Pollen is the sole protein source for bees and the largest component of their larval diets. Most bee species restrict their pollen foraging to a subset of the available floral resources – ranging from generalists that forage across multiple plant families to specialists that forage within a plant genus. An advantage of generalization is it reduces dependency on individual plant taxa. However, some bees may be restricted to a narrow range of hosts due to physiological constraints that prevent larval development on other pollens.
In subalpine habitats where short seasons and low temperatures already limit bee foraging opportunities, reliance by bees on late-flowering plants poses a challenge, since it allows little time to complete larval development before the onset of winter. At our study site, two mason bee species, Osmia subaustralis and O. montana, are specialized on late-blooming asters and we investigated whether a physiological constraint prevents these species from developing on other pollens. We transferred eggs and larvae from the Asteraceae specialists as well as two other native bee species, O. iridis (Fabaceae specialist) and O. tristella(generalist), to host and nonhost pollen provisions in a factorial design and monitored their survival and development.
Results/Conclusions
We found that the Asteraceae specialists could survive on both host (Asteraceae) and nonhost (Fabaceae and generalist) pollens (proportion surviving was 67% and 75% respectively). Conversely, none of the Fabaceae specialist or generalist bees survived when reared on Asteraceae pollen. The development rate of the Asteraceae specialists was actually higher on nonhost pollens than host pollen with 44% and 17% respectively reaching the spinning larval stage by the end of the field season. However, the difference was not significant. Since the Asteraceae specialist developed on both host and nonhost pollens, it is unlikely that a physiological constraint prevents these species from exploiting earlier-flowering resources. We are currently examining the amino acid composition of the different pollen provisions to determine whether amino acid deficiencies in Asteraceae pollen can explain the observed patterns.
In the future, our larval feeding study should be paired with host selection experiments to assess the willingness of adult bees to exploit novel pollen sources in the absence of their host plants. Understanding the potential host range of bees is important since changes in the availability of host plants could lead to pollen shortages, potentially forcing bees to visit nonhost plants in order to provision their nests.