Parasitism in wild and managed populations of the almond pollinator Osmia cornuta Latr. (Hymenoptera: Megachilidae)
The potential threat of parasites to Osmia cornuta populations managed for almond pollination was analysed. Rates of parasitism were measured in both managed and wild trapnested populations. Only two parasites, the drosophilid fly Cacoxenus sp., and the cleptoparasitic mite Chaetodactylus osmiae, were found in wild populations, but their rates of parasitism rarely exceeded 10%. Cacoxenus was never found in managed populations, but the torymid wasp Monodontomerus obsoletus caused serious damage in some populations when bee cocoons were made accessible by extricating them from nesting materials in the laboratory. Chaetodactylus osmiae was also present in managed populations but in low numbers. In 1991, a population of O. cornuta was released in an almond orchard. As opposed to other releases in which 'clean' populations were used, in this release some cells infested with C. osmiae, Cacoxenus sp., and M. obsoletus were intentionally introduced with the bee population. No cell was reparasitized by Cacoxenus sp., because by the time these drosophilids became adults the flying season of the O. cornuta population was almost over. On the other hand, the number of cells infested with C. osmiae mites increased to six times the number of infested cells originally introduced. Most M. obsoletus developed normally, and parasitism by this wasp increased slightly (from 20 to 30 cells). As O. cornuta flies very early in the season when few species of parasites or nest destroyers are active, the diversity of associated fauna is usually lower in O. cornuta nests than in nests of other, related, later-flying species such as O. ruta and O. tricornis.