Open-cell parasitism shapes maternal investment patterns in the Red Mason bee Osmia rufa

Abstract

Brood cell parasitism inflicts high fitness costs on solitary, nest-constructing bees. Many of these parasites enter open cells during its provisioning, when the mother bee is absent. Therefore, females can reduce the risk of open-cell parasitism by limiting the time they are away from the nest. However, provisioning efficiency (provisioning time per unit of progeny body mass) decreases due to aging. To limit the increasing risk of open-cell parasitism as the nesting season progresses, female bees could optimize their maternal investment strategy by shifting the sex ratio and the body size of offspring during the nesting season. This prediction was tested in the Red Mason bee Osmia rufa (O. bicornis), a stem- or hole-nesting, polylectic, univoltine megachilid bee. In O. rufa, the risk of open-cell parasitism was found to be correlated with cell provisioning time. Additionally, the provisioning efficiency of females declined during the nesting season to one-fourth of the initial value. However, cell-provisioning time did not increase correspondingly. Bees dealt with their decreasing provisioning efficiency by reducing the amount of stored larval food, leading to a reduction of offspring size and a seasonal shift toward males in the offspring sex ratio. The influence of provisioning efficiency and risk of open-cell parasitism on optimal offspring size was analyzed by means of a statistical model. The observed maternal investment pattern of Red Mason bees is an adaptive strategy to reduce open-cell parasitism.