DULUTH – In the seconds that Peggy Flynn walked toward Lake Superior for a swim, her leg was covered with a black mass of swarming flies.

Those near the North Shore have reported bothersome batches of stable flies, biting insects that feast on human and animal blood.

“You couldn’t even enjoy the lake,” said Flynn, who was camping in Gooseberry Falls State Park with her husband Sunday, Monday and Tuesday. “We’ve seen some flies in the past, but this was more than we’ve ever seen before.”

Experts said stable flies often appear in July and August in Minnesota, though they don’t know why one year’s fly population may be worse than the last.

“Some years we don’t notice them at all, and other years they go ka-boom, and we don’t really understand why,” said Roger Moon, a former entomology professor at the University of Minnesota.

Bill Dalin, owner of North Shore Superior Pest Management, said his company received calls from restaurants in Grand Marais asking for help keeping the flies away from customers. On Thursday, he’s planning to install experimental traps designed to attract the insects to patterned panels covered with a poison.

The 35-year veteran of the pest control industry said he hasn’t noticed anything out of the norm this year.

“But wherever you are, if you’re outside and not moving, these guys can make your life miserable,” Dalin said.

But what about bug spray, which every good Minnesotan carries in the car for crises like this?

Unfortunately, experts said stable flies are a hearty breed and not easily deterred from a meal.

“There really are no repellents for these flies,” said Jeff Hahn, a University of Minnesota entomologist.

The flies are sometimes called “ankle biters” and tend to attack during daylight hours. Hahn said wearing pants and long sleeves is the best way to protect from bites, which can sting or itch. The insects are also known to go after pets, as evidenced by one poor pup sporting a swollen face in a Facebook post.

Stable flies like to breed in damp organic matter, such as animal bedding and seaweed. They are often found near livestock, though Moon suspects some material from the Great Lake is responsible for this year’s shoreside crop of pests.

“There are records of stable flies chasing Florida beach­goers away,” said Moon, adding that the flies have been known to move along shores. “They can be tormenting.”

Flynn, who lives in Elk River, is having second thoughts about her camping reservation on the North Shore next weekend, remembering the swarm of flies that chased her husband while he was out for a morning run.

Strong breezes can help, but until late summer those wishing to enjoy the North Shore may have to brave the bugs.

“They will go away,” Hahn said. “Eventually.”