Pollen specialisation is associated with later phenology in Osmia bees (Hymenoptera: Megachilidae)

Abstract

1. Species exhibit a range of specialisation in diet and other niche axes, with specialists typically thought to be more efficient in resource use but more vulnerable to extinction than generalists. Among herbivorous insects, dietary specialists seem more likely to lack acceptable host plants during the insect's feeding stage, owing to fluctuations in host-plant abundance or phenology. Like other herbivores, bee species vary in host breadth from pollen specialisation (oligolecty) to generalisation (polylecty). 2. Several studies have shown greater interannual variation in flowering phenology for earlier-flowering plants than later-flowering plants, suggesting that early-season bees may experience substantial year-to-year variation in the floral taxa available to them. 3. It was therefore reasoned that, among bees, early phenology could be a more viable strategy for generalists, which can use resources from multiple floral taxa, than for specialists. Consequently, it was expected that the median dates of collection of adult specimens to be earlier for generalist species than for specialists. To test this, phenology data and pollen diet information on 67 North American species of the bee genus Osmia was obtained. 4. Controlling for latitude and phylogeny, it was found that dietary generalisation is associated with significantly earlier phenology, with generalists active, on average, 11–14 days earlier than specialists. 5. This result is consistent with the generalist strategy being more viable than the specialist strategy for species active in early spring, suggesting that dietary specialisation may constrain the evolution of bee phenology—or vice versa.