Wild Side: The buzz on bumble bees

Foraging tirelessly, these insects are prodigious pollinators.

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Everybody knows what bumble bees are: big, hairy, black-and-yellow insects flitting from flower to flower. At least eight species occur on the Vineyard, with two others known from historical records and a couple others remotely conceivable here. But as a practical matter, if you see a bumble bee on the Island, the odds are good it belongs to one particular species: Bombus impatiens, the common Eastern bumble bee.

The dominance of this species here is impressive. It has the longest season of activity here of any bumble bee (perhaps of any native bee, period), with queens on the wing as early as the end of March, and the lingering workers, males, or members of the next generation of queens present, well into October, even early November. B. impatiens visits nearly any type of flower and can turn up in any habitat, including heavily built-up human environments. And in terms of abundance, the number of this species observed generally exceeds the number of all other bumble bees put together. As I write this on the last day of September, Bombus impatiens remains common, but it has been weeks since I’ve seen any other bumble bee species.

From most perspectives, this success means that the common Eastern bumble bee ranks among our most beneficial insects. Foraging ceaselessly, these insects are prodigious pollinators. To provision their colony, workers pack pollen onto specialized areas of their hind legs — broad, flat sections surrounded by long, robust hairs. Pollen transported to the colony in these so-called “corbiculae,” of course, are a loss as far as the plants are concerned. But the hairy bodies of bumble bees also transport pollen, and these grains rub off on other flowers the bee visits, completing the pollination process. Native flowers and shrubs as well as crop plants benefit hugely from the efforts of this bee.

Identifying bumble bees visually in the field is challenging; distinguishing marks can be subtle or simply invisible, and one needs to rely on a combination of field marks and be willing to settle for probability, not certainty, in one’s identification. But a couple of features mark Bombus impatiens fairly reliably. The hairy abdomen is generally almost all black, with yellow hairs confined to the first, basal segment of the abdomen. And while the thorax — the middle body section — may show a dark “bald spot” in its center, the hair that surrounds that spot is all yellow. There is no transverse band of dark hair on the thorax of this species, though some other species show one.

In terms of size, B. impatiens varies widely. A large queen approaches an inch in length and is a mighty impressive insect. At the other extreme, the smallest workers are lightly built bees well under a half-inch in length. You might not even take them to be bumble bees.

As is often the case with social bees and wasps, only queens survive the winter, snuggled up under the leaf litter. (Back in the days when I still maintained ornamental flower beds, I’d rouse many queens during early spring clean-up.) Waking up with the first warm days of spring, these sturdy bees begin to forage and seek out a suitable nest site, often an abandoned rodent burrow or similar hole in the ground. There they build waxy cells and begin laying eggs, foraging to feed the workers that result. As the stock of workers grows, the queen stops foraging and devotes herself full-time to reproduction.

From here, a complex system of behavioral and hormonal control governs what sex and caste of offspring are produced, with both fertilized queens and unfertilized workers capable of laying viable eggs. Suffice it to say that, eventually, both males and potential queens are produced in addition to female workers, and at the end of the season, males and queens leave the colony and mate. The males don’t survive long; the fertilized queens overwinter with a supply of sperm already on board, ready to begin the process again in spring.

Is there a downside to the success of this species? Well, perhaps. Other bumble bee species have roughly similar habits and behavior, though they have shorter seasons of activity and may be fussier about habitat and pollen sources. But to the extent that bumble bee species compete with each other for food or nesting sites, the adaptability of Bombus impatiens may give it a competitive edge. The subject does not appear to be well studied, but especially in fragmented or degraded ecosystems, the success of Bombus impatiens may come at the expense of related species.

But on balance, the common Eastern bumble bee is an easy insect to love. Sure, females will sting in defense of their nest, or if you molest them. But foraging bumble bees are docile, genial creatures, intent on their work and not at all inclined to take offense. And both our native ecosystems and our food supply benefit enormously from the efforts of these insects.

 

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