Sources and frequency of brood loss in solitary bees

We surveyed the literature for reports of parasites, predators, and other associates of the brood found in the nests of solitary bees. Studies were included in this survey if they reported the contents of all the bee brood cells that they examined. The natural enemies of solitary bees represented in the studies included here were taxonomically diverse. Although a few studies report high loss of solitary bee brood to a species-rich set of natural enemies, most studies report losses of less than 20% to few natural enemies. Brood parasitic bees are the greatest source of mortality for immatures of pollen-collecting solitary bees followed by meloid beetles (Meloidae), beeflies (Bombyliidae), and clerid beetles (Cleridae). Most groups, however, are reported from only a few host species and attack a low proportion of brood cells. Mortality due to unknown causes is also common. The suite of natural enemies that attack ground- and cavity-nesting solitary bees is very different. The cavity-nesting species have higher reported mortality due to unknown causes perhaps related to how nests are manipulated and handled by researchers.