Thursday, September 29, 2005

Let's Give Three Bronx Cheers for Bumblebees!

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. Many of us have childhood memories of watching these 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.

Bumblebees occur throughout the Americas and Europe. They are primarily found in northern temperate, sub arctic and sub alpine regions, although a few species are endemic to South America as well. Similar to many other bees and wasps (the Hymenoptera), bumblebees are social insects although, unlike the domesticated honeybee, Apis mellifera, where a certain percentage of the entire hive successfully over winters, only new queen bumblebees survive the winter. Early each spring, the young queens forage and build their colonies after they awaken from hibernation. New colonies quickly attain sizes of at least 50 individuals and certain species’ nests may grow to as many as 300 to 500 individuals. In areas with long warm seasons, some colonies may surpass populations of 1000.

Despite my general 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 birdhouses. 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 sting only rarely, even when provoked, unlike the more aggressive domesticated honeybees. The bees buzzed inquisitively around my face for a minute or so before wandering off to pursue their pollen- and nectar-gathering activities for the day.

Bumblebees are important pollinators for native food plants, 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 plant family that includes economically important crops such as tomatoes, peppers and tobacco. Approximately 8% of the world’s described 250,000 species of flowering plants, the angiosperms, rely exclusively on bumblebees for pollination.

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 endemic flower species 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, bumblebees are cold tolerant so they visit flowers much earlier in the year than 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 extracts pollen from tomato blossoms hundreds of times faster than a honeybee can. Their energetic high-pitched buzzes are produced by rapid contractions of their flight muscles. 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 body fuzz captures this airborne pollen Some of this pollen 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 bubmblebee'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 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 create 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 honey stockpiles are seasonally raided by beekeepers with a sweet tooth. Despite the fact that bumblebee honey is delicious, the small amount that they produce each season ensures that bumblebees are not very attractive to commercial beekeepers. However, several species of bumblebees have been domesticated for use as pollinators, especially for “hothouse tomatoes”, including Bombus impatiens, which is the main species currently used in North America, 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, habitat destruction and the inadvertent introduction of foreign pests and diseases. This has resulted in a similar reduction in native plant species that depend upon the unique pollination abilities of bumblebees.


Thanks to my pal, Liz, for fact-checking this document.

Included in the Circus of the Spineless, Issue the First,
the Best of Invertebrate-related Blog Writing.

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