Pollen Harvest by Sonoran Desert Honey Bees: Conservation Implications for Native Bees and Flowering Plants

Abstract

Managed and feral honey bee colonies (Apis mellifera) harvest immense quantities of nectar and pollen within kilometers of their nests whether they live in relatively undisturbed or agricultural habitats. Within the Sonoran Desert of southern Arizona, pollen collection by European honey bee colonies was monitored by the use of apicultural pollen traps. Managed colonies near Tucson, Arizona routinely collected from 20 to 50 kg of pollen each year. Flowering pulses (phenology) in the local flora was closely tracked by the colonies, and pollen influx into their nests usually occurred as three to four distinct seasonal peaks, although some pollen was actively harvested during 48 or more weeks every year. The range of flowers visited for pollen by the honey bee is likely the most diverse for any social or solitary bee yet studied, largely due to their massive food requirements, efficient scouting and recruitment to ephemeral flower patches, and persistence of their colonies as perennial units for many years. At most Sonoran Desert sites, honey bee colonies took pollen from at least 12 and as many as 40-50 dominant angiosperm taxa. Additionally, pollen diet breadth of feral honey bee colonies was determined microscopically from blackened below-nest refuse deposits known as bee middens. One such deposit from the Arizona-Mexico borderlands is thought to represent more than a half century of accumulated materials. Honey bees are dominant invertebrate herbivores in desert regions taking pollen and nectar in massive amounts from at least 25 percent of the local flora. Had this pollen remained on its host plants, it would have been available for transport by co-adapted insect, bird and bat pollinators which are often better at depositing viable pollen, effecting subsequent fertilization, fruit and seed set on native flowering plants. Sonoran Desert bees are predominantly specialist feeders and depend upon certain plants more than honey bees which can switch hosts at will and have a highly mixed diet. Thus, in direct competition with these alien social bees living in large colonies, native desert bees are often at a disadvantage in acquiring pollen and producing replacement offspring. Desert flowering plants, especially rare, threatened and endangered species are also adversely affected since honey bees remove most of the pollen and often are responsible for setting fewer seeds or dispersing pollen at different distances than their original pollinators once did.