Pollen Foraging in a Community of Osmia Bees (Hymenoptera: Megachilidae)

Abstract

Thirteen species of Osmia bees were studied for 2 yr to determine their pollen foraging behavior in terms of individual and population constancy. Two species, Osmia californica Cresson and O. montana Cresson, were family-level oligoleges on Asteraceae. Two of the eleven polyleges were generalist pollen foragers: O. densa Cresson showed no statistical preference and O. albolateralis Cockerell showed a significant preference, but for two different pollens. Nine Osmia polyleges showed a local specialization for either a genus or family of flowering plants that was shown to be statistically significant: O. raritatus Michener and O. sladeni Sandhouse for Trifolium (Fabaceae); O. brevis Cresson for Penstemon (Scrophulariaceae); and O. bruneri Cockerell, O. calla Cockerell, O. indeprensa Sandhouse, O. kincaidii Cockerell, O. lignaria Say, and O. nanula Cockerell for Phacelia (Hydrophyllaceae). Individual constancy measures were statistically higher for the oligoleges. Niche breadths and the number of flower species visited were not statistically different among the foraging groups. When tested against the expected oligolege and polylege niche breadths, observed oligolege niche breadths were not statistically different; however, polylege niche breadths were statistically lower than expected. The Osmia species showed flower partitioning and overlap; the overlap occurred on the most abundant flowering species in the study area. We have no indication or evidence of competition for pollen resources among the 13 species studied.