PARENTAL INVESTMENT PER OFFSPRING BY A SPECIALIST BEE: DOES IT CHANGE SEASONALLY?

Abstract

Torchio and Tepedino (1980) have shown that the investment per offspring of the generalist solitary bee, Osmia lignaria propinqua Cresson, decreases over the foraging season. Average adult body size of offspring of both sexes from late provision masses was smaller than average adult body size of offspring from early provision masses. Also, a greater proportion of male offspring were produced late in the season than early in the season. Because males are smaller than females, increasing the sex ratio (males: females) leads to a further reduction in average investment per offspring late in the season. Two selective pressures suggested by the authors may favor a reduction in investment per offspring over the season: a) a reduction in the quality and quantity of floral resources, and b) reduced foraging rates of aging females. As a result of such seasonal changes, the mother bee presumably reduces the size of provision masses for each offspring. Torchio and Tepedino predict that such changes should be especially common among specialist bees. The specialist megachilid bee, Hoplitis anthocopoides (Schenck), forages for pollen and nectar only from flowers of Echium vulgare L. (Boraginaceae). Because of this bee's dependence on a single, seasonal resource, one might expect females to reduce their investment in individual offspring as flower resources decline over the season. An alternative response to seasonal changes might be the production of fewer provision masses of the same size by constructing fewer cells per individual and/or by a decline in the foraging population. The data reported here were collected as part of a study comparing foraging efficiencies of H. anthocopoides and four generalist bee species (Strickler, 1979). This study was not designed to detect seasonal changes in investment per offspring, so data are not as complete as those of Torchio and Tepedino. In particular, sex ratio changes over the season cannot be determined. However, adequate data are available to test the predictions that average investment per offspring and foraging rate decrease over the season.