Megaselia scalaris

 What can you tell me about phorid flies? We’ve had a horrible problem with these flies in our house, by the hundreds. We finally had them identified and were told they could be from our drains. Is that right? T.W., Uxbridge, MA

Phorid flies belong to a group of small flies that I like to call gunk gnats because they are all associated with decaying organic matter in a variety of sites. Two other related gunk gnats are drain flies and fruit flies. Phorid flies are also called “humpbacked flies” because, in side view, their head is bent down.

If It’s Decaying, They Will Eat It!

Around homes, tiny phorid fly larvae can be found feeding on all kinds of gross stuff: dead animals, rotting fruit or vegetables, dead insects, and animal feces including human sewage. We look for them in clogged drains and dirty garbage cans, too. Out in the real world, phorid flies even feed on human wounds in hospitals and on corpses, which is why morticians call them “coffin flies.” “Mushroom flies” is yet another nickname because they infest the compost of commercial mushroom growers.

Sewer Breaks Can Be a Source of Phorid Flies

Phorid flies can be associated with scummy drains just like drain flies, but when we see phorid flies in large numbers like you mention we always have to consider a sewer line break (see Phorid Flies and Sewer Breaks). A break or leak in the line allows fly larvae to feed on sewage that accumulates under the slab. This possibility is considered only after multiple pesticide treatments have provided only temporary relief. The flies in the home will be killed, but more will shortly emerge from under the slab and will find their way up into the living space. A sewer line break can provide a steady supply of phorid flies as long as the sewage keeps flowing.

You definitely need a professional, and maybe two, to inspect your home. If you’re lucky, an exterminator can find a much simpler and cheaper source of the flies (although it may be a secondary source). If a sewer break is suspected, however, you may need the services of a plumber who can check the lines (see Phorid Flies – How to Find a Sewer Line Break), and possibly a contractor who can make the repairs and haul away sewage-contaminated materials.

Photo by Charles Schurch Lewallen (bugguiude.net) [CC BY 3.0], via Wikimedia Commons