NEST USURPATION AMONG FEMALES OF AN INTRODUCED LEAF-CUTTER BEE, MEGACHILE APICALIS (HYMENOPTERA, MEGACHILIDAE)

We compared female nest usurpation and size variation in a population of the introduced leaf-cutter bee Megachile apicalis Spinola. Both beh avioral and morphological evidence was gathered from three types of fe males: 1) those that occupy and defend nest holes, 2) those that patro l the nesting substrate while attempting to displace the latter group and 3) pollen-foraging bees that apparently had already established ne sts elsewhere in the environment. Females occupying holes and patrolli ng nesting sites were, on average, larger than foraging females. In ad dition, female densities at nest sites correlated positively with the average size of females found at those sites.