Invader effects in a community of cavity nesting megachilid bees (Hymenoptera: Megachilidae)

Temporal and spatial characteristics of a community of cavity nesting megachilid bee species and their introduced competitors were examined at a site in the Central Valley of California. Two of the introduced bee species, Megachile apicalis Spinola and Megachile rotundata (F.), were frequent nest site occupants and accounted for 19.6 and 3.4% of all artificial nesting cavities (trap-nests) monitored during the 1st and 2nd yr of this study, respectively. These species were most active before and after the peak in native bee nesting activity and tended to occupy xeric habitats, despite significantly higher larval mortality associated with such environments. The ability to occupy a wide array of habitats may account for the invasion success of M. apicalis in California. Another invader, the European earwig, Forficula auricularia L., was a frequent occupant of nesting cavities in mesic habitats, accounting for 59 and 85% occupancy of trap-nests during the Ist and 2nd yr of the study, respectively. Exclusion experiments indicate earwig densities were sufficient to displace female bees at nest sites with 1/3 as much nesting activity occurring in sampling units that allowed entry by earwigs as those made inaccessible to them with sticky barriers. In comparison, all native species (including bees and wasps) never accounted for >25% of all occupied nesting cavities of either monitoring period of the study.