Caste‐specific patterns of flower visitation in bumble bees (Bombus kirbyellus) collecting nectar from Polemonium viscosum

Abstract

ABSTRACT. Foraging routes of worker and queen bumble bees (Bombus kirbyellus Curtis) collecting nectar from flowers of the alpine sky pilot, Polemonium viscosum Nutt., were followed and the corolla tube length, corolla diameter, floral scent, and number of flowers on plants visited or bypassed by bees were monitored. Additionally, the number and proportion of flowers visited per inflorescence and distance flown from each to the next were recorded. Queens and workers differed significantly in choice of flowers. However, intra‐inflorescence visitation rates and departure distances were similar between castes. Castes differed in the extent to which visitation reflected patch quality versus individual floral traits. Both queens and workers failed to visit skunky‐flowered plants more often than they failed to visit sweet‐flowered ones, and preferred large over small inflorescences. However, queens visited large‐flowered plants more often than small‐flowered ones, while workers preferred flowers with shorter corolla tubes, regardless of their diameter. Although a number of studies have documented caste specialization on alternate species of host plants, ours is one of the first to show that morphological preferences promote comparable foraging differences between castes on monospecific plant resources. Queens, once on a plant, responded to floral traits by probing more flowers on large inflorescences, as well as on those with broader floral form. Workers did not alter intra‐inflorescence visitation rate in response to floral traits. For workers, no significant relationship was demonstrated between the likelihood of passing by a plant and the number of flowers probed on the previous inflorescence visited. Thus, workers appeared to accept or reject each plant of P. viscosum independently. However, queens passed by fewer plants when leaving rich inflorescences than poor ones. These results suggest that workers use only individual plant acceptability in choosing which plants to visit, whereas queens base plant choice on patch and individual attributes. Such differences between castes in foraging rules when exploiting the same floral resource have received little attention, and provide insights into the heterogeneity of harvestable reward distributions from the perspective of the forager population.