Why Are Bumble Bees Endangered?

Are bees endangered? Worldwide, many bee species are declining, and with them, our food supply. Several bumble bees (Bombus spp.) are on that list. They are the round, fuzzy bees you see zig-zagging close to the ground early in spring and crawling around on flowers throughout the summer. There’s no single factor responsible for bumble bee losses. Rather, many interacting threats have converged, putting them in a dangerous situation. We’ll describe the issues and what it means for bumble bees in the future as well as what people can do to help protect them. But first, why should you care?

Why we need bumble bees? 

Why do we need bumble bees? Simply, we would have a lot less food without them.

There are over 250 species of bumble bees. Contrast that with the fewer than 10 known species of honey bees.

Nearly 50 species of bumble bee live in the United States alone. This diversity of bumble bee species means diversity in proboscis (tongue) length, which means they can pollinate a wide variety of flowers. Bees pollinate flowers while they forage for nectar and pollen, using their proboscis to reach down into the flower and suck out the nectar. While they perch on a flower, pollen becomes stuck to the hairs covering their bodies. Bumble bees have a special pollen-collecting structure on their hind legs called a corbicula. After visiting a flower, a worker bee grooms herself, pushing pollen down her body into the corbicula. This helps her carry more than just one flower’s worth of pollen. When she visits another flower, some of the pollen falls off into the flower, pollinating it.

Some flowers can only be pollinated by bumble bees because their pollen is tightly stuck in the anthers. To collect this pollen, a bumble bee grabs onto the flower with her mouth and vibrates her wings rapidly (over 130 beats per second), shaking the pollen out, in a process called buzz pollination. Tomatoes, peppers, potatoes, eggplants, blueberries, strawberries, cranberries, and kiwis are pollinated this way.

Bumble bees are active at times that other pollinators are not. They begin foraging earlier in the year (February) and earlier in the day than other bees. They also remain active later in the year (November) and later in the day. They can fly in weather conditions (cold, cloudy) that other bee species can’t.

These characteristics (diversity, buzz pollination, long activity time, and weather tolerance) make bumble bees extremely important pollinators without which we would have less food and fewer wildflowers.

Bumble bees in crisis

Are bumblebees endangered? Bumble bee populations are doing poorly worldwide, with declines reported in North and South America, Asia, and Europe. A combination of climate change, pesticides, monoculture planting, habitat loss and fragmentation, competition, and disease transmission are responsible.

Climate change endangers many species in a variety of ways. Bumble bees, being adapted for colder weather, have trouble adjusting to a warming planet. In response to climate changes, bumble bee ranges, abundance, and activity have changed. Some species have moved towards the poles while others seek higher elevation. In some cases, this leads to population mixing, which reduces community diversity and makes populations more vulnerable to further change. In Europe and North America, bumble bee ranges are contracting as they are squeezed into ever-tighter zones of suitable habitat. Some changes occur too fast for bee populations to find new habitat. As a result, bumble bees have declined over the past 110 years. Evidence showed that some were unable to move with warming temperatures at the northern limits of their range and others lost habitat and moved to higher elevations in southern habitats. In the United States alone, three native bumble bee species showed significant population declines over a 140-year period. Between 1973 and 2007, bumble bees shifted an average of 80 meters up in elevation in the Colorado Rockies. In Europe, bumble bees have moved uphill in Spain’s Cantabrian Mountains over the last 20 years, resulting in local extinctions and mixed populations.

Pesticides and other agricultural practices are also detrimental to wild bumble bees. Between the 1940s and 1960s, four bumble bee species disappeared from Illinois. This was likely due to the increase in agricultural practices in the state in the mid-20th century, which lead to reduced habitat and food availability, and increased pesticide exposure. Today, half of all bumble bee species once found in Illinois have disappeared or declined.

Pesticides are formulated to kill insects, making them an obvious threat. However, agriculture (and human living more generally) can have less obvious effects on bumble bees. Changes in land use, including the cultivation of lawns, urbanization, and farming, lead to habitat fragmentation and resource loss. Monoculture (cultivating a single plant species can be very dangerous for pollinators. Low diet diversity in queens slows worker bee maturation by several days, affecting nest establishment.

Important!

Genetically-modified crops have mixed effects on bumble bees. Herbicide-tolerant crops lead to increased pesticide use and decreased weed abundance. Many weed plants are wildflowers that diversify bee diet. Insect-resistant crops lead to decreased pesticide use but have some negative effects on bees.

The cultivation and international transport of bumble bees for greenhouse and other agricultural pollination has hurt native bumble bees. Today, over 1 million bumble bee colonies are managed worldwide each year. Nonnative bees compete with native bees for resources and transmit disease to native bee populations. European bees have been introduced to New Zealand, Israel, Chile, Asia, Central America, and North America. From these places, they have secondarily spread to Japan, Tasmania, and Argentina. Some species are shipped overseas and then shipped back after having picked up parasites and/or pathogens from another country. Japanese bumble bee mites have been introduced to Europe while European bumble bee mites have in turn infested Japanese bees.

In North America, over 25% of bumble bee species are at risk of extinction. Four have already decreased nearly to extinction while suffering range contractions of up to 87%. Some of these declines have taken place in just the last two decades. Much of this reduction is due to disease transmission. In the United States, the rusty patched bumble bee (Bombus affinis) was declared critically endangered by the International Union for the Conservation of Nature (IUCN) in 2015 after its population decreased over 90% in the preceding decade, likely due to disease. The yellow-banded bumble bee (B. terricola) is considered vulnerable by the IUCN and possibly extinct in Illinois. In the western United States, Franklin’s bumble bee (B. franklini) may have disappeared completely. Once found from Oregon to California, recent surveys have been unable to locate a single bee. In Canadian blueberry fields, the western bumble bee (B. occidentalis), which made up about a quarter of the bumble bee community in the early 1980s, represented a scant 1% by the early 2000s.

In South America, the giant bumble bee (B. dahlbomii) has disappeared from most of its native range as a result of the spread of the European bees B. ruderatus and B. terrestris. This has been blamed on a parasite that didn’t exist in the region until B. terrestris appeared.

In Europe, bumble bee communities have changed dramatically. 24% of European bumble bee species are on the IUCN Red List of Threatened Species. 8 are vulnerable, 7 are endangered species, and 1 is critically endangered. In Swedish clover fields, two short-tongued bumble bee species that made up about 40% of the community 90 years ago now make up 89%, while long-tongued species have drastically decreased. This reduction in community diversity has negative implications for pollination. There may have been as many as 10 times more bumble bees in the clover fields during the 1940s-1960s than there are today. Nearby, in Danish red clover fields, of 12 native bumble bee species identified in the 1930s, only 7 remain. All of the species that disappeared were long-tongued species.

What we can do to protect bumble bees?

While commercial bumble bee operations can do a few things to reduce the spread of disease (e.g., not importing bees from other countries, disease screening, preventing cultivated bee escape), individual citizens have a role to play as well. Simply providing ample food and habitat space can help bumble bees.

Farmers can introduce more diversity by planting different plants together, practicing crop rotation, or creating wildflower strips and hedgerows.

Reducing pesticide use in favor of less toxic pest control can help bumble bees.

Diversify your home garden with some native wild species in addition to your ornamentals, fruits, and/or vegetables. Native bumble bees necessarily thrive on native flowers. Plant a diversity of colors (bumble bees prefer purple, blue, and yellow) and sizes. It’s recommended to focus on perennials rather than annuals as they typically have more nectar. The UK-based Bumblebee Conservation Trust maintains an online tool that you can use to see how bee-friendly your garden is.

Bumble bees need somewhere to live and overwinter. How long do bumble bees live? Most of the colony lives for only a few months, while queens live for about a year and hibernate through the winter. They typically nest and hibernate underground, so maintaining large tracts of property undisturbed by mowing, grazing, planting, or tilling will provide bumble bees with habitat. You can also provide an artificial nest site for bumble bees by using an overturned flower pot with the drain hole covered and a length of hose or pipe serving as an entrance and exit. Additional artificial options include nest boxes or brush piles.

Conclusion

Bumble bees are critical for our food supply and native environments. They pollinate plants that other species can’t and are active at times and in weather that other species can’t tolerate. However, they are in trouble. Worldwide, they are suffering population declines, mainly due to disease and habitat loss. However, there are things we can do to protect these indispensable creatures, including planting diverse flowers and providing suitable bee habitat. If we work together, we might be able to prevent bumble bee extinction.

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