UNIVERSITY PARK, Pa. — When they first showed up in 2012, Anne Kelly had no idea what the insects were.

“I had a day off from work and decided to clean my deck doors, and I saw all these little bugs,” Kelly, a resident of a development in southern Chester County, recalls. “They look like tiny gnats. I cleaned them up and went about my business. But they were back within an hour, and then they were all over my ceilings and windows. That was just the beginning.”

Kelly hired several exterminators, but they could not figure out where the insects were coming from. At her wits’ end, she took her dog and cat out of the house and had a bug bomb set off. But the mysterious “gnats” were back the next day — even more than before.

Kelly and her neighbors did not realize the swarms of tiny insects were mushroom phorid flies that came from a nearby mushroom farm. And she didn’t know that the U.S. Environmental Protection Agency had just ruled that diazinon — an insecticide that mushroom growers had used for decades to control the pest — could no longer be used in mushroom production because of its toxicity. That decision left mushroom farmers without a control product.

At another Chester County development near a mushroom farm, resident Colin Carson had a similar experience. He was capturing an estimated 2,000 phorid flies a day on lighted sticky traps. “It got so bad that some people here had to put off their Thanksgiving dinner,” Carson said. “It was just not a habitable kind of situation for a holiday.”

Carson and his wife became so frustrated, they formed a community group called the Phorid Fly Action Committee, which met regularly with local officials and state legislators. Some of those raucous sessions drew more than 200 people; in the audience were Penn State entomologists and Penn State Extension educators who were asked to help solve the problem.

Finding a solution was critical because Pennsylvania leads the nation in mushroom production. The state’s 67 mushroom farms — mostly clustered in a few counties in the southeast — last year produced more than 500 million pounds of mushrooms. In addition to besieging residents in ever-growing swarms, phorid fly infestations also damage mushrooms, limit crop yields and make the job of picking mushrooms onerous.

In 2016, a team of researchers from the Department of Entomology in Penn State’s College of Agricultural Sciences launched an intensive effort to solve the problem. Mike Wolfin, a postdoctoral scholar in entomology, worked with mushroom growers to devise a procedure to give beleaguered residents relief from hordes of the tiny flies. Wolfin was supported by entomology faculty at the university.

By 2020, the phorid fly invasions were affecting thousands of residents in more than 150 neighborhoods, covering more than 200 square miles of Chester and several adjoining counties, even stretching into parts of Maryland and Delaware.

The Penn State team knew that many chemical products would be effective in killing phorid fly larvae, but they cannot be used because they are not permitted by EPA for use in mushroom crops. “It was a complicated challenge because anything that you add to the compost may be taken up by the mushroom," said team co-leader Tom Baker, distinguished professor of entomology and chemical ecology.

The team started screening products for potential use in the compost for larval control, but they did not find anything that was effective. The researchers then started looking at products for control of adults, as these could be targeted away from the compost/crop. They also studied the biology and behavior of the adults to determine if the flies were establishing populations in the residential areas. They found that populations were sustained only on the mushroom farms.

“This enabled us to focus on how to target the adults at the source,” said research team co-leader Nina Jenkins, research professor in entomology. “Our pesticide-screening activities identified a product called EcoVia, made by Rockwell Labs, that is exempt from EPA registration, making it available for immediate use in mushroom production.”

The product contains botanical oils that kill adult flies when they come into contact with it. But it was the innovative application of the biopesticide at the mushroom-growing houses that made it so effective.

Drawing on a technique that Penn State scientists helped develop to channel and kill malaria-carrying mosquitoes in Africa, Wolfin worked with several mushroom farmers to implement targeted placement of electrostatic screens at key entry and exit points. EcoVia is applied to the screens and held in place by the electrostatic coating. Depending on the structure and design of the mushroom house, EcoVia is applied to other key areas, such as attics and return vents.

“By November of 2020, we were seeing almost complete reductions in phorid flies on our test farms,” Wolfin said. “On the test farm where we implemented all of the controls mentioned, we didn’t find any live flies.”

Wolfin praised the determination to wipe out the phorid flies shown by the workers, managers and owner at that test farm, Sher-Rockee Mushroom Farms. “Within one week, they installed new windows in all 34 of their growing rooms with the screens, which was no easy task,” he said. “Their workers followed my recommendations and achieved outstanding results.”

Wolfin is confident that these methods will make a huge difference for both the residents and the mushroom farmers.

“We will continue to work with Sher-Rockee, and we have set up experiments at Phillips Mushroom Farms and Brownstone Mushrooms, among others,” he said. “Eventually, we’d like to roll this out to all the mushroom growers in the state.”

According to James Ciarrocchi, owner of Sher-Rockee, Penn State’s expertise was vital to solving the pest problem.

“Mike Wolfin’s assistance has been invaluable with the outbreak of this massive fly problem in Chester County,” he said. “With mushrooms being the largest cash crop in Pennsylvania, this program will be such an asset to our industry. Mike has been a great help in adding tremendous value to our integrated pest management program.”

Seeing the greatly reduced phorid fly numbers has been “life changing,” Kelly said.

“When you are sealed in your home, doing everything you can to try to keep bugs out — and they still get in, hundreds and in some homes, thousands a day — you can't imagine the horror of it. So, for them to be able to figure out how to get rid of these insects at the source — it’s phenomenal.”