Cerambyx miles (Coleoptera: Cerambycidae) and Oobius rudnevi (Hymenoptera: Encyrtidae): a new host–parasitoid association revealed by laboratory tests and sentinel eggs in the wild

Summary The parasitism of the encyrtid wasp Oobius rudnevi (Nowicki, 1928) on the eggs of the longhorn beetle Cerambyx (Cerambyx) miles Bonelli, 1812, is recorded for first time through field and laboratory trials. Field trials with sentinel eggs of C. miles were conducted in SW Spain oak forests in which O. rudnevi was known to occur, using as positive controls sentinel eggs of both Cerambyx (Cerambyx) cerdo Linnaeus, 1758, and Cerambyx (Cerambyx) welensii (Küster, 1845), the only two O. rudnevi hosts known to date. Laboratory assays were conducted using non-choice tests and choice tests (C. miles vs. C. cerdo). Results showed that C. miles eggs were attacked and successfully parasitised by O. rudnevi in the wild, uncorrected parasitism rates (PU) being similar to those of C. cerdo and C. welensii (2.2–5.2%), and net parasitism rates (PN) even significantly higher in C. miles (8.9%) than in C. welensii (3.3%). Interestingly, C. miles sentinel eggs were parasitised in holm oak forests in which C. miles was unknown to occur, suggesting that egg-derived volatile cues were important in host location. In the laboratory, non-choice tests showed no differences in PN among C. miles, C. cerdo and C. welensii (80–88%), signifying that O. rudnevi performed alike in the three hosts. Choice tests showed that O. rudnevi did not exhibit host preference between C. miles (53%) and C. cerdo (47%). Similar tests proved that eggs of Cerambyx (Microcerambyx) scopolii Fuessly, 1775, and Prinobius myardi Mulsant, 1842, were never selected as hosts by O. rudnevi. Our results and the chorological, ecological and phylogenetic available evidence suggest that O. rudnevi behaves as an oligophagous egg parasitoid, which could be associated to other Cerambyx (s. str.) species over the Western Palaearctic realm.