Pest (some sort of fly?) id please

Hi. Any idea what this is? We've been seeing them in our newly re-finished basement, but had never seen them in the past (in the 8 years we lived here).

They are less than 1/4" (smaller than a traditional house fly). We mostly find them dead on the floor, this one happened to be on the wall (not moving). We hardly ever see them actually flying around.

We're in Southern Connecticut.

Thanks for any thoughts.

 

If it looks like a tiny humpback house fly it might be a pforid fly. They breed in your sewer system so can come out of floor drains or sink traps.

 

Search "psychoda" and see what you think. Commonly called sewer flies, drain flies, etc. They like decaying organic matter, slow moving drains just as phorid flies do.

Wherever their density is the greatest, you will be close to the source. In a basement, my first suspicion would be a sewer pipe crack where it exits the basement wall, either just inside or just outside the wall. Another culprit would be a sump pump if present, especially units that don't often run as they could have stagnant water in them. Also check air conditioning condensate pump and eventual drain.

 

Thanks so much and that definitely could be what it is (a drain fly...).

That never even crossed my mind or I would have mentioned in original post that we did have a bathroom installed (sink and upflow toilet). I haven't ever actually seen a fly in the bathroom, or the utility room behind it where most of the plumbing is, but I'll keep a closer eye out. I also don't see any leaks or water anywhere though.

All that being said, you mentioned a condensate pump and we do have one of those for our old Sante Fe dehumidifier which hasn't been working so we haven't been using it. Therefore there is likely stagnant water in there (or even in the Santa Fe itself?)...

 

Yep, the dehumidifier tank could be a source. You've got the idea now, as far as harborage to consider.

Make sure all toilet connections are tight and sealed. In a traditional toilet installation, sometimes the wax ring will shrink, pull away or the toilet may not be tight enough, so see if any of this applies to your installation. I'm not real familiar with how upflow toilets are assembled.

Make sure the sink has water in the p trap. In bathrooms that aren't used much, the p trap can dry out and will be an open connection to the sewer/septic system. In such cases, a little mineral oil on top of the water in p trap will slow evaporation. Good trick to remember for families that travel for long periods of time.

If possible, close all doors and isolate rooms as best you can as that can help you locate the room(s) that the problems are coming from.

 

Thanks so much for the follow-up/detailed info/suggestions.

In fact, after we returned from a 5 day vacation, the flies were everywhere... So maybe it is the p-trap drying out as you suggested.

That being said, and I will google this as well, but how do I make sure it has water in it? Do you just mean to make sure to use the sink periodically, or do you mean I should somehow actually check to make sure water is properly accumulating in it?

Thank you again.

 

Good question as to checking water in P trap. Hoping that members with plumbing knowledge will chime in.

Another thing to check is where the sewage/septic pipe leaves the basement. Sometimes a crack just outside the foundation wall will cause the flies, too. If the basement wall is finished where the pipe exits, then a damp wall won't be obvious. I've also seen the top of the older style cast iron pipes crack, but no leakage it's a drain pipe, not under pressure, but the odor and flies were escaping through the crack

 

Drains that don't get used can eventually evaporate. This most commonly happens in a basement floor drain, sink that doesn't get used or toilet that doesn't get flushed. You can just pour a bucket of water down a floor drain periodically to prevent that from happening. Then as mentioned, a little vegetable oil hinders evaporation.

You don't need to "check" anything. A p trap only holds about 4 oz of water... so running the water is all that is needed to refill it.

 

Traps can dry out and seldom used traps can be a breeding ground for insects. You can pour a small amount of vegatable oil down the drain of rarely used fixtures. The oil will float on top of the water slowing evaporation and will stop most insects from breeding or drinking the water. If you use that fixture it will wash the oil away so you will need to repeat if it gets used.

 

Great, thanks again everybody!

 

I've heard that in some situations the water in the P trap can be sucked out by improper plumbing. I was thinking that the upflow toilet, being a non-traditional toilet, may create more force/suction than a traditional toilet and possibly suck out the P trap water. Be alert to that possibility.

Another thought; lightly coat a drinking glass with vegetable oil or similar and invert over the sink drain. If the flies are coming from the drain, they should get stuck inside the inverted glass.

 

S traps are more likely to be siphoned which is why they were phased out. P traps have a minimum trap weir length that is supposed to break the siphon effect.