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

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