Parasitization of a wild and reared population of the solitary bee Osmia cornuta Latr. by the parasitoid Anthrax anthrax Schrank (Diptera, Bombylidae): comparison between two types of artificial nest

A nest-trapping (NT) and a releasing and rearing (RR) campaign were performed in two similar areas of Tuscany (Italy), towards nest inducing and reproduction of Osmia cornuta Latr. For the NT campaign, artificial nests built with reed segments (Arundo donax L.) (reed nests) and grooved hardboard plates were used. The latter type was termed Artificial Assembled Trap Nest (AATN). The RR campaign utilized the same nest types and a reared O. cornuta population, with 100 cocoons having a sex-ratio of 2:1 (female:male). Nests were removed from fields during the first week of August, placed in mesh bags and kept in the laboratory at room temperature up to the month of October, when each nest was opened in order to analyse its content. Results showed that in both investigated areas, the wild and reared population were parasitized by the bombylid dipteran Anthrax anthrax Schrank. A. anthrax parasitization of O. cornuta was found to be greater in assembled (AATN) compared to reed nests. Highest prevalence of parasitization was found in the innermost portion of the tunnel, where Osmia cocoons usually contains females. Additional damage is cocoon crashing, caused by A. anthrax armed pupae as it forces its way out of the tunnel. Both O. cornuta population (reared and wild) resulted parasitized by A. anthrax specimens. In October, when broodnests were opened, exuviae of newly emerged specimens, armed pupae and mature diapausing A. anthrax Schrank larvae were, in some cases, found inside together, suggesting that the life cycle of this dipteran is parsivoltine.