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All About Monitoring Mushroom Flies

Information about monitoring mushroom flies for the mushroom industry.

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Updated:

March 18, 2026

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Monitoring phorid flies with light and sticky traps

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What is fly monitoring? Why should farms monitor their flies?

Pest monitoring is a foundational aspect of an Integrated Pest Management program. It is the process of accurately observing and recording the fly population, both on a farm and in a particular growing room. Farms should monitor flies because it helps a grower more efficiently manage their flies. Because one mushroom phorid fly can lay 50 eggs every 18-22 days, and one fungus gnat can lay 170 eggs every 18-22 days, populations can get out of control quickly. Monitoring fly populations can help a grower make management decisions to alert them of a potential outbreak before it occurs.

How does monitoring flies save money?

It is estimated that Pennsylvania mushroom growers lose between 10-40% of their crop during peak fly populations.Proper monitoring allows a grower to act accurately in response to real mushroom fly populations on their farm much more accurately than with their naked eye. For example, it is easy to see flies on a monitoring trap when fly populations are low, and the grower can use this early information to act proactively to prevent outbreaks and crop losses.

Monitoring can also help a grower decide on a pesticide dosage/frequency based on the number of flies on the monitor. Additionally, growers can see in real time if their interventions are working, and can scale up or down accordingly. Proper monitoring can allow growers to not waste pesticide/labor on applications that are not needed.

supplies for creating a monitoring trap — Figure 1. Sample monitoring trap materials. In this setup, wax paper will be attached to the wings of the trap and the spray adhesive will be applied to the wax paper. The UV light in the middle is shatterproof. The trap is hung by a chain.

What resources do I need to monitor flies?

A fly monitor is needed to make this process happen. The monitor has three major parts: the light, the paper, and the base (Figure 1).

The standard PA fly monitor has a shatterproof (to minimize food safety hazards) UV bulb (to maximize fly attraction) that is at least two feet in length. The light should be securely attached to the base, and the electrical cable in good condition.

The base needs to fit both the light and the papers used to trap the flies.

There are several effective options for fly papers. The cheapest method is applying a strong adhesive spray to parchment paper. There are also premade adhesive trap cards.

Where should I put my fly monitor?

While fly monitors can be used in most any part of a farm, they are most useful in the growing rooms. They should be on a second floor of the growing room, if possible, and off the ground kept at a height where they are easy to access. The monitor should also be out of the way for any workers. Lastly, while extension cords are almost always needed, the monitor should be reasonably close to an electrical outlet. Monitors should be in the same place in the growing rooms for accurate comparisons. Always follow safety regulations when placing fly monitors.

What maintenance does a fly monitor need?

The monitor requires minimal maintenance. The most frequent form of maintenance is replacing the fly paper. This should be done when the fly paper gets saturated with flies, or a fly count is performed.

Possible repairs include replacing the bulb/light source, readjusting the monitor side flaps, resetting the wall attachment, and repairing the cord connection. All of these are relatively minor repairs and can be done in growing room with minimal need for additional tools.

How should a farm monitor their flies?

A worker should count the flies on the monitor depending on how quickly the monitor fills up with flies. Fly papers should be counted and changed before they fill up with dead flies. Counts should occur at least once per week to monitor changes in invading and emerging flies. These fly counts should be used to determine the best course of action for what treatment method is needed.

How should we perform a fly count?

A fly count is as simple as just counting the number of each species of fly in a particular house. If the flies on the monitor are low and the papers don’t need to be changed, workers can mark counted flies on fly paper by squashing the flies on the monitor with their finger as they count them. Workers should wash their hands after touching the flies to avoid spreading disease around the farm. Workers can be trained to identify the two prominent species of flies for: sciarids and phorids.However, other than the use of Insect Growth Regulators (IGRs) and different species of nematodes, many interventions are the same for the two species. It is up to the individual farm to decide whether they will differentiate the two species of fly.

To perform the count, simply count all the flies around the fly monitor. In many cases, the flies will be in low enough numbers where a simple count will suffice. Workers can estimate the number of flies if there are many flies on a paper. For example, workers can count ¼ of a fly paper and multiply that number by 4. Alternatively can count the first 100 flies and estimate how much of the fly paper those 100 flies occupy. The worker can then multiply the 100 flies by the proportion of the paper they counted.

Figure 2. Examples of fly monitors in growing rooms. Some traps can be counted (left) while others may make more sense to estimate. A worker can estimate the flies in a growing room by counting the number of flies in the red box, then multiplying by 4 because the red box is 25% of that side of the trap.

What do these numbers mean?

Fly counts within a growing room should be divided into two categories: "Invaders" and "Emergers."

Invaders are adult phorid flies that initiate infestations by invading the grow room typically during the first ~18 days of the crop. Invasions are highest during vulnerable stages such as filling and casing on Phase III farms or spawn run on Phase II farms. Gravid females lay eggs near the advancing mycelium front in the compost or casing layer because they are attracted by the scent of actively growing mycelium. If these flies originate from older crops, they may also carry pests and pathogens, posing a risk to new crops. Increased numbers of invading flies may be detected shortly after casing and can indicate how effective exclusion efforts are. For example, spikes in invader numbers may occur if a door is left open. While the exact number of invaders is often outside the grower's control, these counts help growers anticipate pressure and prepare interventions. Large numbers of invaders typically lead to higher risk of future emergers.

Emergers are the next generation of flies that develop from the eggs laid by invaders in the compost. After completing their larval and pupal development, they typically appear as adults around days 21-26 days after invasion depending on temperature. These flies exit the room to mate outdoors and later return to other grow rooms as invaders, continuing the infestation cycle and spreading disease to younger crops. Monitoring emergers helps determine whether a management intervention within the room was successful and provides growers with early warning that new invaders may soon appear in younger rooms (Figure 3). Distinguishing between these two stages is critical because each represents a different point in the infestation cycle and requires different management strategies (Gill & Allan 2025).

Graphic showing the phorid fly infestation cycle (adult, egg, larvae, pupae, adult) — Figure 3: Phorid fly infestation cycle a) Phorid Adults form mating swarms at dusk and dawn along the bottom of the wall or more often along the roofline outside from rooms b) Gravid female invaders, attracted by the odor of actively growing mushroom mycelium, enter the grow room and lay up to 50 eggs close to the colonizing mycelium c) Larvae develop from the eggs, which directly feed on the mycelium impacting yield d)The larvae pupate and mature into adult flies which emerge from the compost e) Emergers then leave the room to the congregate outside in a mating swarm to continue the infestation cycle. Image Warwick Gill

Effective phorid fly management, therefore, focuses on two complementary approaches: reducing the number of invaders entering the room and suppressing the development of emergers within the compost. Preventing invaders from laying eggs can be achieved by treating outside mating swarms with knockdown sprays, applying an insecticidal fog immediately after doors are closed following filling and casing, and reinforcing exclusion measures within the growing rooms to reduce the number of adults entering the facility (Gill & Allan 2024). At the same time, suppressing emergers helps reduce the number of flies that will later become invaders.

Because rooms in close proximity can experience drastically different fly pressure, raw fly counts alone can be misleading. Monitoring programs typically rely on sticky traps or electronic zappers placed in the room immediately after doors are closed, following filling and casing. Flies caught during the first few days are generally considered invaders that have survived exclusion and knockdown measures, while flies caught after approximately 18 days represent emergers that developed from eggs laid within the compost (Figure 3). To standardize the data, growers should calculate the emerger-to-invader ratio by dividing the number of emergers by the number of initial invaders caught in the room. This ratio acts as a proxy for larval survival within the room.

How can we use the Emerger:Invader ratio to make decisions?

Consider the following example:

A grower is trying a new IPM technique in a growing room. The grower pairs one room with standard management with a room using the new technique.The flies were monitored properly, and the grower produced the following table:

Room Type	Invading Flies	Emerging Flies	Total Flies
Standard Management	500	25,000	25,500
New IPM Technique	1,000	35,000	36,000

At first glance it may appear that the new IPM technique did not work. There were more emerging flies and total flies in the room with the new IPM technique. The number of invading flies is critical to consider when evaluating a new IPM technique because this number can change so much between rooms. In this example, the Standard Management room had half of the invading flies than the room with the New IPM Technique. We must consider the Emerger:Invader ratio to understand the effect of the new technique:

Standard Management:

25,000 emerging flies/500 invading flies = 50 emerger:invader ratio

With the New IPM Technique:

35,000 emerging flies/1,000 invading flies = 35 emerger:invader ratio

What does this mean?

Under standard management, each invading fly produced 50 emerging flies in the next generation. With the new IPM technique each invading fly produced 35 emerging flies. Therefore, the new IPM technique reduced the ability of the invading flies to produce the next generation of adults by 30%. This 30% reduction can have an impactful effect on your farm and is evidence that the IPM technique was successful. Of course, these comparisons need to be replicated in many room pairs to have confidence in the numbers.

When should I start counting flies?

A fly count should be performed starting from the day a house is filled through the entire crop cycle, counting the flies at least once per week depending on fly pressure. Ideally flies should be counted every other day to maximize the amount of information for the grower to make decisions with.Shatterproof lights should be used to keep fly lights in a growing room through harvest. Workers should not enter houses if conditions are unsafe.

How often should I perform a fly count?

This depends on fly pressure. The paper needs to be changed when it is saturated with flies, or if it is no longer sticky. When populations are low, weekly counts in each fly room may be enough to monitor potential outbreaks. Some farms count flies every day while others count done every few days. At a minimum, accurate fly counts during spawn run and the first few days of case hold are recommended because these invading flies most influence the number of emerging flies in later stages of the crop. These accurate fly counts allow the grower to act quickly to kill the flies before they emerge from the compost.The more accurately flies are monitored the better the grower sets themselves up for success.