Nest selection, usurpation, and a function for the nest entrance plug of Centris bicornuta (Hymenoptera : Apidae)

Centris bicornuta Mocsary nests in preexisting holes in wood. We present da ta to suggest that females prefer previously used nest holes of the same sp ecies. Further, females appear to be intensely competitive and sometimes us urp nests of other females, using the resources of the last provisioned cel l by opening and replacing the extant egg. Completed nests have a series of provisioned cells followed by a solid plug near the entrance to the nest. Immediately outside the solid plug is a 3-5 mm thick plug of a custardlike oily material. We provide evidence that this oily material may play an impo rtant role in reducing nest usurpation.