Effects of pollen species composition on the foraging behaviour and offspring performance of the mason bee Osmia bicornis (L.)

Abstract

The effects of floral species composition on offspring performance of solitary bees are rarely studied under conditions where foraging behaviour of mothers is allowed to play a role. In a semi-field experiment, we restricted foraging choices of the polylectic mason bee Osmia bicornis L. to flower species belonging to plant families presumably used to different extent: Borago officinalis L. (Boraginaceae), Centaurea cyanus L. (Asteraceae) and Brassica napus L. (Brassicaceae). We quantified the foraging behaviour and brood cell production by mother bees, and compared the quality of offspring in pure and mixed flower species stands. Offspring survival in pure stands was expected to reflect the mothers’ foraging preferences in the mixed stand. Pure stands of B. napus supported highest offspring survival, body mass and fraction of females produced. Offspring survival on C. cyanus and B. officinalis was very low. Larval mortality occurred earlier in brood cells provided with B. officinalis pollen than in brood cells provided with C. cyanus pollen suggesting different effects of pollen quality on early larval and later development. The time spent on different foraging activities correlated with lifetime reproductive output. However, in mixed stands, the proportion of time the bees were foraging on the different flower species did not differ significantly. Foraging behaviour may therefore not generally be a good proxy for the quality of floral resources for offspring production. Our results suggest that resources collected from one plant species may influence the usefulness of resources from another plant species. Bees may therefore overcome potentially deleterious effects of the suboptimal resources by mixing low- and high-quality resources. This may help generalist bees, such as O. bicornis, to cope with an unpredictable environment.