New host-parasitoid association and the Trojan horse strategy adopted by the cuckoo wasp Elampus bidens with two vectors of Flavescence Dore: Scaphoideus titanus and Orientus ishidae

Chrysidid wasps adopt different strategies to enter host nests whilst evading detection. However, the specific oviposition strategies of most species are still largely unknown, and it has only become evident in recent years that some brood parasitic species are actually endoparasitoids in intermediate hosts that serve as the prey of their final host. Here we report for the first time the biology of Elampus bidens and describe a novel association between this cuckoo wasp and two invasive leafhopper species: Scaphoideus titanus and Orientus ishidae (Cicadellidae). Both leafhoppers are prey of the cuckoo wasp's principal host and are common invasive pests of European vineyard agro-ecosystems that are vectors of 16SrV phytoplasma agents of grapevine Flavescence Doree, an economically important disease. This association is characterised by the "Trojan horse" strategy, in that the chrysidid lays an egg in the body of 3rd to 5rd instar nymphs of O. ishidae or S. titanus, which are later captured by an apoid wasp and carried into the wasp's nest and placed in a brood cell. Biological control of these two vectors has not been effective so far, because the use of entomophagous insects has had little impact on their population growth in both their native range and Europe. Here we present the results of a survey of the parasitisation rate of E. bidens in north-western Italy, finding that the rate varies according to the sampling region and can reach an average of over 25% in areas characterised by less-intensive agriculture. A similar parasitisation result has never been reported for the two invasive leafhoppers until now, thus has potentially valuable economic implications. We did not discover any other Hemipteran host for E. bidens, therefore the original host of this cuckoo wasp is still unknown.