Invasive flies have never been worse in Pennsylvania's mushroom capital, homeowners say. But eliminating them is a challenge

PHILADELPHIA — When Brendan Nerney wakes up, it's not the sound of his alarm clock he dreads, but the dull buzzes from bug zappers strung throughout his Chester County home.

Throughout the historic Avondale property, flies gather in black masses across the windows, while flypaper traps, once a crisp white, are instead blotted out with hundreds of the tiny insects.

"There's flies in the exhaust vents, there's flies attached to the light fixtures," Nerney said. "I'm in the shower, I lift up the shampoo bottle, and they're hiding underneath. They're everywhere."

Nerney purchased the home in 2023, thinking its location in the heart of the nation's mushroom capital would offer an idyllic setting and a sense of peace.

But Nerney and his wife's home is now one of the latest in the area to be invaded by thousands of phorid flies, which thrive in the soil of the mushroom farms near his property.

The invasion is part of a trend that has gotten worse for residents near Kennett Square, Avondale, and Toughkenamon in recent years, as neighbors complain on social media and in town halls that the phorid fly population was markedly more noticeable in 2024.

Meanwhile, the insects are causing headaches for members of the $1.1 billion mushroom industry, as growers report millions in damage from phorid flies in the facilities that supply more than half of the nation's mushroom crop.

Even the insect researchers backed with state grant money to eliminate the fly are feeling the impact, as residents' complaints raise the pressure to figure out a science-backed solution to the problem.

"Each year it has gotten a little bit worse and a little bit worse," said Kennett Square Mayor Matt Fetick. "I would get two or three complaints every year. This year, by mid-October, it was almost 40 people."

A mayor's cry for help

Over four terms as Kennett Square's mayor, Fetick has never seen this many flies.

"It was so bad that a woman who had fly strips in her house pulled all the strips down and brought them into Borough Hall and handed them to a receptionist," Fetick said. Another resident told the mayor they held Thanksgiving dinner at a restaurant due to a fly invasion at home.

It wasn't always this bad.

For years, researchers say, the potent chemical pesticide diazinon kept Chester County's phorid fly population in check. But in 2012, the Environmental Protection Agency barred mushroom growers from using diazinon, citing negative human health effects.

The phorid fly also reproduces exponentially, and because the insect thrives in warmer weather — a common occurrence under climate change — more females are giving birth in warmer months, a phenomenon researchers say may be contributing to the noticeably larger population.

The phorid fly is harmless to humans — physically, that is.

The mental toll the invasion has wrought on Nerney is palpable. In a video that the work-from-home IT contractor showed The Inquirer, a flip-flop clad Nerney meanders across his attic, letting out a deep-bellied sigh with each cluster of phorid flies he inevitably comes across.

Because the flies spawn in mushroom soil — not the homes they invade — pest-control experts have had little positive news for neighbors like Nerney.

"There's nothing you can do," one exterminator recently told Nerney, apart from the bug zappers, fly traps, and mesh screens that cover nearly every conceivable entry point to his home, to no avail.

Beyond Nerney's peace of mind, he's also fretting over the potential hit to his property value. Nerney said he bought the home when the flies weren't out in full force, and believed the few insects he did see were due to the house being vacant.

"This is apparently an open secret," Nerney said of the neighborhood fly problem, frustrated that he had not been adequately warned.

Fetick has cast himself as a crusader for beleaguered residents, and in early December, issued an emergency declaration on the invasion with hopes of prompting Harrisburg leaders to devote more resources to help.

The plea called for the Department of Agriculture to increase funding for the entomologists from Pennsylvania State University who have been studying how to mitigate the fly since the mid-2010s, as well as for uniform guidelines for mushroom growers to help manage the insect population at their facilities and the creation of a multiagency phorid fly task force.

But the cry for help was mostly just that; as the leader of a 6,500 person borough with only one mushroom grower within its borders, Fetick has little power to enforce measures throughout the larger region.

State Rep. Christina Sappey, whose district covers Kennett Square and other mushroom towns, hopes other municipalities will pass similar emergency declarations.

"This has gone on long enough," the Democratic lawmaker said, mentioning constituent complaints about the flies dating well before the pandemic.

The four-term representative, who helped secure around $128,000 in grants for phorid fly researchers in 2022, said maintaining that level of funding in Harrisburg each year was a fight.

"It's hard for people who don't deal in [entomology] to understand," Sappey said. "We're used to seeing a pest and being able to squish it to get rid of it."

Still, any major relief for homeowners will ultimately require those Penn State researchers to solve the problem in the first place.

Eliminating the fly for good

Michael Wolfin, an assistant research professor at Penn State, believes his team is close to that goal.

As lead entomologist on the research team, Wolfin has spent recent years traversing the mushroom region to test a variety of fly mitigation tactics.

"2020 and 2021 were really good years," Wolfin said, crediting his team's use of pesticide-laced window screens at growing facilities for a brief and promising crackdown that led to fly population reductions at 59 farms participating in the study.

But the experiment came to a halt after the company that sold the electrostatically charged screens — hardly a widely available product — stopped manufacturing them.

After "starting from scratch," several alternative tactics look promising, according to Wolfin.

Those include introducing microscopic nematodes and mites — natural predators that damage phorid fly larvae before they hatch (without harm to the mushrooms that make it to the shelves, Wolfin stressed) — to mushroom soil and applying the eco-friendly pesticide OrganiShield.

But no solution is immediate. Testing can take two years or more. Meanwhile, any new proposal for a licensed food facility would also require approval from the EPA, according to Wolfin.

Then officials like Fetick would face another challenge: persuading the scores of mushroom-growing facilities to follow such a plan.

Lori Harrison, a spokesperson for the American Mushroom Institute in Avondale, said that the national trade association is taking the phorid fly problem seriously, and that it has been cooperating with Penn State researchers for nearly a decade.

"Phorids are a shared, persistent problem with our neighbors that all farms combat," Harrison said. "Farms want to be good neighbors. They and their families also live and work in these communities."

The phorid fly is a nuisance to mushroom facilities, too.

Researchers estimate that the flies damage 10 to 40% of the mushroom crop when present in the soil, and Harrison said farmers have spent millions attempting to mitigate the insect's impact.

Whatever the solution may be, Fetick stressed that it will need to be both affordable and widely implemented by mushroom farmers.

"We can put in all the best management practices in the world, but if a small independent grower can't afford it, there's no way to require them to do it," Fetick said. "And without 100% compliance, it's almost futile."