Agriculture Secretary Russell Redding announced an updated quarantine order, expanding proactive efforts to control Phorid flies, or Megaselia halterata, a destructive pest causing crop losses on Southeast Pennsylvania mushroom farms and creating a nuisance for nearby homeowners. Pennsylvania has made $1 million in state funds available through the Chester County Conservation District to support farmers in meeting new quarantine order requirements to control the pests.
Phorid flies do not harm humans and are not a public health threat. The flies and required treatments to control them do not affect the quality of mushrooms in the marketplace.
The Pennsylvania Department of Agriculture (PDA) revised its January 2025 quarantine, broadening the requirements to apply to more farmers and better protect the region’s most valuable crop. The revision is based on industry feedback and crop monitoring since January 2025.
“The department worked with mushroom industry leaders and Penn State researchers to develop safe, effective ways to control this pest, and demonstrated their effectiveness over the past year,” Secretary Redding said. “We know these methods are safe and effective, but they only work when every grower is following the best management practices every day.”
The revised order, effective March 23, 2026, requires all mushroom growers in Chester County to steam-treat mushroom houses between crops, as well as the medium used to grow prior crops. Steam treatment, or pasteurization, kills adult flies, larvae, bacteria, fungus, and pathogens, preventing spread to adjacent farms or subsequent mushroom crops. The method is an industry-proven, effective practice for managing the spread of the pest.
Growers must keep records of treatments and the number of insects present. They must also implement one of five additional treatments proven effective in controlling the pests:
- Exclusion and Inclusion Method – sealing entry points and eliminating attractants
- Parasitic Nematodes
- Predatory Mites
- Adulticides – insecticides that kill adult flies
- Long Lasting Insecticide Nets
Phorid flies feed on fungi and reproduce in dark, humid environments, making mushroom houses and the substrate used to grow mushrooms the ideal environment for them to grow and reproduce. Warming temperatures and increased year-round humidity have increased fly infestations. The pest has been shown to reduce mushroom crop yields by up to 40%, and pests that escape mushroom houses have created a nuisance for surrounding residents.
Pennsylvania leads the nation in Agaricus mushroom sales and production. The center of the state’s mushroom industry is Chester County, where approximately 2,000 mushroom houses are located. According to Penn State University, Pennsylvania produced 69% of mushrooms sold in the U.S. in the 2024-25 growing season. The value of the Agaricus crop was estimated at $1 billion.
