Native bees face a range of environmental pressures that drive evolution of adaptive behaviors, including interactions with predators. We examined a predator-prey interaction between developing carpenter bees and their woodpecker predators. In southeastern Arizona, carpenter bees (Xylocopa californica) build their nests in the dried inflorescence stalks of plants in the family Agavaceae, including sotol (Dasylirion wheeleri), Agave, and Yucca. Woodpeckers prey upon carpenter bee larvae developing inside the stalks. Sotol has a dense rosette of barbed leaves at the base of the stalk. We asked whether these leaves protect carpenter bee larvae from woodpeckers, and whether predators bypass carpenter bee nests built within sotol leaf rosettes, which we hypothesized would be more difficult for them to reach. To test this hypothesis, we recorded carpenter bee nest entrance hole height, the location of any bird peck marks, and the height of the tallest point of each leaf rosette, of 68 sotol stalks containing nests.
Results/Conclusions
Carpenter bee entrance holes were observed most often within the basal rosette of sotol leaves (Chi-square test, Χ2 = 59.569, p = 1.181e-14, n=68). Peck heights too were significantly more often within rather than than above the height of the rosette (Chi-square test, X2 = 21.253, p = 4.025e-06). This could be due to birds pecking where nests are present. Indeed, a disproportionate 25.2% of peck marks were centered within 10 cm of the maximum leaf height, a region constituting only 12.3% of the range of peck heights. The median carpenter bee nest entrance is 9.5 cm deeper within sotol rosette height than is the median peck, indicating that carpenter bees commonly place nests lower on stalks than woodpeckers peck. However, when we compared the height distributions of carpenter bee entrance holes and woodpecker pecks, we found that the two distributions were not significantly different (Mann-Whitney U Test, W = 6170, p = 0.256). While bees’ nest placement choices may be driven by the risk of woodpecker predation, we could not find evidence that nesting deeper within sotol leaf rosettes effectively evades predation.