Pollen collection and plant identity
Rates of pollen foraging were low on some days, and ultimately, we collected 486 pollen pellets from bumble bees over 34 days, and 1048 from honey bees over 33 days (Appendix Table S1). Because no honey bee pollen foragers were caught at site 1 during period 3, this site/period combination was excluded from pollen analyses for honey bees. Over the experiment, bees collected pollen from 20 distinct plant morphotypes and 16 plant families (Table 2). The morphotype classifications below family included genus (e.g. Hemerocallis ) and species (e.g. Lotus corniculatus ). When we could distinguish distinct pollen morphotypes within a plant family but these morphotypes could not be accurately associated with a genus or species, we subdivided the family using defining features (e.g. Fabaceae_Tricolporate).
Over the flowering season, 65% of pollen returned to honey bee colonies by foragers was from plants in the Fabaceae family, in contrast to bumble bees who brought back 38% of pollen from Fabaceae, and 34% from Brassicaceae (Fig. 1a). Furthermore, over both bee species, Fabaceae was the most abundant pollen collected during all periods except for period five, when Asteraceae became the most abundant (31.3%) plant family along with Brassicaceae (26.2%), the latter being predominantly collected by bumble bees (Fig. 1b).