Nesting Preferences of the Solitary Bee Osmia sanrafaelae (Hymenoptera: Megachilidae)

A series of cage studies was used to investigate nesting preferences of Osmia sanrafaelae Parker. Bees preferred nest holes of smaller diameter in blocks in the southwest corner of cages. Within multiple-block arrays, nests were more frequent in the lower left corner. The number of cells per nest was lowest in cages with the fewest nests in some experiments, but did not vary in others. Nest diameter affected neither the proportion of females produced per nest, nor the size of offspring of either sex. Rather, nesting females constructed cells of varying orientations within nests of different diameters, a process that resulted in approximately equal cell volumes. In one experiment, the sex ratio of bees released within cages was varied and influenced the sex ratio of offspring. Nests within cages in which a higher ratio of males to females was released contained a higher proportion of female offspring.