Modification of Bumblebee Foraging: The Effect of Milkweed Pollinia

Abstract

Bumblebees (Bombus terricola Kirby and B. vagans Smith) rapidly accumulated pollinia after contacting milkweed (Asclepias syriaca L.), with losses soon balancing gains. Vagans carried significantly more pollinia than did terricola. Often there were more pollinia on the mouthparts than on the legs, especially on vagans, but there were always more pollinia and corpuscula (combined) on the legs than on the mouthparts. Turnover of pollinia on bees ranged from ½ to 2 d. Proportions of pollinia on the mouthparts often increased over the season. Pollinia—bearing bumblebees foraging on cow vetch (Vicia cracca) carried fewer pollinia on their mouthparts and more pollinia on their feet than did bees captured on milkweed. Bees foraging on milkweed regularly lost claws or segments of their legs. Pollinia slowed the foraging speed of both bee species about 25%. Corpuscula alone had less effect on foraging than did pollinia. Difficulties in foraging included losing footing and extricating feet or mouthparts from flowers. Pollinia—bearing bumblebees moved to new plants more frequently than unimpeded individuals, enhancing the possibility of outcrossing.