, follows phylogenetic evolution much more closely than previously suspected from the existing host-lists. Aulacophora represents the New World genus equivalent of Diabrotica in the Old World, and there is a remarkable resemblance between the two genera on larval, pupal and adult structures, breeding habits and food plants. In the current tests of non-target species within Galerucinae, only A. foveicollis was found to be parasitised by C. compressa. This suggests that phylogenetically closely related non-target species would be most likely at risk when C. compressa would be selected in a classical biological control programme.

Although A. foveicollis was occasionally attacked by C. compressa in the laboratory, the parasitism was extremely low compared to that of D. v. virgifera. Only a single puparia was obtained from 22 parasitised A. foveicollis and from a total of 1,110 A. foveicollis tested. Encapsulation of the tachinid larvae was found in A. foveicollis, showing an immune response of the resistant host toward C. compressa. It suggests that A. foveicollis is sub-optimal as a host for development of C. compressa, although it is possible that tachinid larvae in A. foveicollis require a longer development time than that in D. virgifera. Besides two more Aulacophora species in Russia, A. foveicollis is the only Aulacophora species present in Europe and is restricted to southern Europe. Thus it is unlikely that C. compressa would encounter A. foveicollis in the field in Central/Middle Europe if future field release is implemented in Hungary. In addition, there were only two natural enemies found attacking A. foveicollis, one is a mite Histiostoma sp. that attacked larvae in Greece and another is Rhinocoris fuscipes preying on adults in India. Therefore, the impacts of C. compressa on A. foveicollis and other organisms would be extremely low even if they are encountered in the ecosystem.

Learning is very common in Hymenoptera parasitoids and learning effects could increase the acceptance of a low ranked host for Hymenopteran parasitoids. It was also suggested that learning is also likely to be common in Diptera parasitoids, although it has been only approved in one species, Drino bohemica. Our results of sequential no-choice and choice test all indicated that the parasitism pattern of C. compressa did not change in over time. No difference was found between the host acceptance of naive and experienced C. compressa. Thus increased oviposition pressure for C. compressa and the experience effects of sequential exposure had no influence on host range. Learning might play a minor role in C. compressa behaviour, and would likely further reduce the risk level to non-target species in the field.

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