Intra- and interpopulational variation in the ability of a solitary bee species to develop on non-host pollen: implications for host range expansion

Pollen host choice in bees is in many cases highly conserved, which might partly be due to physiological limitations of bee larvae to digest non-host pollen. These limitations need to be overcome in order to incorporate new pollen hosts; however, the mechanisms underlying such host expansion are poorly understood. In this study, we examined intra- and interpopulational variation in the ability of larvae of the solitary bee species Osmia cornuta (Megachilidae) to develop on a non-host pollen diet of Ranunculus acris (Ranunculaceae) by comparing larval performance within and between five geographically distant European populations. The majority of bee larvae from all tested populations died when reared on the Ranunculus pollen diet. Between 10% and 43.5% of all larvae per population reached the cocoon stage, and 48% of these emerged as viable adults from the cocoons, indicating that the physiological ability to cope with the unfavourable properties of Ranunculus pollen exists in each population. The bee larvae of one population exhibited significantly reduced survival on the Ranunculus pollen diet compared with three of the four other populations. Although bees that successfully developed on the Ranunculus pollen diet showed a distinctly prolonged development time, exhibited higher mortality during diapause and reached a considerably lower adult weight compared with individuals fed the control pollen diet, several of the Ranunculus fed individuals were able to reproduce and to sire viable offspring. This study provides the first evidence for both intra- and interpopulational variation in the physiological ability of solitary bees to digest non-host pollen. This variation might enable host expansion and subsequent host shifts in response to natural selection.