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BUMBLE BEES IN DECLINE |
Threats | Biology | What you can do In the late 1990's, bee taxonomists started to notice a decline in the abundance and distribution of several bumble bee species. Three of these species (Western Bumble Bee, Rusty-patched Bumble Bee, and Yellow-banded Bumble Bee) were once very common and important crop pollinators over their ranges. Franklin's Bumble Bee was historically found only in a small area in southern Oregon and northern California, and it may now be extinct. The dramatic decline in wild populations of these species occurred about the time that a disease outbreak was reported in populations of commercially raised Western Bumble Bees, which were distributed for greenhouse pollination in western North America. The timing of this suggests that an escaped exotic disease organism may be the cause of this widespread loss. To better understand what has happened, the Xerces Society is working with many scientists and other individuals throughout the U.S. and Canada to document the former and current ranges of these species. This detailed information on past and present distribution and current search efforts will help determine the best methods for protecting those remaining populations. If you have any information on the distribution of any of these four species of bumble bees, please contact us. |
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Yellow-banded Bumble Bee Bombus terricola |
Commercial bumble bee rearing may be the greatest threat to B. affinis, B. occidentalis, B. terricola, and B. franklini. In North America, two bumble bee species have been commercially reared for pollination of greenhouse tomatoes and other crops: B. occidentalis and B. impatiens. Between 1992 and 1994, queens of B. occidentalis and B. impatiens were shipped to European rearing facilities, where colonies were produced then shipped back to the U.S. for commercial pollination. Bumble bee expert Robbin Thorp has hypothesized that these bumble bee colonies acquired a disease (probably a virulent strain of the microsporidian Nosema bombi) from a European bee that was in the same rearing facility, the Buff-tailed Bumble Bee (Bombus terrestris). The North American bumble bees would have had no prior resistance to this pathogen. Dr. Thorp hypothesizes that the disease then spread to wild populations of B. occidentalis and B. franklini in the West (from exposure to infected populations of commercially reared B. occidentalis), and B. affinis and B. terricola in the East (from exposure to commercially reared B. impatiens). In the late 1990's, biologists began to notice that B. affinis, B. occidentalis, B. terricola, and B. franklini were severely declining. |
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Where these bees were once very common, they were nearly impossible to find. B. impatiens has not shown a dramatic decline; Robbin Thorp hypothesizes that B. impatiens may serve as a carrier of an exotic strain of Nosema bombi, although it may not be as severly affected by the disease as B. affinis, B. occidentalis, B. terricola, and B. franklini. B. affinis, B. occidentalis, B. terricola, and B. franklini are closely related to each other (they all belong to the subgenus Bombus sensu stricto). B. impatiens is not as closely related, which may explain the difference in sensitivity to a pathogen. This hypothesis is still in need of validation, although the timing, speed, and severity of the population crashes strongly supports the idea that an introduced disease caused the decline of bees. The rearing company Koppert recently applied for a permit to transport the eastern species Bombus impatiens to California for crop pollination. The Xerces Society worked with bumble bee researchers to prepare comments to the USDA/APHIS discouraging the movement of these bees into new areas. Read about our comments on the introduction of nonnative Bombus impatiens to California. Besides the threat posed by the commercial bumble bee industry, there are many other threats to wild bumble bee populations. Bumble bees are threatened by many kinds of habitat alterations which may destroy, alter, fragment, degrade or reduce their food supply (flowers that produce the nectar and pollen they require), nest sites (e.g. abandoned rodent burrows and bird nests), and hibernation sites for over-wintering queens. Major threats that alter landscapes and habitat required by bumble bees include agricultural and urban development. Livestock grazing also may pose a threat to bumble bees, as animals remove flowering food sources, alter the vegetation community, and likely disturb nest sites. As bumble bee habitats become increasingly fragmented, the size of each population diminishes and inbreeding becomes more prevalent. Inbred populations of bumble bees show decreased genetic diversity and increased risk of decline. Insecticide applications on farms poses a direct threat to foraging bumble bees. Insecticide application on Forest Service managed public lands for spruce budworm has been shown to cause massive kills of bumble bees and reduce pollination of nearby commercial blueberries in New Brunswick. Broad-spectrum herbicides used to control weeds can indirectly harm bumble bees by removing the flowers that would otherwise provide the bees with pollen and nectar. Bumble bees are threatened by invasive plants and insects. The invasion and dominance of native grasslands by exotic plants may threaten bumble bees by directly competing with the native nectar and pollen plants that they rely upon. In the absence of fire, native conifers encroach upon many meadows, which removes habitat available to bumble bees. The small hive beetle (Aethina tumida) is an invasive parasite of the honeybee, yet it also infests bumble bee colonies. Its actual impact on bumble bee colonies could be severe, although it has not been well studied. Global climate change also poses a real threat to bumble bees; anecdotal evidence has suggested that some of the bumble bee species adapted to cool temperatures are in decline, whereas warmer adapted species are expanding their ranges. Baseline data and long term monitoring are needed to better understand the true impact of climate change on bumble bees. |
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All bumble bees belong to the genus Bombus within the family Apidae. The family Apidae includes the well-known honey bees and bumble bees, as well as carpenter bees, cuckoo bees, digger bees, stingless bees, and orchid bees. B. affinis, B. terricola, B. occidentalis, and B. franklini all belong to the same sub-genus of Bombus, Bombus sensu stricto. Bumble bees are important pollinators of wild flowering plants and crops. As generalist foragers, they do not depend on any one flower type. However, some plants do rely on bumble bees to achieve pollination. Loss of bumble bees can have far ranging ecological impacts due to their role as pollinators. In Britain and the Netherlands, where multiple bumble bee and other bee species have gone extinct, there is evidence of decline in the abundances of insect pollinated plants. |
Bumble bees are also excellent pollinators of many crops. Bumble bees are able to fly in cooler temperatures and lower light levels than many other bees, and they perform a behavior called “buzz pollination,” in which the bee grabs the pollen producing structure of the flower in her jaws and vibrates her wing musculature causing vibrations that dislodge pollen that would have otherwise remained trapped in the flower’s anthers. Some plants, including tomatoes, peppers, and cranberries, require buzz pollination. Read more about bumble bee life cycle and natural history. |
WHAT YOU CAN DO TO HELP Download a fact sheet on Bumble Bees in decline in North America Funding for our efforts to conserve bumble bees in decline has been generously provided by the CS Fund and Xerces Society members. Much of the content for this page was developed from a status review, co-authored by professor emeritus Robbin Thorp (U.C. Davis Department of Entomology), Elaine Evans, and Scott Hoffman Black (Xerces). A number of individuals have contributed their unpublished records of bumble bees or other infromation to this effort; we greatly appreciate their collaboration: Adriean J. Mayor, Great Smoky Mountains National Park |