Natural history and habitat selection by Osmia lanei, a solitary bee

Field observations of the nesting biology and foraging ecology of Osmia lanei (Megachilidae), a solitary native bee species were made in northern California (40˚ 47' 30" N, 123˚ 41' 00" W). O. lanei nests in dense aggregations within rocky clearings in meadow habitat. Detailed observations of marked, nesting females were made at two sites. Females generally dig their own underground nests which include between one and three cells. Cells are constructed solely out of masticated leaf material which bees gather and bring back to the nest. Analysis of leaf mastic reveals that O. lanei selectively gathers leaf materials from its environment. 66% of the leaf materials analyzed were Gayophytum humile (Onagraceae), 18% Viola halli (Violaceae), 12% Sidalcea malviflora (Malvaceae) and three of the eighty samples could not be identified. 92 pollen provisions were collected from O. lanei cells revealing that the species is oligolectic on legume pollen. 86 provisions contained more than 95% Trifolium longipes (Fabaceae), 4 contained mixtures of T. longipes and Vicia americana (Fabaceae), and 2 contained mixtures of Vicia americana and an unidentified species. Habitat modeling was performed to ascertain why bees were nesting in certain locations and not others. 18 nest locations were compared with 27 non-nest locations by a series of habitat variables. Initial evaluations with t-tests revealed that nest sites and non-nests could be distinguished by the percent sand, percent silt, percent clay, mid-sized sand fraction, coarse sand fraction, very coarse sand fractions, percent stone cover and bulk density of the soil. Percent sand, percent silt, the mid-sized sand fraction, coarse sand fraction, and very coarse sand fraction were multicolinear and all of these variables except percent sand were eliminated. 11 candidate logistic regression models were developed and ranked using their Akaike Information Criterion corrected for small sample size values (AICc), and Akaike weights. The best model according to these metrics included percent clay and percent stone cover, and had a correct classification rate of 81%. Model averaging revealed that stone cover was by far the most important variable in determining whether a location was a nest site or non-nest. Nesting in rocky areas likely offers O. lanei protection from the weather, predators, and cleptoparasites and allows females nesting underneath rocks to extend their foraging windows.