Foraging activity of Bumble Bee, Bombus spp., workers on Yellow Hedysarum, Hedysarum sulphur escens, in a subalpine meadow

Abstract

The diel activity of bumble bee workers foraging on Hedysarum sulphurescens Rybd. (legumionosae) in a subalpine meadow was studied in relation to ambient temperature and nectar production. Foraging activity was significantly temperature-dependent for all seven Bombus species observed in the meadow. Workers of B. mixtus were significantly smaller than those of B. melanopygus and B. sylvicola and showed a greater temperature dependence because of a higher cost of thermo-regulation at low temperatures. Sugar concentration in nectar varied from 22% to 95% sucrose equivalents and was inversely related to humidity. The activity of B. melanopygus and B. sylvicola workers was significantly, though weakly, correlated with sugar concentration; however, the activity of B. mixtus workers was not. Therefore, of the variables examined, temperature was the most important factor influencing foraging activity of the bumble bee workers.