An adult black fly feeding on a human. (Courtesy Griffin Dill) Credit: Courtesy of Griffin Dill

This story was originally published in May 2020.

Jokingly referred to as the “Maine State Bird,” black flies are one aspect of the Maine outdoors that most people would prefer didn’t exist. Dotting the air in early spring, these tiny bugs have an annoying habit of flying into people’s eyes and ears. And their bites can be especially itchy.

Particularly numerous in the spring and early summer, black flies persist in Maine well into the fall. So as the weather warms, you can anticipate a close relationship with them for months to come — especially if you enjoy spending time outdoors.

In recognition of the state’s long black fly season, we’ve compiled some interesting facts about these tiny pests, some of which may help you tolerate them just a little bit more.

Not all black flies bite.

Maine is home to several species of black flies, and many of those species do not have a taste for human blood. Instead, they prey on other animals, such as other mammals or birds. And on top of that, only female black flies bite. So while it may seem like every black fly in the area is out to get you, that’s not actually the case.

“There are between 40 and 50 different [black fly] species in Maine, and just a handful of them actually go after people,” said Clay Kirby, insect diagnostician at the University of Maine Cooperative Extension. “I don’t know the exact number, but it’s not the majority of the species.”


To the everyday person, black flies of different species look identical, Kirby said. They’re generally small, black or gray, and have short legs and short antennae.

The female black fly feeds on blood for the nutrients it needs to lay hundreds of eggs. In addition, both male and female blackflies feed on flower nectar. And black fly larvae filter organic matter and small invertebrates out of the flowing water they hatch in.

Maine is home to a unique black fly species.

In the late ‘70s, biologists discovered a new species of black fly breeding in the tributaries of the Penobscot River. This species was notable because it did — and continues to — swarm and bite people late in the season, when other black fly species tend to drop off. This species, given the scientific name of Simulium penobscotensis, is especially active in July and August.

A couple of other black fly species found in Maine are also busy late in the summer, which makes one wonder if there really is a true “black fly season.”

Black flies tell us that our water is clean.

Black flies lay eggs in moving water, such as brooks, streams and rivers. And upon hatching, black flies spend two stages of life — as larvae and pupae — in the water, filtering food from the current. Therefore, clean waterways with a high level of dissolved oxygen are especially important to black fly production.

“I can remember back when I was in college here as an undergraduate in Maine, there was a black fly season,” said Jim Dill, a pest management specialist with UMaine Cooperative Extension. “It went from maybe the middle of May to the middle of June and that was it. But the reason was the waters of Maine were so polluted at that time. The blackflies couldn’t survive. Now that we’ve cleaned up our water, we have about 40 species of black flies.”

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Black flies also contribute to a healthy freshwater ecosystem, serving as food for a wide variety of larger animals, including Maine’s celebrated native brook trout.

“[Black flies] are a good indicator species,” Kirby said. “As Maine’s streams and rivers clean up, your black flies can become more common. Plus a lot of animals and other insects feed on the larvae and the airborn adult.”

Black flies are attracted to a variety of sights and scents.

People have long been studying what attracts black flies. Over the years, studies have supported the ideas that black flies are attracted to certain colors, scents and carbon dioxide, which is expelled when a person breathes. Considering all this, it’s nearly impossible to avoid them.

“If you were standing exactly still and didn’t breathe, they probably wouldn’t find you,” Dill joked.

One study observed how many black flies landed on clothes of different colors. It found dark blue to be the most attractive color, while white was the least attractive. Indeed, most sources on black flies hold true to the idea that black flies are most attracted to dark colors.

Another study demonstrated how black flies are attracted to carbon dioxide by altering the amount of carbon dioxide around test subjects. Unfortunately, in everyday life, it’s not feasible to control the amount of carbon dioxide you exhale — but it does explain why black flies might be more attracted to people who are exercising or especially active.

“There is some body chemistry that goes into it too, and also probably foods that you eat may actually exude odors [that attract or repel black flies],” Dill said. “Garlic keeps the vampires away. Maybe it keeps the little vampires away, too.”

People react differently to black fly bites.

Black flies have a special saliva that stops a person’s blood from naturally clotting, which allows the fly to feed more efficiently. When bitten, a person’s body reacts to that foriegn substance. This can cause itching and swelling at the bite site, but this reaction differs depending on the person.

“Most people who have been in Maine for a while don’t seem to react as badly as people who have just come to the state for their first time,” Dill said. “They can have some real nasty reactions.”


Severe reactions to black fly bites that collectively are known as “black fly fever” include headache, nausea, fever and swollen lymph nodes in the neck, according to material on black flies provided online by the Entomology Department at Purdue University. This source also states that while black flies are capable of transmitting a number of disease agents to livestock, none cause disease in humans.

Most of the time, black fly bites are only irritating and the itch they cause is short-lived.

There are several ways to protect yourself from black flies.

While black flies can’t be avoided entirely in Maine, there are many things you can do to lessen the number of times you’re bitten by them when spending time outdoors.

First and foremost, you’ll want to invest in some good insect repellent. The options can be daunting, but many people in Maine will be happy to give you their opinions. Some people prefer all-natural repellents, though they generally have to be added to the skin more frequently. Other people prefer repellents that contain man-made chemicals such as DEET and picaridin.

In addition to wearing repellent, it can be helpful to wear a hat, which will prevent black flies from crawling into your hair and biting your scalp. Glasses or sunglasses are also a good choice, as they will shield your eyes, which black flies tend to be drawn to. Along the same vein, clothing can help protect you from black flies. That’s just one reason why many Maine outdoors-people love wearing jeans and flannel shirts.

“Actually, what we suggest to people who are gardening is to put a hardhat on and smear it with baby oil,” Dill said. “Those [black flies] that are just bouncing off you and driving you crazy will get stuck in the baby oil. They’re kind of delicate little creatures, so that’s enough to stop them.”

In especially black fly infested areas, it may be most effective to wear a bug headnet. And if it’s too hot to cover up with a long-sleeved shirt, you can always invest in a bug net jacket as well.

Also, keep in mind that black flies don’t bite at night. During the day, black fly activity tends to peak in late morning and early evening, according to an online resource published by the University of New Hampshire Cooperative Extension. They also tend to be more active on humid, cloudy days and just before storms. And they hunker down when it rains, since one drop of water can easily knock them to the ground.

Whatever you do, don’t let these tiny flies ruin your time outdoors. With a little planning, they’re entirely manageable.

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Aislinn Sarnacki can be reached at Follow her on Facebook:, Twitter: @1minhikegirl, and Instagram: @actoutdoors. Her guidebooks “Family Friendly Hikes in Maine,” “Maine Hikes Off the Beaten Path” and “Dog-Friendly Hikes in Maine” are available at local bookstores and wherever books are sold.

Aislinn Sarnacki is a Maine outdoors writer and the author of three Maine hiking guidebooks including “Family Friendly Hikes in Maine.” Find her on Twitter and Facebook @1minhikegirl. You can also...