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Pest Control/DRAIN FLIES


Dear Jack,

I live in an apt. building and have a problem with drain flies that seem to congregate by the front door in the two windows on either side of the door.  I have checked and blocked EVERY DRAIN on the first floor including the laundry room and cannot find the source.  I firmly believe they are coming from the apt. across from me because the previous tenant had a problem with them.  I believe they are coming from inside the building from the neighbors apt.  About a week ago I smelled a STRONG odor of bleach in the hall and noticed the drain flies all of a sudden disappeared but they are coming back a little at a time and they look crazy like spinning around and such and I think that they have been poisoned so to speak and are still breeding some kind of "deformed" drain flies because they are really acting crazy.  I know also that bleach alone will not eradicate them.

My question is can these drain flies come from the outside?  There are no drains and only the sewer grates several feet from the building but I have NEVER seen them come in from outside so I'm just asking this question to get an answer and eliminate this possibility.  Thank you for your time I appreciate it.


I think it is too cold in NH at this time of the year for drain flies to survive outside. See for general info about these flies. The most common source of these flies, besides drains, are sewage ponds or sewer line leaks. These sources would be more common in summer.

Jack DeAngelis

Pest Control

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Jack DeAngelis


I can answer questions about the control of pest insects, spiders, mites and related arthropods. These household pests include termites, carpenter ants, powderpost beetles, nuisance ants, bed bugs, cockroaches, fleas, wasps, and many others. I can also answer questions about using pesticides and other pest control tools such as baits and traps.


I am a retired university extension entomologist. I've taught and conducted research in urban and agricultural entomology. I've published over 70 extension publications, 20 research publications and several books about insects.

Ph.D. in Entomology

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