This year and in springs past we visited interesting spring pollinators including honeybees, plasterer bees, carpenter bees, and mason bees. While exploring a meadow last week, a group of naturalists happened upon gaggles of busy pollinators darting among patches of dandelions and hovering near native wildflowers in a meadow. At first glance, these appeared to be some kind of furry bee with a wickedly long tongue used to probe the depths of the florets. Closer inspection revealed the fancy flier’s flight gear to include one pair of wings not two, a sure sign that this was a fly and not a true bee. The close resemblance of these hairy flies to pollinators such as honeybees and bumble bees has earned them the name bee fly.
Bee flies vs. Bees
Bee flies have remarkably long mouthparts called a proboscis that are modified to reach deep into flowers to sip the carbohydrate rich nectar which is an important source of energy for these hyperactive fliers. Although they do not deliberately collect pollen as a source of food for themselves or their young as do bees, their hairy coat traps pollen and provides convenient transport of pollen from one plant to another. The fact that bee flies are common around flowers during this season of high bee activity is more than just a coincidence. Bee flies have a seamier side that often proves deadly for solitary bees such as the mason bees and plasterer bees we met in previous episodes.
Taking advantage of bees
When solitary bees visit a flower and get a full load of nectar and pollen they head back to their nest to provision it with food for their young. The wily and agile bee fly follows the bee back to its nest and deposits an egg in or near the burrow of the bee. After hatching, the fly larva enters the gallery of the bee. Some species of bee flies first consume provisions left behind by the solitary bee before turning their attention to the developing baby bees. They attach to the skin of the developing bee larva and suck its blood, which is the source of nutrients for the developing larva of the bee fly. A fascinating account by the great naturalist Jean-Henri Fabre (1823 – 1915) describes the attack of a bee fly larva in the genus Anthrax (described below as the worm and nursling) on a leafcutter bee in the genus Chalicodoma (called the nurse). “The worm is fixed by its sucker to any convenient part of the nurse, plump and fat as butter. It is ready to break off its kiss suddenly, should anything disquiet it, and to resume it as easily when tranquility is restored. No Lamb enjoys greater liberty with its mother's teat. After three or four days of this contact of the nurse and nursling, the former, at first replete and endowed with the glossy skin that is a sign of health, begins to assume a withered aspect. Her sides fall in, her fresh color fades, her skin becomes covered with little folds and gives evidence of an appreciable shrinking in this breast which, instead of milk, yields fat and blood. A week is hardly past before the progress of the exhaustion becomes startlingly rapid. The nurse is flabby and wrinkled, as though borne down by her own weight, like a very slack object. If I move her from her place, she flops and sprawls like a half-filled water bottle over the new supporting plane. But the Anthrax' kiss goes on emptying her: soon she is but a sort of shriveled lard bag, decreasing from hour to hour, from which the sucker draws a few last oily drains. At length, between the twelfth and the fifteenth day, all that remains of the larva of the mason bee is a white granule, hardly as large as a pin's head.” Yikes! Bee flies are a large diverse group known to attack and kill caterpillars, eggs of grasshoppers, and larvae of beetles as well as baby bees. So bees beware, ‘tis the season of bee flies.
We thank the Master Naturalist Program of the University of Maryland for providing the inspiration for this episode. The wonderful references “Insects: Their natural history and diversity” by Stephen Marshall and “The Life of the Fly” by J. Henri Fabre were used as references for this episode.