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 b ee species, Megachile apicalis Spinola and Megachile rotundata (F.), w ere frequent nest site occupants and accounted for 19.6 and 3.4% of al l artificial nesting cavities (trap-nests) monitored during the 1st an d 2nd yr of this study, respectively. These species were most active b efore and after the peak in native bee nesting activity and tended to occupy xeric habitats, despite significantly higher larval mortality a ssociated with such environments. The ability to occupy a wide array o f habitats may account for the invasion success of M. apicalis in Cali fornia. Another invader, the European earwig, Forficula auricularia L. , was a frequent occupant of nesting cavities in mesic habitats, accou nting for 59 and 85% occupancy of trap-nests during the Ist and 2nd yr of the study, respectively. Exclusion experiments indicate earwig den sities were sufficient to displace female bees at nest sites with 1/3 as much nesting activity occurring in sampling units that allowed entr y by earwigs as those made inaccessible to them with sticky barriers. In comparison, all native species (including bees and wasps) never acc ounted for >25% of all occupied nesting cavities of either monitoring period of the study.