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Farms in Kennett and New Garden townships have been ordered to steam-treat between crops to stop the spread of phorid flies, the state agriculture department said in a news release on Thursday, Jan. 30. The quarantine order requires growers to treat mushroom houses and soil between harvests to kill the flies, larvae, bacteria, and pathogens before they spread.
The state is helping farmers fight phorid flies by making $500,000 in grants available through the Chester County Conservation District.
"We have been working closely with the mushroom industry and Penn State researchers for many years, seeking safe and effective solutions to this persistent pest plaguing one of Pennsylvania’s most valuable crops," said agriculture secretary Russell Redding. "Steam treatment has proven to be safe and effective, but the method only works if 100% of growers use it."
More than 60 percent of all U.S. mushrooms are now grown in Kennett Square and surrounding areas, stretching into Berks County and parts of northeastern Maryland, according to the American Mushroom Institute (AMI). The Keystone State produced 66 percent of the nation’s agaricus mushrooms in 2020, selling 796 million pounds.
Mushroom farming has been a southeastern Pennsylvania tradition for more than a century. The industry took root in Kennett Square, which has earned the nickname of the "Mushroom Capital of the World."
In the late 1800s, florist William Swayne experimented with growing mushrooms beneath his greenhouse benches and his success led to the construction of the region’s first mushroom house. His son, J. Bancroft Swayne, expanded the operation into a commercial powerhouse, developing a spawn plant and cannery that fueled industry growth.
The industry generates $1.1 billion in economic impact and supports 8,600 jobs. Mushroom growing also brings millions in revenue to Pennsylvania businesses through the purchase of spawn, casing, compost, and other materials.
Phorid flies are tiny, humpbacked pests that thrive in mushroom farms, laying up to 50 eggs per female near fresh fungal root growth. Unlike sciarid flies, phorid flies prefer warmer temperatures and tend to invade mushroom houses later in the crop cycle.
Their larvae—creamy-white maggots up to a quarter-inch long—feed on mushrooms, reducing yields by as much as 40 percent.
"Significantly more phorid larvae can be tolerated-perhaps as much as 50 to 100 times more than sciarids-before economic damage can occur to the crop," the AMI's website said. "Phorid adults are very capable of transmitting fungal and bacterial diseases, however, and control of the adults is necessary to maintain crop health."
Beyond harming crops, phorid flies have become a major nuisance for nearby homeowners, escaping farms and spreading into residential areas. Warmer temperatures and increased humidity have exacerbated the infestations, making control even more critical.
The AMI, headquartered in Avondale and a longtime advocate for phorid fly solutions, welcomed the state’s investment.
"AMI members continue to test and develop strategies to combat phorids and they stand ready to adopt new tools and practices based on the research conducted with the industry," said AMI president Rachel Roberts.
The order requires growers to document all treatments, submit reports, and comply with state inspections. The state Department of Agriculture will also host educational sessions for farmers in English and Spanish, in partnership with Penn State Extension and AMI.
The state has invested $445,000 in phorid fly research, working with the US Department of Agriculture and Penn State to find safe, non-chemical pest control solutions. Mushroom farmers needing help implementing the new steam-treatment requirements can apply for grant funding through the Chester County Conservation District at 610-455-1381.
The full quarantine order, treatment procedures, and more information can be found at agriculture.pa.gov.
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