Impacts of Asteraceae pollen spines on bumble bee survival and larval growth

The flowering plant family Asteraceae is diverse and its constituent species are common in many habitats worldwide. However, its pollen, although widely available, is rarely collected by generalist bee species, suggesting it possesses properties that deter these bees from over-exploiting it. Previous studies have proposed that the conspicuous spines on Asteraceae pollen grains might serve as a deterrent against exploitation, but these studies have not separated the effects of pollen morphology from those of pollen chemical composition. Here, we assessed whether Asteraceae pollen spines are detrimental to generalist bees independently of any effects of pollen chemistry. To do so, we administered three pollen diet treatments to queenless bumble bee microcolonies: one treatment incorporated sporopollenin exine “shells” extracted from spinous (echinate) sunflower pollen; a second incorporated chemically identical shells from non-spinous mixed pollen; and the third was a control diet with no added exine. We found that the presence of pollen spines in the diet significantly reduced larval growth, beyond the reduction caused by adding exine to the diet. However, we observed no effect of pollen spines on the survival of adult workers or larvae, nor did we find substantially increased intestinal damage in adults. Our results show that Asteraceae pollen spines impair larval development of generalist bumble bees, but further investigation is required to elucidate the mechanism behind this phenomenon.