Jeffrey Hahn, Assistant Extension Entomologist
Moth flies can be present anywhere in a home, especially in bathrooms, basements, and kitchens. These flies lay their eggs in moist, organic matter where the larvae, small, slender, legless insects, feed on decaying organic matter, fungi, algae, and similar material. They are commonly found associated with the gelatinous film found in sinks, shower and bathtub drains, and similar places. Moths flies can also be associated with sewage from sewer line breaks. Moth flies are primarily a nuisance because of their presence. They don't bite people but they can potentially be a mechanical vector of disease because of their association with filth.
The best control of moth flies is to remove the source of the infestation. You can not eliminate a problem by just spraying the adults that are out in the open, First check drains and basins for the presence of an infestation. If you are not sure, place some tape over the openings (sticky side down); flies will get stuck on the tape as they try to fly out. If you suspect a sewer line break under a floor or slab, it may be necessary to break through the floor or concrete to verify this.
If you are dealing with a drain, you need to remove the gelatinous gunk that has accumulated. You can do that by taking a brush with stiff bristles and physically removing it. Another effective option is to use biological drain cleaner which breaks down and removes he organic material. However the use of hot/boiling water, bleach, and chemical drain cleaners is not effective. Attempts to try to drown the larvae is difficult and is unlikely to be successful. If you are dealing with sewage from a broken pipe, it is critical to fix the break and remove the sewage and any contaminated soil that is present.