The Great Sunflower Project

photo by Rich Hatfield

Bombus – bumble bees (family Apidae)
by Lisa Schonberg and Mace Vaughan (Xerces Society) and Gretchen LeBuhn (SFSU)

Genus summary: All bees commonly known as “bumble bees” are in the genus Bombus. There are approximately 250 species of Bombus in the world (Williams 1994), 49 in the United States, and about 27 in the northwestern portions of both the U.S. & Canada (Stephen et al 1969). Bombus are primarily found in temperate climates and are particularly important pollinators in alpine and artic environments. There is a subgenus of Bombus called Psithyrus that are social parasites on other Bombus species. A Psithyrus female enters the nest of a non-parasitic Bombus and kills the queen. The workers of the raided colony then provide for the Psithyrus female and her offspring.

Floral relationships: Bombus are among the first bees to emerge in the spring. They are generalist foragers, and visit a succession of flowers throughout the foraging season. Many Bombus species have longer tongues that enable them to access nectar from deep flowers such as monkshood, foxgloves, and lousewort (Kearns & Thomson 2001). Some Bombus species are managed for the pollination of crops. Bombus are more efficient pollinators than honey bees (Apis mellifera) for many crops, including tomatoes, cranberries, raspberries, blueberries and field beans (Kearns & Thomson 2001). Bombus are able to work under cooler temperatures than Apis mellifera (Kearns).

Nesting habits: Bombus nest socially in annual colonies that they build in abandoned rodent nests or grass tussocks. Queens overwinter alone and establish a new nest in the spring. Bombus brood cells are unique among all bees; they are closed and grow with the larvae. Sometimes there is more than one larvae within each cell, and the cell buds as the larvae mature (Michener 2000). The queen forages for a short time to provide provisions for her first brood. Once those workers are ready to take over foraging, the queen remains in the nest to lay eggs (O’Toole & Raw 1999).

Diagnostic characteristics: Bombus are very hairy and cute, with yellow, black, orange or red bands. They range in length from 0.4 to 0.9 inches (Michener 2000). Species have very similar body shapes, but can be distinguished by their color patterns. Members of the Bombus subgenus Psithyrus lack pollen-gathering structures because they do not visit flowers and instead use pollen gathered by workers of Bombus species. Bombus females carry pollen wetted with nectar on their corbicula, a smooth, bowl-shaped structure ringed with long hairs on the upper part of their hind legs. Bombus spp. make a low buzzing sound when flying, usually in a characteristic “bumbling” pattern .

Known conservation concerns: In the late 1990's, bee taxonomists began to notice a decline in the abundance and distribution of several bumble bee species, including many bumble bees that were formerly among the most common species in North America. There are a number of threats facing bumble bees that may be leading to the decline of these species, including: spread of pests and diseases through commercial bumble bee rearing and transportation, habitat destruction or alteration, pesticide use, invasive species, and climate change. Bumble bee expert Dr. Robbin Thorp (Professor Emeritus, U.C. Davis) has hypothesized that wild populations of Bombus occidentalis, B. affinis, B. terricola, and B. franklini were infected by an introduced disease carried by commercially reared colonies of B. occidentalis and B. impatiens. In the early 1990s, B. occidentalis and B. impatiens were shipped to Europe and reared in the same facility as a European bumble bee, B. terrestris, then returned to the U.S. Thorp suggests that B. occidentalis and B. impatiens were exposed to a pathogen B. terrestris for which they had no prior resistance. Upon returning to the U.S., B. occidentalis and B. impatiens may have spread a highly virulent disease to wild populations of many other bumble bees. Five Bombus species are on the Xerces Society Red List of Pollinator Insects. Franklin's Bumble Bee (Bombus franklini) is listed as Critically Imperiled and Possibly Extinct. Four species are listed as Vulnerable, and three of these, B.terricola, B.occidentalis, and B.affinis, are in sharp decline. Here is a link to more information about the bees that are in trouble.
→ The Yellow-banded Bumble Bee (Bombus terricola) is a northeastern bee that was once commonly distributed throughout the east and upper Midwest of the United States and throughout most of southern Canada, but has steeply declined in recent years. The Yellow-banded Bumble Bee is an excellent pollinator of potatoes, alfalfa, berry crops, and cranberry, as well as a wide variety of wildflowers and other crops.
→ The Rusty-patched Bumble Bee (Bombus affinis) was once very commonly distributed throughout the east and upper Midwest of the United States, but has nearly disappeared in recent years. The Rusty-patched bumble bee is an excellent pollinator of wildflowers, cranberries, and other important crops, including plum, apple, alfalfa and onion seed.
→ The Western Bumble Bee (Bombus occidentalis) was once very common in the western United States and western Canada. They can still be found in the northern and eastern parts of their historic range, but the once common populations from southern British Columbia to central California have disappeared. This bumble bee is an excellent pollinator of greenhouse tomatoes and cranberries, and has been commercially reared to pollinate these crops. In the past, it has also been an important pollinator of alfalfa, avocado, apples, cherries, blackberries, and blueberry.
→ Franklin's Bumble Bee (Bombus franklini) has the smallest range of any bumble bee in the world; existing only in a small area in southern Oregon and northern California between the Coast and Sierra-Cascade mountain ranges. This bee is a generalist forager on many types of wildflowers, including lupines and California poppies. Since 1998, Franklin's Bumble Bee populations have drastically declined and the bee is likely extinct.
→ The American Bumble Bee (Bombus pensylvanicus) was formerly very common in the eastern and Midwestern parts of North America. In more recent surveys (2004-2006), it has shown a dramatic decline, especially in the eastern parts of its range.
→ The Yellow Bumble Bee (Bombus fervidus) was very common in the 1970s in the eastern U.S. More recently, it has significantly declined in at least part of its range.

Similar taxa: Carpenter bees (Xylocopa spp.) are the only native bees as large as bumblebees, but they are shiny and black and not as hairy as bumblebees.
Interesting fact: Some short-tongued Bombus species that cannot access the nectar in deep flowers practice “nectar robbing.” They cut a hole in the petals of the flower and steal nectar. Thus, they never come into contact with the reproductive parts of the flower and do not pollinate it (Inouye 1982).
Additional resources: www.xerces.org/bumblebees; Benton (2006), Kearns & Thomson (2001) Thorp et al. (1983). For Psithyrus: Fisher (1987).

Worldwide map of Bombus species

Key to bumble bees at Discover Life