Phorid fly infestation frustrates Chester County, Pennsylvania, residents

For some homeowners in Chester County, Pennsylvania, fall brings more than cooler temperatures and changing leaves. It also brings the return of phorid flies.

Residents on Heatherly Lane in New Garden Township said the tiny black insects swarm their homes by the hundreds, making it difficult to open windows or even relax inside.

"You can't have a window open," Judy Stoltz, a longtime New Garden Township homeowner, said. "They're really a nuisance, but a real inconvenience, too."

Also known as scuttle flies, phorid flies breed in the compost used by mushroom farms. Homeowners in communities near mushroom-growing operations said the flies have been a nuisance for years.

"They're a definite problem," John Stoltz, Judy Stoltz's husband, said. "We've noticed it this year, I'd say relatively early, early in the season, and it's only going to get worse."

The frustration has brought some residents together. Homeowners have started a Facebook group where they share information and pictures of traps covered with phorid flies.

"We had no idea when we bought the house that phorid flies were an issue," New Garden Township homeowner Michele Hanna said.

Hanna said the flies have swarmed her kitchen and living room.

"We were watching TV the one night, and they just kept coming and getting in front of my face," she said. "And literally, I'm swatting them while I'm trying to watch TV."

The Pennsylvania Department of Agriculture said it is closely monitoring the phorid fly population. The flies are also a concern for mushroom growers.

The American Mushroom Institute said mushroom farms are taking several steps to mitigate the problem, including upgrading seals on doors and vents, using targeted chemical treatments, removing contaminated substrate and working with Penn State on best practices.

"Not all farms have flies," Lori Harrison, vice president of communications and industry relations at the American Mushroom Institute, said. "Penn State University research has concluded that phorid flies can be carried by winds and there is no way to tie any flies to any specific mushroom house."

Harrison added that the mushroom industry, which employs about 7,000 people in Chester County, has taken aggressive steps to reduce fly populations.

According to the American Mushroom Institute, in fall 2025, the EPA approved emergency use of specialized fly netting for mushroom farms after data from growers and their partner organizations showed it could help control phorid flies.

The institute said it is exploring other pesticide products that could be suitable for residential use. In the meantime, residents are still searching for a lasting solution.

"Maybe one day, we'll just have two come in the house," Hanna said. "Last year, I probably had 1,000 come in."