Over 50 species of bumble bees or humble bees ( Bombus spp.), named for the humming sound they make while flying, occur throughout North America (Canada, the United States, and Mexico). Forty-four bumble bee species are known to be native to the United States and Canada (Natural History Museum, 2009). These bees are large and robust, and are generally black and yellow often with white or orange bands. They are covered in branched hairs that pick up and transfer pollen. Female bumble bees have pollen baskets or corbiculae - a broad concave shiny segment rimmed with long hairs and found on the back legs. The pollen baskets are used to carry pollen back to the nest. Additionally, they have relatively long mouth parts and are able to pollinate plants with deep nectaries, such as the blueberry (Vaccinium spp.). Bumble bees engage in a behavior called sonication, or buzz pollination. The bee places the anther in its jaw and vibrates each flower with its flight muscles, causing pollen to be dislodged. Tomatoes (Lycopersicon spp., syn. Solanum spp.) require buzz-pollination and bumble bees are important pollinators of this crop.
Bumble bees are generalist foragers and do not rely on one particular flower type. They are important pollinators of several wild flowering plants and crops like blueberry, tomato, eggplant (Solanum melongena), and pepper (Capsicum spp.), which is also a member of the Solanaceae family. They are also effective pollinators of some orchard crops like almonds (Prunus dulcis, syn. Amygdalus dulcis), apples (Malus domestica), and cherries (Prunus spp.). These bees are even considered better pollinators than honey bees ( Apis mellifera ) in some instances because bumble bees can fly during rainy, cool, cloudy, and windy weather and they have longer tongues than honey bees so they can pollinate flowers with long, narrow corollas or flowers.
In North America, bumble bees are most important as pollinators of greenhouse crops. Bumble bees do not fly against windows like other bees and require smaller hives than honey bees. Often times they achieve close to a 100% pollination rate with tomatoes in greenhouses. Two species that have been domesticated for use in agriculture are the western bumble bee ( Bombus occidentalis ) in western North America and the eastern bumble bee ( Bombus impatiens ) in eastern North America.
There are several other bumble bee species of particular environmental value or concern in North America. The American bumble bee (Bombus pensylvanicus), native to North America, is an indicator species in the Chihuahuan Desert. Other native species, such as the Franklin's bumble bee ( Bombus franklini ), the rusty patched bumble bee ( Bombus affinis ), the Sonoran bumble bee ( Bombus sonorous ), and the yellowbanded bumble bee ( Bombus terricola ), have all undergone severe declines throughout their ranges in recent years. The buff-tailed bumble bee (Bombus terrestris) is native to Europe, North Africa, and Asia but has found its way into North America, despite its importation into the US being prohibited by law.