residential phorid fly infestation - Ask Extension
We have been dealing with a phorid fly infestation in our 1940's brick house since 2022. The usual pest extermination company declined to service us a...
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residential phorid fly infestation #925101
Asked February 26, 2026, 2:50 PM EST
We have been dealing with a phorid fly infestation in our 1940's brick house since 2022. The usual pest extermination company declined to service us and suggested we call a plumbing company. We did. The plumbing company scoped all of our pipes, decided the problem lay in our main sewer drain, dug up the old cast iron sewer line under the basement cement floor and replaced it. This did not solve the fly issue. They did a smoke test with negative results. They gave up and left. What we have done is put UV-light fly traps on the 4 walls of the partially-finished basement and keep the basement door closed. This reduces the number of flies in the rest of the house. However, we have not been able to pinpoint a particular direction/wall as the ingress route. We have noticed that when it rains or if the outside temperature is <40F, we don't see many flies at all. We suspect that the source of the flies must be outdoors close to the foundation and that the flies are flying/burrowing in through cracks in the foundation. Does that sound feasible? If so, do you have any advice?
Montgomery County Maryland
Expert Response
Just submitted a question about phorid flies and realized that the photo uploaded needed to have a ruler included. Hence submitting a new photo.
Thank you for your patience as we consult with our entomologist. Until we have her feedback, are you able to share how you determined they are phorid flies? We ask because some other indoor fly pests, like fungus gnats, can easily be confused with phorid flies. (Note that one of the fungus gnat images on the linked page isn't a type usually found indoors. The dark-winged fungus gnat, pictured further down that page, is the typical type.) Magnification of the insects you're seeing, especially the veins in their wings, can be very helpful in separating similar-looking fly families. Unfortunately we can't see enough fine detail in the images you provided to determine which family of fly these belong to. We're checking to see if your county Extension office has a microscope, where they could take closer images for you and upload them.
Fungus gnats and phorid flies can overlap in their habits, in terms of what resources the larvae need to reproduce. Damp, decaying organic matter is required for them to feed, so in addition to the possibility of a pipe leak that you investigated, you can also look for that food source in places like a compost pile located against the house, and any roof downspout pipes that empty close to the foundation. Phorid flies are also sometimes called "coffin flies" due to their ability to decompose dead bodies, and human or animal remains buried near the structure might be supporting them, though presumably only if something was only recently buried before the infestation became noticeable. As you noted, an outdoor source of the fly population may explain why you don't tend to see them in cold weather. Cracks in a foundation, or worn-out door or window weather-stripping or insect screening might allow them entrance, but we think it's unlikely they would seek to enter the building if they are not already breeding somewhere indoors, since indoor conditions generally have lower humidity compared to outside.
Fungus gnats might be breeding in a biofilm layer in slow-draining pipes, but also could be reproducing in houseplant potting soil, if you happen to grow plants under lights (or in a window) in the room(s) where you often find the flies. Is any potting soil stored inside, for example?
Does the home use a septic system, and if so, has it been inspected recently? If present, that might be a breeding site for the flies, which then have found a way inside.
Although our Phorid Fly web page is very short at present, it links to a thorough page by NC State Extension that has more detail about breeding sites for this group of flies, which may help you narrow-down where else to look. We'll also get back to you as soon as we have more feedback from our entomologist.
Miri
Fungus gnats and phorid flies can overlap in their habits, in terms of what resources the larvae need to reproduce. Damp, decaying organic matter is required for them to feed, so in addition to the possibility of a pipe leak that you investigated, you can also look for that food source in places like a compost pile located against the house, and any roof downspout pipes that empty close to the foundation. Phorid flies are also sometimes called "coffin flies" due to their ability to decompose dead bodies, and human or animal remains buried near the structure might be supporting them, though presumably only if something was only recently buried before the infestation became noticeable. As you noted, an outdoor source of the fly population may explain why you don't tend to see them in cold weather. Cracks in a foundation, or worn-out door or window weather-stripping or insect screening might allow them entrance, but we think it's unlikely they would seek to enter the building if they are not already breeding somewhere indoors, since indoor conditions generally have lower humidity compared to outside.
Fungus gnats might be breeding in a biofilm layer in slow-draining pipes, but also could be reproducing in houseplant potting soil, if you happen to grow plants under lights (or in a window) in the room(s) where you often find the flies. Is any potting soil stored inside, for example?
Does the home use a septic system, and if so, has it been inspected recently? If present, that might be a breeding site for the flies, which then have found a way inside.
Although our Phorid Fly web page is very short at present, it links to a thorough page by NC State Extension that has more detail about breeding sites for this group of flies, which may help you narrow-down where else to look. We'll also get back to you as soon as we have more feedback from our entomologist.
Miri
Hi Miri, thanks for getting back to me. Answer to questions/comments:
1. The flies were identified by the pest control company technician. I have not paid attention to the wing vein pattern; however, they do have that rapid zigging and zagging walking behavior on windowpanes which I've read is typical. 2. There is no compost pile against the house. 3. Roof and downspouts were completely replaced last spring - no change in number of flies. 3. I've looked for flies in the houseplants but have never noticed any in or nearby. Sometimes, when one waters a houseplant, little flies may fly out of the soil, I know. I've looked for these and have not seen any such activity. Potting soil is stored outside. I only have orchid potting medium (which is dry bark) indoors. 5. We do not have a septic system. The hookup is to county sewer. We have at one point covered the basement drain with clear tape for several weeks. Never saw any activity there.
My husband and I have just put clear packing tape over the basement cove joints, trying to see if we can pinpoint an ingress point. Maybe that'll give us some new info...
Appreciate your help!
Catherine
Thank you for the additional information. Since the Montgomery County Extension office should have a microscope available for them to assist with IDs, we recommend you bring a sample of the flies (ideally not stuck to the sticky trap cards, if any have died on a windowsill, for example) to their office for help with confirming their ID. Our entomologist agrees that they don't look like phorid flies from what we can see (it looks more like a member of the gnat/midge group of flies), but it really needs microscopic inspection of a few different body parts to be more certain.
You can reach out to the office before bringing samples in case there's a best time to drop by; you can use the page linked above or inquire with the Master Gardener Coordinator or the MG administrative assistant. They can pass along the sample to our entomologist (in a different office) if needed. We might not be able to ID it with great detail, but if we can at least determine what group of flies it belongs to, that can help narrow-down where to look for a breeding site. That said, you've done a lot of the investigative work already, as many flies breed in damp, decaying material or puddled water.
Miri
You can reach out to the office before bringing samples in case there's a best time to drop by; you can use the page linked above or inquire with the Master Gardener Coordinator or the MG administrative assistant. They can pass along the sample to our entomologist (in a different office) if needed. We might not be able to ID it with great detail, but if we can at least determine what group of flies it belongs to, that can help narrow-down where to look for a breeding site. That said, you've done a lot of the investigative work already, as many flies breed in damp, decaying material or puddled water.
Miri
