THE FORAGING SPECIALIZATIONS OF INDIVIDUAL BUMBLEBEES

Bumblebees of any one species in Maine forage for pollen and/or nectar from a large variety of morphologically diverse flowers, but individuals have limited foraging repertoires at any one time. Unspecialized individuals were sometimes unsuccessful in ex- tracting nectar and/or pollen from highly rewarding flowers. In any one area with a variety of concurrently blooming plants, the bumblebees had ap- parent species preferences. Superimposed on the species preferences were individual prefer- ences. Individuals had primary foraging specialties (their majors) and secondary specialities (their minors). Minors were often bridges to new majors. Queens in Maine necessarily have several successive majors during their lifetime since the blooming time of the plants they utilize are brief relative to their life-span as foragers. However, the blooming time of most plants available to Bombus fervidus workers are long relative to their lifetime. Switching was rarely observed in these bees, even in some individuals observed daily for up to 1 mo at the same foraging area containing other plant species in bloom that were highly attractive to other individuals of the same bumblebee species. On a per flower basis, those flowers producing the most food rewards generally had the largest number of bees majoring from them, and the food rewards available were roughly comparable between different kinds of flowers, regardless of their differences in rates of nectar production. Specializing was usually preceded by sampling a variety of rewarding as well as non- rewarding flowers. When the flowers from which bees were majoring in an area were experi- mentally removed many of the bees sampled flowers of other concurrently blooming plants, but they generally did not switch to flowers from which the food rewards were being depleted by specialists, unless these were experimentally fortified with syrup. Upon finding superior food rewards in enriched blossoms, they switched immediately. Flower-specificity is related to site-specificity. Many bees shared the same foraging area, but different individual bees at the same site utilized the flowers of different plant species. When the foraging area contained landmarks, the bees visited clumps of flowers in a sequence (foraging path) that was generally repeated several times on the same foraging trip when the foraging site was small. The foraging behavior of bumblebees is discussed from a comparative standpoint with other bees and in relation to food distribution and availability in the environment.