Researchers at Penn State developing strategies to reduce mushroom phorid flies

UNIVERSITY PARK, Pa. — In southeastern Pennsylvania, where 60% of U.S. mushrooms are grown, phorid flies are posing a threat to the industry, destroying up to 40% of the crop and invading homes. To combat the pest, Penn State researchers and Penn State Extension specialists are working alongside state officials and industry leaders to establish science-based integrated pest management solutions.

The Pennsylvania Department of Agriculture recently issued a quarantine order and allocated $500,000 in grant funding to support affected farmers. As part of these emergency measures, Pennsylvania Department of Agriculture Secretary Russell Redding announced that mushroom farms in Kennett and New Garden Townships in Chester County must implement steam treatment between crop cycles, following best practices developed by researchers at Penn State. This process involves raising the temperature of mushroom growing houses to the point that neither phorid flies nor their larvae can survive.

Phorid flies have been a problem on mushroom farms since the 1940s, and for decades farmers used a pesticide called diazinon to eliminate them from their crops, according to Michael Wolfin, assistant research professor of entomology at Penn State. But in 2012, the Environmental Protection Agency ruled that the pesticide was too toxic and could no longer be used in mushroom production.

“Since then, Pennsylvania has seen year-to-year exponential growth of the flies in the Kennett Square area of Chester and Berks counties,” Wolfin said. “Kennett Square is unique — more than half of the mushrooms in the country are grown in this 250-square-mile area in which there are hundreds of mushroom farms in close proximity, so the flies always have a constant food source.”

The tiny insect is attracted to the aroma of the growing mycelia — the root-like structures of fungi — in the mushroom compost, Wolfin explained, adding that the only thing that these mushroom phorid larvae can eat is mycelia. Most of the flies go outside the mushroom houses to mate and then return to lay eggs in the compost to reproduce.

“It’s impossible to estimate the number of flies in the area during the peak phorid fly breeding seasons,” he said. “Although they are not known to harm humans, they create a major nuisance, pouring into the vents of homes in places like Kennett Township, Kennett Square Borough, New Garden Township and Avondale.”

Funded by grants from the Pennsylvania Department of Agriculture and the U.S. Department of Agriculture to Penn State, the research team has been working to develop an integrated pest management plan comprising multiple approaches to control mushroom phorid flies safely. They are on the verge of rolling out in mushroom farms four potential strategies to complement the current recommended method of steam treatment, which can eliminate bacteria and fungi in addition to adult flies and larvae from mushroom houses. 

The team also plans to present research updates at the Mushroom Pesticide Credit Meetings that will be hosted by Penn State Extension twice this year, and at the Mushroom Short Course this fall. Wolfin’s lab also has provided bilingual educational webinars and on-farm training sessions to educate growers.

The four strategies are:

• Adding entomopathogenic nematodes — nearly microscopic roundworms that are natural enemies of insects — to mushroom compost to kill phorid fly larva
• Introducing predatory mites — tiny soil-dwelling arthropods, related to ticks and spiders, that are generalist predators — to mushroom compost to kill phorid fly larva
• Using a pesticide already registered for use in mushroom farms as a compost drench as a spray or fog to kill adult flies
• Hanging long-lasting insecticide-impregnated mosquito netting — used in Africa and Asia to protect people from malaria-causing mosquitoes in their homes — around mushroom-growing rooms to control adult phorid flies

Together, these four strategies represent a comprehensive integrated pest management plan approach to the mushroom phorid fly problem, and that multipronged line of attack is the best way to provide a solution, Wolfin said.

“We think our research will bring relief soon,” he said.

The team will continue to test the approaches with plans to publish their work, as well as a fact sheet on phorid fly suppression methods, in the future. The team also plans to present research updates at the Mushroom Pesticide Credit Meetings that will be hosted by Penn State Extension twice this year, and at the Mushroom Short Course this fall. In addition, Wolfin’s lab has provided bilingual educational webinars and on-farm training sessions to educate growers.