Black Fly Season

May 22, 2009

Seven black-fly seasons have passed since I moved away from the Northeast. This time of year, when I am missing the Red Sox and the swimmable ocean, I find comfort in thinking about the black flies.

Though black flies’ thick swarming, painful chomping, and blood-sucking have ruined many a Memorial Day outing in the North Country, there are a few good things about these critters. First, their lives are very short—a few days to a few weeks long. Second, they likely play a part in pollinating blueberries. And last, unlike mosquitoes, they don’t bite at night.

About 2,000 species of black fly (a.k.a. buffalo gnat) range from the South Pole to the North Pole, 40 in Maine alone, where they have unofficially been dubbed the state bird. Legend holds that they are worst between Mother’s Day and Father’s Day, but many will attest to a longer season, especially farther north. The ones we see most often are likely from the genus Similium.

Abatement techniques in the past included pouring diesel oil and gasoline into rivers and spraying the broad-spectrum chemical insecticides methoxychlor and Dibrom, which also killed most other flying insects. And it was pollution and raw sewage that also held the flies at bay through the 1970s. Because the water must be pristine for eggs to hatch, the clean rivers of the past several decades have brought back the flies in droves.

Enter Bti (bacillus thuringiensis israelensis), a natural soil bacteria and “biopesticide” discovered in a stagnant pond in an Israeli desert in 1977. Shortly thereafter, scientists at the New York State Museum began an extensive study of Bti to determine its effectiveness and safety in several towns in the Adirondacks. They taught local volunteers and town employees how to broadcast Bti, in the form of microscopic crystals, in carefully determined amounts across moving streams where female black flies lay their eggs. (Mosquitoes lay eggs in stagnant water.) While feeding, the larvae ingest the crystals and because of the particular alkalinity of their digestive tract, the crystals dissolve, become toxic, and destroy the stomach walls, killing the insects.

One of the scientists, Daniel Molloy, says they found Bti to be nontoxic to other aquatic life—it has even earned a reputation as one of the safest insect-control agents ever developed. Remarkably, the acidic environment of the stomachs of fish, humans, and other mammals does not activate the toxins. And, he says, “it had no effect at all on the trout streams. All research indicates it is incredibly selective. Because of its toxicity to black flies and mosquitoes, it’s a once-in-a-lifetime biopesticide.”

In Pennsylvania, Bti application has become an annual $6 million business. And 70 towns in the Adirondacks have continued to use it. Molloy says that, with virtually no black flies at all in spring, the village of Lake Placid, N.Y., is one of the success stories.

Despite the abundance of the flies in their state, Maine officials remain adamant about not using Bti. “We do not favor anything that is toxic to one organism because we often find out down the road they are toxic to others,” says David Littell, commissioner of the Maine Department of Environmental Protection.

So, while Mainers may be slapping their heads and cursing the black flies today, they may be grateful later on.

Search AMC Outdoors and Blogs


Search for:

Madeleine Eno

AMC Outdoors, the magazine of the Appalachian Mountain Club, inspires readers to get outside and get engaged. Learn more.