Some plants need the bumblebee’s buzz

Everyone notices that autumn brings shorter days, cooler temperatures and bright fall colors. But there are other, just as reliable indicators of the transition from summer to winter — rusty and black banded woolly bears crossing country roads, yellow-and-black garden spiders roosting inside aging Queen Anne’s lace umbels, and seas of brilliant goldenrod fading from the late summer stage.

It was in a field of goldenrod where just a few plants retained their bright color that I recently came upon another sure sign of fall. On an evening walk something caught my eye about one particular still bright goldenrod. I looked closer and found a bumblebee hugging the underside of the leaning inflorescence. I moved the stem, and the bee responded lethargically. The chill of the evening air had already moved the bumblebee to retire for the evening. Upon the first hard frost, this bee, like most bumblebees will die. The fate of the species resides with recently impregnated queens that winter underground.

Unlike honey bees, which overwinter in enclosed hives well stocked with food (honey), bumblebees rely on individual queens to make it through the winter and renew the population in the spring.

Queen bumblebees mate in the fall. The “lucky” males die shortly after mating, and the queens find an underground hibernaculum to spend the winter. They may use abandoned chipmunk or mouse burrows.

In the spring the queen searches for a place to establish a new colony. It may be underground, or she may use an old above-ground vole nest or just a dense tuft of grass. She lines the nest with fine plant fibers and secrets wax from abdominal glands to form a “honeypot.”

Then she visits early spring blooming flowers and fills her crop with nectar, which she regurgitates into the honeypot. Meanwhile, she’s also collecting protein–rich pollen at the flowers she visits. The pollen is collected on her furry body and on pollen baskets on her hind legs.

Back at the honeypot, the queen sheds the pollen into a waxy mass and lays up to a dozen eggs. (Remember, she mated back in the fall.) She stays with the eggs and actually incubates them with her warm, fuzzy body. Unlike many insects, bumblebees can actually generate their own internal body heat via complex muscular activity.

After the eggs hatch, the queen provides additional nourishment in the form of nectar and pollen as needed. During the larva’s two- to three-week period of development, the queen builds another honeypot and repeats the process. This continues throughout the spring and summer until the colony reaches a size of two to three hundred individuals.

Until late summer, all the queen’s offspring are sterile females. They tend the honeypots and feed each new brood of bumblebees. As fall approaches, the workers provide a near constant supply of food to the final generation of bumblebee larvae. It is these late summer bumblebees that develop into fertile adult males and females.

That brings the colony full circle to early fall. Fertile males and females mate, the males die, and the next generation of mated queens retire to underground dens for the winter.

Pollination by most bees, wasps, butterflies, beetles, bats and hummingbirds is mechanical and incidental. As pollinators visit flowers for nectar, pollen adheres to body parts, and they carry it to subsequent flowers where pollination occurs.

But bumblebees also use another fascinating pollination technique. Some plants require a sonic stimulus to release pollen. Bumblebees oblige by

grasping the pollen-containing anthers and producing an audible buzzing sound as they work the flowers. The sound is caused by rapid fire contractions of the flight muscles, vibrations of which are transmitted inside the hollow anthers. Bumblebees act as living tuning forks and cause the pollen to be sonically discharged as an explosive cloud. Stephen Buchmann, co-author of The Forgotten Pollinators (1996, Island Press) calls this “buzz pollination.”

Among the 20,000 worldwide species of plants that require buzz pollination are several important food crops — blueberries, cranberries, some peppers, eggplants, kiwi fruits and tomatoes. The dinner table just wouldn’t be the same without bumblebees.

XSend questions and comments to Dr. Scott Shalaway, RD 5, Cameron, WV 26033 or via e-mail to sshalaway@aol.com