Let's Give Three Bronx Cheers for Bumblebees!

tags: Bumblebees, Bombus species, Hymenoptera, insects, entomology, natural history

The Bumblebees, Bombus species, are among the most popular of all insects. Their black-and-yellow fuzz, large round bodies, and bumbling, buzzing flight make them appear almost cuddly, almost like the "teddy bears" of insects. I have many childhood memories of watching these appealing gentle giants move from one bright flower to another, carefully gathering pollen grains and sipping nectar along the way, gently rebuffing occasional pokes from inquisitive fingers.

A plumose hair.
Image: Orphaned.
Please contact me with attribution information. Bumblebees are easily identified on the basis of several physical characteristics; their distinctive color patterns and their rotund, fuzzy bodies. Nearly all species have at least some black on their bodies, and most have alternating black and yellow bands or splotches, but some species have white, orange or red in addition to, or instead of yellow, while others may be entirely black. Their large round bodies are completely covered with soft fuzzy plumose hairs, a characteristic that they share with several other hymenopterans according to Brian L. Griffin's delightful little book, Humblebee Bumblebee: The Life Story of the Friendly Bumblebees & Their Use by the Backyard Gardener. However, Bombus are distinguished from other bees by the shape of the female's hind legs. Each hind leg is specially modified in females to carry pollen; it has a bare concave surface surrounded by hairs, known as a corbicula or more commonly, as a pollen basket. In contrast, other similar bees have completely hairy hind legs.

Bombus corbicula (arrow).
Image: Matthew P. Kweskin,
The Evergreen State College. Bumblebees occur throughout the Americas and Europe. Bombus is comprised of more than 250 known species that are primarily found in northern temperate, subarctic and subalpine regions, ranging into cold areas where other bees are not found, although a few species are endemic to tropical regions of South America as well. The reason for this wide distribution is because Bumblebees regulate their body temperatures through solar radiation, shivering, and radiative cooling from their abdomens, as you will learn when you read the excellent book, Bumblebee Economics, written by one of my favorite scientist-writers, Bernd Heinrich.

Similar to many other bees and wasps (the Hymenoptera), Bumblebees are social insects. However, unlike the domesticated honeybee, Apis mellifera, whose nests are comprised of hundreds of thousands of individuals, Bumblebee nests typically number between 50 and 100 individual and rarely exceed 500 individuals, although some colonies may surpass 1000 individuals in tropical climes. Additionally, while a generally large percentage of the entire honeybee hive successfully overwinters, only newly fertilized Bumblebee queens survive the winter. Early each spring, young Bumblebee queens forage and build their colonies after they awaken from hibernation. The young queens raise their first offspring alone while also foraging, until this first generation can take over babysitting duties and then can begin collecting nectar and pollen themselves.

Despite my fondness for and attentiveness to Bumblebees, I consider myself lucky when I discover one of their nests. Bumblebee nests are typically located in abandoned mouse holes, but I have found them in all types of objects, including attic insulation, compost piles, abandoned teapots and even in styrofoam refrigerator boxes. Early one spring several years ago, I discovered an active bumblebee nest after I bumped into a precariously dangling chickadee nest box. Within seconds, approximately one dozen small black Bumblebees with yellow and orange stripes greeted me with a soft hum. Fortunately for me, most Bumblebee species rarely sting, even when provoked, unlike the more aggressive domesticated honeybees. The young bees buzzed inquisitively around my face for a minute or so before wandering off to pursue their pollen- and nectar-gathering duties for the day.

As you will learn from Stephen Buchmann and Gary Nabhan's important book, The Forgotten Pollinators, Bumblebees are important pollinators for a variety of food plants that are native to the northern latitudes such as potatoes, blueberries, cranberries, eggplants, kiwi fruits (Chinese gooseberries) and raspberries, and for important crop plants such as red clover, alfalfa, and cotton. Further, Bumblebees are the only insect capable of pollinating the Solanaceae, a family of plants that includes economically important crops such as tomatoes, peppers and tobacco. Approximately 8% of the world's known 250,000 species of flowering plants, the angiosperms, rely exclusively on Bumblebees for pollination.

Bumblebee on a Foxglove blossom.
Image: Mike Montalvo.Bumblebees have several special attributes that uniquely adapt them for pollinating "their" flowers. First, Bumblebees have longer tongues than domesticated honeybees. Their tongues allow Bumblebees to pollinate flowers with long, narrow corolla tubes -- which are found in many flower species that are endemic to North America such as foxglove and fuchsia. Additionally, the large size of Bumblebees enables them to push their way into flowers that protect their nectar reservoirs with "trap doors" or other barriers, such as snapdragons. Further, because Bumblebees are cold tolerant, they visit flowers much earlier in the year than can domesticated honeybees, which are native to Africa. In fact, Bumblebees can be found flying when the cloud cover is more than 70% or when ambient temperatures are cooler than 15 degrees Celsius -- either condition is sufficient to keep honeybees snuggled together in their hives. Amazingly, bumblebees have been reported to actively forage during the winter!

The most remarkable Bumblebee characteristic is their special ability to release tightly held pollen from many important crop plants using sonic vibrations. This ability -- unique to Bumblebees -- is commonly known as "buzz pollination" or "sonication." Using sonication, a buzzing Bumblebee's body shakes rapidly, thereby releasing pollen from tomato blossoms hundreds of times faster than can a honeybee. Their energetic high-pitched buzzes are produced by rapid contractions of their flight muscles while they are decoupled from their wings. These muscular contractions produce physical vibrations of approximately 400 Hz that are transmitted throughout the hollow pollen-containing anthers of the flower, releasing clouds of golden pollen. The Bumblebee's fuzz captures this airborne pollen, some of which is distributed to nearby flowers by the bee, thereby guaranteeing a new crop of tomatoes for humans to enjoy. But most of this pollen is gathered into so-called "pollen baskets" on the Bumblebee's hind legs and they deliver this collected pollen to the hive where the bees later consume it. Because Bumblebees consume pollen in addition to nectar, they visit these so-called "buzz blossoms" that are typically ignored by honeybees, who actively seek out flowers that provide a nectar reward for their pollinators.

The Bumblebee's intense buzzes also makes a strange noise, somewhat reminiscent of the sound produced when one person gives another a "raspberry" or a "Bronx cheer" (which also suggested the peculiar title for this essay).

Considering their environmental and economic importance as pollinators, one might expect that many Bumblebee species have been domesticated, as were their cousins, the honeybees. Unfortunately, this is not the case: with the exception of several tropical species, Bumblebee hives do not over winter so their colonies are much smaller than honeybee hives. As a result, they do not amass large stores of honey necessary to support their large populations through the winter -- these are the same honey stockpiles that are seasonally raided by beekeepers with a sweet tooth. Despite the fact that Bumblebee honey is delicious, they produce such small amounts that they are not attractive to commercial beekeepers. However, several species of Bumblebees have been domesticated for use as pollinators, including Bombus impatiens, which is the main species currently used in North American greenhouses especially those producing "hothouse tomatoes", and the large Earth Bumblebee, Bombus terrestris, which is native to Europe.

Unfortunately, most Bumblebee species are declining or are endangered in the wild due to indiscriminate use of broad-spectrum pesticides, as well as habitat destruction and the introduction of foreign pests and diseases. This has resulted in a concomitant reduction in native plant species that depend upon the unique pollination abilities of Bumblebees. But gardeners and property owners can help conserve Bumblebees by providing them with a safe home: by not using any pesticides on your property, by leaving rock and wood piles and compost piles intact and undisturbed, and by growing plenty of wildflowers. Further, if you do discover a Bumblebee nest on your property, please leave it alone, except to move it to a more private location, if necessary. If you absolutely must move a Bumblebee colony out of their chosen nesting container, then you should provide them with a small wooden or styrofoam box that has a small entrance hole, preferably a chickadee or wren nestbox.

It is my hope that this little essay will help people to become aware of the importance and the plight of Bumblebees so they will act to preserve these amazing insects and the many native plants that depend upon them.

Sources:
NOTE: I highly recommend all of these well-written and informative books to interested readers

Bumblebees (Naturalists' Handbooks 6) by Oliver E. Prys-Jones & Sarah A. Corbet (Richmond Publishing, UK; 1991). Obtained in Seattle courtesy of Flora and Fauna Books.

Bumblebee Economics: Revised edition, by Bernd Heinrich (Harvard University Press; 2004; originally published in 1979).

Humblebee Bumblebee: The Life Story of the Friendly Bumblebees & Their Use by the Backyard Gardener, by Brian L. Griffin (Knox Cellars Publishing; 1997).

The Forgotten Pollinators by Stephen L. Buchmann and Gary Paul Nabhan (Island Press; 1997).

The Natural History of Pollination by Michael Proctor, Peter Yeo and Andrew Lack (Timber Press; 2003).

Insects and Flowers: The Biology of a Partnership by Friedrich G. Barth (Princeton University Press; 1985).