If black flies are swarming around you when you’re out and about this summer — experts say you can thank Maine’s clean, running water.

In the early 1970s, Maine had just a handful of black fly species — according to Jim Dill, pest management specialist and director of the University of Maine Cooperative Extension Diagnostic and Research Laboratory. He said there are over 40 species of black flies in Maine — though not all of them bite.

According to the Natural Resources Council of Maine, Maine’s rivers are much cleaner today than they used to be — largely due to 1972 legislation known as “the Clean Water Act.”

“Once the Clean Water Act came into play, the rivers and streams in the state of Maine and other areas got cleaner and cleaner and cleaner,” said Dill. “As the streams got cleaner, we got more species of black flies.”

Dill also said one by-product of that clean water is a longer black fly season.

“It used to be a very limited season,” said Dill. “We used to call it black fly season, that was usually about from the middle of May maybe to the middle of June — part of the reason was there were only a couple of species of actively biting black flies that would come out. They need nice, clean, running water.”

Dill said black flies can sometimes be seen flying around even in the fall.

“Now we have basically black flies from early in the spring until November,” said Dill. “I’ve seen them out when I’ve been in the woods in November hunting. There will be black flies swarming all around your face.”

Thankfully, Dill said black flies in the United States aren’t known transmit disease to humans and are primarily a nuisance.

“The good thing about it — if there is a good thing — is that they do not transmit any diseases in this country,” said Dill. “In other countries they do. But in this country, the U.S., there’s not black fly species yet and no cases of anything being transmitted by black flies.”

Dill said only some species of black flies are known to bite humans, though other species may still swarm around you.

“Now, some of those are more just nuisance ones,” said Dill. “They’ll swarm around you but they’re not human biters. They’re kind of attracted to us because of movement or colored clothing or whatever, but they don’t bite. There are only just a handful of species that bite humans.”

However, Dill also said some people — particularly those who are not accustomed to black flies — may notice reactions to black fly bites.

“It seems to be that people coming to visit Maine who don’t live in Maine can really react to black fly bites,” said Dill. “We’ll get calls where people say ‘what can I do about these black flies.’”

Dill recommends wearing repellant, covering up your skin and wearing a head net for some relief.