Predictors of body size under field conditions in the alfalfa leafcutting bee, Megachile rotundata

Body size strongly predicts fecundity and flight performance. In solitary bees, mothers provision the amount of food available for larval growth. Mothers determine the conditions of the nesting location, and the sex of offspring. In Megachilid bees, heritability of body size is low. Therefore, mothers may be able to determine the body size of offspring and make life history choices accordingly that maximize fitness.  However, the amount of variation in body size determine by maternal nesting behaviors has not been well established. We measured provision size, position of the brood cell in the nest, diapause status, temperature during the larval stage, sex, adult body weight and intertegular span in field-collected nests of Megachile rotundata.  Mothers gave diapausing offspring, female offspring, and those laid first in the nest larger provisions. Provision size alone predicted 18% of the variation in adult body size. Adding other significant predictors of body size including sex, diapause status, and position in the nest increased the R² to 48%. Temperature of the nest during the larval stage did not affect adult body size even though some nests spent multiple hours above 40°C, which is known to be a stressful temperature. The size of provision did not affect survival. Our results suggest that M. rotundata mothers are able to determine about half the variation in the adult body size of their offspring through provisioning and nest building behaviors.