100s of Small Flies Problem in Home Solved

Phorid Flies Identified

Phorid fly under a microscope.
Phorid fly under a microscope. photo by Andrea S

A reader had hundreds of tiny black flies swarming the kitchen and making their way to other areas of the house. They look like fruit flies, but they don’t have red eyes and are not even remotely interested in a bowl of vinegar – one of the most common fruit fly attractants. After reading through numerous threads, the reader sent a photo and a plea:

Reader: "I have hundreds of these coming into my kitchen and don’t know what they are, so I’m finding them impossible to deal with! No one seems to know – they are black and about 2-3mm long.  Their wings overlap completely when they are not flying and they only seem interested in resting on windows and light sources.  I have put out a bowl of vinegar but they are not remotely interested in it so I assume they are not black fruit flies (although they look just like them!)  They crawl into cracks in my window and door seals and then reappear.  My electric fly killer light is catching lots but I need to know what else to do!
I have spent a long time googling and found many threads that discuss the same flies but no one has come up with the name. They are definitely not drain flies, nor brown like fruit flies. Can you help?"

Common Small Flies [fruit fly, drain fly, phorid fly]

There are three common small flies that can be a problem in homes: fruit flies, drain flies, and phorid flies. From the information and photo that was sent, my inclination was to identify it as a phorid fly. But because that fly has a very distinctive physical attribute and behaviors, I wanted to ask a couple more questions before making a true identification. So my first response was:

"I’m thinking that you may have phorid flies – particularly if you have ruled out fruit flies and drain flies. I know they are very small to try to see any detail, but when you look at it at rest,

  • Does it look as though it has a bit of a humped back? and …
  • Does it move very quickly and rather erratically?

"If so … this is one of the most difficult of the small flies to eliminate, as the source can be broken or leaky pipes, even beneath the surface. Phorid flies feed on and breed in putrid sources – even if you’re not noticing there is one. This would be why your fruit fly treatments aren’t working."

While communicating with this reader, I had also forwarded the information and my assessment to an entomological expert and friend, and forwarded his response along with mine: "From the description of the infestation, I recommend that the sewer line from the house to the street connection be inspected. My thought is that there is a disconnect or break somewhere in the system and these flies are likely infesting that area and getting into the house. Another possibility is that there is something very rotten somewhere, and considering the species that I think it is (Dohrniphora cornuta), it is likely inside the house. So, there are two possibilities: a break or gap in the sewer line, or something decaying indoors."

Humpbacked Fly Identified

So the reader took a closer look and responded: "I looked at some of the flies and yes, they do have a very small hump on their back. I'd never have noticed if you hadn't asked. Having googled them straightaway, the behavior is spot on! They do walk around a lot, and I told my husband this morning that I had stepped on a couple which were just walking on the floor!"

She also added, "We have had a spate of dead rats - probably 3, judging by the period of decomposition smell - and have been unable to find the corpses or entry point, but they were somewhere behind our kitchen units. Do you think this might have been the sole breeding/feeding source or is it probably drains too? These flies have been coming from behind our kickboards from the same place where the rotting smell was strongest."

Decaying Rats Attract Flies

With that added information, the identification of the flies was positively confirmed, and the reader was informed that that dead, decaying rats very likely were the sole source. These flies do prefer decaying flesh/dead animals over organic matter. In fact (as I just learned myself), they are sometimes called coffin flies because some of the species tend to be pests at above- and below-ground grave sites.

This fly has a broad range of habits...including feeding and breeding on decaying animals.

The information on the decaying rats also sparked an additional side note: "When attempting to control or eliminate rats or mice in the home (or any building), it is best to use baited rodent traps instead of poison bait, so that the rodents are captured and can be disposed of. Otherwise, the rats or mice will take the bait their nest in the wall or other hidden area and die there. Their decaying bodies will then attract secondary pests, such as the phorid flies, as well as carpet beetles, mites, and other flies.

Eliminating the Small Phorid Flies

After all our communication, it was great to receive a note from the reader that the problem was, indeed, due to the decaying rats, and the problem was now solved:

We assume that the rat carcass has decomposed and the breeding has therefore ceased - but we have literally vacuumed up and caught in the electric fly trap over a thousand of them. I truly cannot tell you how grateful I am for your contact and identification. I was getting desperate with not knowing what I was dealing with.

Identifying Phorid Flies

If you have small flies that you think may be phorid flies, look for these common signs:

  • A humped back on the fly.
  • Erratic, skittering movement; while running as well as flying.
  • A putrid source: decaying animal or organic matter in an open or hidden area. Drains are a very common source.

Then follow the recommendations above. If you can’t find the source, it could very well be a break in the pipe, beneath the structure or further out beneath the ground. In this case, you will need to consult a professional to both positively identify the fly and to inspect the pipe and other potential sources.