Mushroom Phorid Fly Infestations in Mushroom Farms and Surrounding Neighborhoods

Pennsylvania mushroom farms lead the nation in domestic production of mushrooms. The mushroom phorid fly, Megaselia halterata, continues to threaten the mushroom farming industry by vectoring fungal diseases and reducing the yields of the crop. Individual farms experience hundreds of thousands of dollars in losses due to mushroom phorid flies. In recent years, mushroom phorid flies have become a significant public nuisance in residential areas in parts of Pennsylvania, Delaware, and Maryland.

What are mushroom phorid flies?

Mushroom phorid flies are pests of mushroom crops. The adult flies are short-lived and do not eat. The life cycle of a phorid fly has four distinct life stages: egg, larva, pupa, and adult. The adult stage is short-lived--they mate, lay eggs, and die. Adult flies do not feed, although they need high humidity to survive. Mated adult females lay eggs in mushroom compost with well-developed mycelium. The larvae hatch from the eggs within a few days and feed on the actively growing mycelium; this is the only life stage that actually feeds. The larvae pupate in the compost, and adult flies emerge approximately 20 to 30 days after the eggs are laid.

How can I tell if a fly is a mushroom phorid fly?

Mushroom phorid flies are very small, usually between 1/64 and 1/8 inch long (Figure 1). They have a steep arch or humped back behind their head. Mushroom phorid fly eyes are dark, not red like fruit fly eyes. Finally, mushroom phorid flies have the distinct odor of anise.

Why are mushroom phorid flies a problem?

Mushroom farmers have limited control options for mushroom phorid flies because the most effective pesticide, diazinon, is no longer approved for use on mushroom farms. Since then, Penn State has been performing laboratory and field experiments to identify alternative effective insecticides and novel control methods to manage this pest.

Are phorid flies a health concern?

Phorid flies are a nuisance pest, but they are not a health concern. They do not sting, bite, or vector human diseases.

Why are mushroom phorid flies in my home?

If you are experiencing mushroom phorid fly invasions, you are not alone. Mushroom phorid flies have invaded more than 150 neighborhoods, covering 242 square miles. Mushroom phorid adults fly around mushroom farms to find the growing rooms with the most actively growing mycelia. Some of these flies get blown by the wind from the farms into residential areas. If possible, the mushroom phorids will return to the mushroom farms. However, not all of the flies can make it back to the mushroom farms and seek refuge in homes.

It is currently unknown why flies invade some homes and not others. Further research is necessary to understand these behaviors. Away from the cool, humid environment inside mushroom growing rooms, adult mushroom phorid flies can only live for three to five days. At temperatures above 75°F, adult flies cannot survive for more than three days, and their survival is even shorter than three days at low humidity (below 60 percent). As a result, when flies enter homes, they are likely to die within two or three days.

Can mushroom phorids reproduce in my home?

They do not reproduce in homes because there is no actively growing mycelia. For the same reason, they do not live in home drains, mulch, or yards. They can mate and lay eggs in residential neighborhoods, but the larvae cannot develop into adults because there is very little actively growing mycelia. This is why we classify the mushroom phorid populations in homes as "invasions" rather than "infestations." Studies conducted at Penn State showed that gravid female flies seek out compost that has well-established mycelial growth for egg laying, and avoid laying eggs where there is little detectable fungal mycelium. Even when gravid females are forced to lay eggs where fungal mycelium is absent, the larvae that hatch cannot develop. Similarly, larvae emerging from eggs laid in compost with less-well-established mycelium develop more slowly than those in well-established mycelium.

Are the mushroom stumps and compost waste seen outside mushroom farms contributing to the fly problem?

No. Penn State conducted specific studies to test this and determined that larvae cannot grow in or on mushrooms, mushroom stumps, or steamed (spent) mushroom compost.

I see phorid flies swarming over lawns and mulch. Do they live and reproduce in these areas?

While it is true that adult flies are frequently seen swarming over lawns and grassy areas, they are most likely taking advantage of the localized humidity provided by these areas. Even if eggs are laid in grass, there isn't enough mycelium to sustain larval development. The Harrogate community conducted a detailed study on this and confirmed that no adults emerged from turf samples.

Which farm is responsible for the flies in my home?

The wind is capable of transporting adult flies over a number of miles, so the flies invading your home may not be from the closest mushroom farm.

What attracts mushroom phorid fly adults?

Mushroom phorid flies are attracted to both ultraviolet and natural light. Some wavelengths of light may be more attractive to the phorids, and research is ongoing to further understand their attraction to light. Additionally, like many insects, mushroom phorid flies have a strong behavioral tendency to walk upward on vertical surfaces. That is why they are often found on the second floor and in the attics of homes. Finally, since mushroom phorid flies are prone to desiccation (drying out), they tend to accumulate in areas of high humidity, like bathrooms.

When are mushroom phorid flies at their peak populations?

Mushroom phorid fly populations are reduced significantly during the winter as the cold temperatures kill off the adults attempting to travel outside between mushroom growing rooms. Mushroom phorid fly populations begin to build when outside temperatures allow for flight between growing rooms. Fly populations reach their peak on farms between September and October. Outside temperature and humidity conditions in the fall (cooler, more humid) also increase the longevity of adults outside the mushroom growing rooms. This contributes to the increase in fly populations in neighborhoods during this period. Unseasonably mild winters kill off fewer flies, which can result in larger numbers of flies reproducing earlier in the season and therefore lead to uncontrollable populations.

When are mushroom phorid flies most active?

Mushroom phorid flies are crepuscular, meaning they are most active during dawn and dusk. It is still unknown at what time of day the flies are entering homes.

What can I do to reduce the mushroom phorid fly populations in my home?

While exclusion is very difficult, it is the most effective way to reduce the mushroom phorid fly populations in your home. Whenever possible, close windows, doors, and other openings to the outside. Additionally, close blinds at night to hide the light from them. Vacuuming the flies that gather in bathrooms and windows is an effective solution. Additionally, sticky traps on windows (Figure 2) and light traps in homes (Figure 3) may also help reduce populations. Chemical insecticides are not recommended since flies that do gain entry to homes usually die within two to three days without any intervention. Furthermore, the use of chemical insecticides that are not labeled for phorid fly control in homes could be extremely hazardous to residents' health. Always follow label directions when using chemical insecticides, even if they are labeled for in-home use.

What is being done to solve this problem?

Scientists on the mushroom fly research team at Penn State have been making significant advances in reducing mushroom phorid fly populations on mushroom farms, which will lead to fewer flies invading homes. This research is being supported by funding from the U.S. Department of Agriculture, the Pennsylvania Department of Agriculture, and the Giorgi Mushroom Company. Novel on-farm control methods have shown great promise, nearly eliminating all flies on test farms. These methods are being expanded to more farms in 2021.

Where can I get more information?

Mushroom Fly Hotline: 833-574-6743
Call the hotline to report an invasion or to receive the most accurate up-to-date information.

Phorid Fly Invasion Map
Note: Addresses on this map are approximate and pins do not reflect homes that are experiencing invasions. Pins have been placed randomly to depict a reported area or neighborhood.

Informational Session on Mushroom Phorid Flies
Hosted by Rep. Christina Sappey and the Pennsylvania Secretary of Agriculture.

Webinar: Overview of Management Strategies for Mushroom Phorid Flies
Hosted by Penn State Extension.

References

Klok, C., and J. Harrison. 2008. "Atmospheric hypoxia limits selection for large body size in insects." Nat Pre.

Reibe, S., and B. Madea. 2010. "How promptly do blowflies colonise fresh carcasses? A study comparing indoor with outdoor locations." Forensic Sci Int. 195 (1-3): 52-57.

Prepared by Dr. Michael Wolfin, Dr. Nina Jenkins, Dr. Tom Baker, and Maria Gorgo-Gourovitch.

This publication was funded by the Pennsylvania Department of Agriculture grant "Innovative Phorid Fly Research and Extension Outreach Efforts Advancing Awareness of Integrated Pest Management (IPM) and Organic Solutions by the Pennsylvania Mushroom Industry and Surrounding Urban Communities."