Size Variation and Aggression among Male Xylocopa virginica (L.) (Hymenoptera: Apidae) at a Nesting Site in Central Oklahoma

Abstract We studied aggression and size variation among males in the carpenter bee Xylocopa virginica (L.) at a nesting site in Central Oklahoma. Male size (estimated by head capsule width), male–male and male–female interactions as well as male site fidelity were recorded at three locations: 1) in the immediate vicinity of the nesting site, 2) at the periphery of the same nesting site and 3) at milkweed flowers (a nectar source) in an adjacent pasture. Nesting site males displayed significantly higher levels of aggression than males at the other two locations. These males were also significantly larger than those at flowers in the pasture while males at the nesting site periphery were intermediate in both size and intrasexual aggression among the three male categories. Male–female interactions (chases and contacts) occurred most frequently closest to the nesting site, but not significantly so relative to males in the nesting site periphery. We also found evidence that some males remain faithful to territories near the nesting site. Unlike the other two locations, no male–male or male–female interactions were observed at the flower site during one-minute focal observation periods. In contrast with several Neotropical Xylocopa species described in the literature, our study indicates that male X. virginica practice female and/or resource polygyny.