Moth flies are also called drain flies, sink flies, filter flies, sewer flies and (incorrectly) just “gnats.” Whatever name they are called, they can cause big problems in both residential and commercial accounts.
It’s often said that small flies are an “inside” problem: they are typically coming from inside a structure and are found close to their breeding source. As the name suggests, moth or drain flies are found in and around drains. That’s a good place to start looking. It’s when they are not in those easy-to-inspect drains that they become particularly challenging. Once you have eliminated the drains as the source, there are other places to start inspecting. Branch out and think “outside the drain.”
POTENTIAL HABITATS. Since these flies breed in moist organic material, any damp area that has some type of food buildup is a potential habitat. Similar to phorid flies, these flies also like sewage and can be found around broken pipes, water treatment sites and sewers. This can result from contaminated soil underneath a building or near structures. Drainage and retention ponds can be likely culprits since waste from nearby areas will wash into them.
In commercial and industrial areas, look “upstream.” Where does the water and the washout travel from and pool into? Moth flies often will appear after rain showers, and sometimes fixing drainage systems and making sure that wastewater flows away from the site can be helpful. One study sampled for moth flies downstream of a “discharge of sewage effluent” and found more moth flies than upstream.
HVAC systems are another point of entry. Especially in commercial facilities, these are typically on the roof and overlooked on inspections. Leaves, debris and even dead animals can be present in these areas. Additionally, if the roof isn’t maintained well, water can pool and create that perfect environment for moth flies. Vents can allow them entry where they get sucked into air ducts and blown out in different locations. Leaks in the roof can lead to organic buildup in ceilings and wall voids where the moth flies take advantage of that breeding material.
Even in residential situations, the areas around HVAC units often aren’t sealed well where they enter the house. The units drip water, keeping the soil and debris moist and perfect for moth fly development. And if filters aren’t installed properly, the units will suck in nearby flies.
INSPECTION TIPS. Moth flies are native insects and naturally live outdoors. After even moderate rainfall, the dead leaves, berries and wildlife wastes become a fertile breeding ground around homes and businesses. Since moth flies take around 21 days to develop, typically around seven to 10 days after a rain event, they can start showing up. The adults will get through tiny cracks and crevices, broken screens and vents. Luckily, they are not strong fliers, so where they are being observed is likely close to where their entry point was. Inspecting those areas and asking the customer to seal them can reduce the problem in the future.
It’s not easy finding moth fly habitat areas because water sources are often hidden behind walls, in voids, down in subbasements or in ceilings. A hospital was finding moth flies in its operating rooms. After inspecting all over the operating room and surrounding areas, finally, the electric outlets were removed, and several hundred dead and living drain flies, as well as larvae, were spotted. The wiring duct could be followed into the cellar, where recent flooding had caused water damage and standing water for several days, immersing drain pipes connected to the ducts that were unintentionally interconnected to the electric wiring ducts.
Even though moth flies aren’t strong fliers, it doesn’t mean they can’t travel distances, especially when aided by air currents.
HEALTH RISKS. Moth flies, like larger pest flies, can carry pathogens that are harmful to people. With the hairs all over their body and wings, they can pick up particles of contaminated material and transfer them to other surfaces. So the moth fly that may be breeding in a sewage leak on the outside of a restaurant can gain entry and land on a food contact surface, contaminating the food being prepped and served.
It doesn’t just affect food sites. Hospitals and health-care facilities need to be aware of the risk of moth flies and other small flies. In a study of hospitals in the United Kingdom, they found Bacillus and Staphylococcus, as well as numerous other bacteria.
When they tested some of these, they found that many strains were also resistant to antibiotics. Moth flies aren’t just an annoying pest; they pose a risk to food and human health.
TREATMENT. Treatment is typically finding and cleaning and removing the food source and breeding site. Moth flies don’t make it easy for us to do that. They have fairly quick reaction times, so their behavior is adapted to avoid when liquids are aimed at them. They quickly fly a short distance away when they detect something coming their way. Their body structure is also evolved to avoid what is targeted at them.
A recent study stated that drain flies’ remarkable ability to evade such potentially lethal threats does not stem primarily from an evolved behavioral response, but rather from a unique hair covering with a hierarchical roughness. This covering, previously unexplored, imparts superhydrophobicity against large droplets and pools, as well as antiwetting properties against micron-sized droplets and condensation.
Basically, their little bodies, with all the hairs, act like a force field to keep liquids off them. If the liquids (say a pesticide) don’t contact them, it can’t affect them. This makes targeting larval sources even more important. If we eliminate the food and shelter for the larvae, they can’t develop into adults and continue the life cycle.
FINAL THOUGHTS. When moth flies are in a simple drain, it’s easy: Clean the drain, and eliminate the flies. When the flies are in hidden areas, it gets tricky to track down their origination point. It is vital to talk to customers about the risks of moth flies and other small flies and how they can spread potentially harmful pathogens. Like all pest issues, dealing with moth fly problems is a team effort, and customers and PMPs have to do their part.
Chelle Hartzer is a board certified entomologist at 360 Pest and Food Safety Consulting.