Nesting biology of the solitary digger bee Habropoda depressa (Hymenoptera: Anthophoridae) in urban and island environments

The nesting biology of Habropoda depressa Fowler is described for urban (Un iversity of California at Berkeley) and island (Santa Cruz Island) populati ons in the state of California (USA). This protandrous species is common in the California foothills where adults are active from late February throug h early June. Larvae do not spin cocoons and pu pate to overwinter as adult s by November. A portion of the population appears to delay development sin ce prepupae were found in nest excavations early in the nesting season. Bro od cells were parasitized mostly by two dipterans: bombyliid flies and an a nthomyiid species, Leucophora fusca Huckett (found only on SCI). Unlike its congeners which nest in sandy soils, H. depressa nests in hard-packed soil s, including clay. During the nesting season, females spend evenings outsid e burrows, roosting on nearby vegetation before returning to their nest the following morning. At the urban locale, female bees subsist almost entirel y upon exotic and horticultural plant varieties while the majority of host plant collection records at Santa Cruz Island were from native species. On average, nests made by bees at UCB were significantly shorter than those co nstructed at SCI, although the average number of cells per nest was greater . These foraging and nest architectural differences may reflect either vari ation among populations or adaptations to urbanization effects of the last century.