Seasonal and spatial patterns of mortality and sex ratio in the alfalfa leafcutting bee, Megachile rotundata (F.)

Abstract

Nests from five seed alfalfa sites of the alfalfa leafcutting bee Megachile rotundata (F.) were monitored over the duration of the nesting season in 2000 and 2001, from early July through late August. Cells containing progeny of known age and known position within the nest were subsequently analyzed for five commonly encountered categories of pre-diapause mortality in this species. Chalkbrood and pollen ball had the strongest seasonal relationships of mortality factors studied. Chalkbrood incidence was highest in early-produced cells. Pollen ball was higher in late-season cells. Chalkbrood, parasitism by the chalcid Pteromalus venustus, and death of older larvae and prepupae , due to unknown source(s) exhibited the strongest cell-position relationships. Both chalkbrood and parasitoid incidence were highest in the inner portions of nests. The “unknown” category of mortality was highest in outer portions of nests. Sex ratio was determined for a subset of progeny reared to adulthood. The ratio of females to males is highest in cells in inner nest positions. Sex ratio is female-biased very early in the nesting season, when all cells being provisioned are the inner cells of nests, due to the strong positional effect on sex ratio.