Centris bicornuta Mocsary nests in preexisting holes in wood. We present data to suggest that females prefer previously used nest holes of the same species. Further, females appear to be intensely competitive and sometimes usurp nests of other females, using the resources of the last provisioned cell 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 important role in reducing nest usurpation.