The Benefits of Black Flies

April 24, 2017
Black Flies
S.H. LeeThough a nuisance to hikers, black flies signal a healthy ecosystem.

Black fly season, which runs from about Memorial Day to mid-June in the North Country, scares off a lot of people. And who could blame folks for not wanting to be surrounded by swarms of blood-sucking bugs? But this is also prime time for viewing alpine flowers and other harbingers of summer, so Nicky Pizzo, an AMC naturalist, suggests making peace with the annoying insects.

How? For starters, she recommends focusing on the positive. “One good thing about black flies is that they’re indicators of clear water,” Pizzo says. “Wherever you have cool, clear mountain streams, you will have black flies.”

That’s because the flies lay their eggs in running water, rather than in the stagnant pools preferred by mosquitoes, and the water must be pristine for the flies to hatch. In the past, pollution and raw sewage held the insects’ population at bay in some parts of the Northeast, but the cleaner waterways of the last few decades have spawned a boom in black flies.

Eat or Be Eaten
If that’s not enough to make you embrace the black fly, Pizzo points out that only the females bite— and only during the day. They need a blood meal in order to lay their eggs.

What’s more, black flies probably don’t hurt you as much as other insects do when they bite. Pizzo argues the piercing sting of a mosquito is more painful, although she admits the sawing action of the black fly tends to cause more bleeding. And she can’t deny the seemingly endless itching afterward or the allergic reactions experienced by people who are sensitive to the anticoagulant black flies inject.

So Pizzo tries a different tack. She notes that male black flies sup on nectar and therefore play a valuable role in the ecosystem as pollinators. There’s some evidence to suggest they even help pollinate low-bush blueberries. Isn’t that great?

Perhaps Pizzo’s strongest argument—and the one offering the most karmic revenge—is that black flies themselves are an essential food source. Their larvae are eaten by fish. When the surviving larvae hatch, the flies are popular with bats and birds, including flycatchers, swallows, and warblers. Dragonflies and damselflies eat both the larvae and the adult black flies.

“Black flies are a major piece of the puzzle,” Pizzo says. “If we take that food source away, it could cause major problems.”

Staying Safe
If you’re still feeling irritated, remember that black fly season is usually short. And in the Northeast, at least, black flies don’t carry any diseases that harm humans. That’s why Pizzo doesn’t support efforts to exterminate them, such as the use by some communities of a pesticide that kills black fly larvae in waterways.

You can also take steps to reduce the likelihood that you will be bitten. Black flies are drawn to sweat and carbon dioxide, Pizzo says, so when you are hiking or engaging in other outdoor activities, slow your pace to avoid perspiring or breathing hard. Apply insect repellent regularly and wear lightly colored clothes, as black flies reportedly favor dark blues, browns, and blacks. Consider wearing a hat with a net that hangs down to your shoulders, protecting your hairline and neck.

Unconvinced? Pizzo points out one more reason to accept black flies. Unlike invasive species that environmentalists often want to remove, she says, “They’re native!”

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Heather Stephenson

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