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dajonx 09-29-2011 01:23 PM

Gnat Invasion

I bought a new home about a year ago and ever since I moved in, I have had a gnat problem around this time of the year. I live about 3 miles away from a mushroom farm, but I don't think they could travel that far just to annoy me, right? And I don't have any plants inside the house. I have seen them all around the upstairs backyard windows (I haven't seen any on the downstair windows!) and somehow they're getting through.

I've been using two FlyWeb Fly Trap and this is how many I've caught in about an hour. (Ugh, I don't know how to post a pic... I can send you the picture if you want to see the gnat covered trap.)

Anyways, I've sprayed Bifen XTS on the outside (I hope I mixed it accurately...) and now I'm starting to tape up the windows with Window Weather Strips. I still catch those pesky gnats in the Fly Trap...

Can someone please help me eliminate this problem?

Any help is greatly appreciated!!


dajonx 09-29-2011 02:49 PM

PAbugman 09-29-2011 05:41 PM

Though I canít enlarge your photos, which are good photos, they look like fungus gnats to me. Compare them to photos on line and see what you think.
That said, you donít have plants and overwatered plants are the usual source. When you see them at upstairs backyard window, are they inside/outside or both? They are not usually found outdoors in any quantity. The mushroom farm could be a culprit, but not likely. Is your house siding yellow by any chance? They are attracted strongly to yellow which you will read on-line. Bifen is an excellent product for many uses, but we need to identify these guys so we can learn their biology and habits. That will lead us in the proper direction. Usually small-fly problems are solved by removing/eliminating their breeding source rather than with chemical means.

They could also be phorid flies; or sphaerocerids (spelling?). Save that glue trap and take it to a county agent/extension office/or local college/university entomologist for proper ID if internet does not help. Keep us posted with info as you get it.

Was your house new construction when you bought it. There are small flies that can emerge from new construction materials for a year or so after a house is built. Doesnít happen often. In this case, drying out the inside with a/c and time will solve the problem, but lets get a good ID first-that will put us in the correct direction.

dajonx 09-29-2011 06:54 PM

I would be more than happy to send that photo along with more photos of the gnats that I captured on the glue trap. If you can private message me your email address, I can send you the actual photos for your review.

The siding of my house is sort of yellow. I guess more like cream-ish yellow? I can take a look tomorrow morning and get back to you.

This house is new construction. I have been using A/C quite regularly and this gnat problem started last year when I moved in.

I forgot to mention that someone from the Department of Environmental Protection captured some specimens last year. This is what he found:

"I looked over the gnat specimens I collected from your home. Most of the specimens I collected were live flies that were running around on the outside of the house.

Of the 48 flies that I collected while I was there, 41 of them were scuttle flies from the Family Phoridae. Three of the flies were fungus gnats from the Family Sciaridae. Three were midges from the Family Ceratopogonidae and one was a small fly from the Family Psychodidae.

The one thing that Phorids, Sciarids, and Psychodids all have in common is that their larvae grow in moist decaying matter. This can vary by species, but most of the species in these families lay their eggs in moist decaying matter. Unfortunately, I do not have resources available that would allow me to definitively identify any of the specimens down the the species level. Since there are hundreds of species in these families of flies (many are poorly documented or understood), it is difficult to say with complete certainty where they may be coming from. Phorid flies in a home are often a problem traced to breeding in a sewer system (where adults then infiltrate a home through drain pipes) or in terraria (they can become a problem for people who keep amphibians and reptiles).

Since the various flies were so prevalent outside the home, I would tend to lean toward some other source, away from your home, providing a breeding ground for the flies. You had mentioned mulch in the neighborhood as a possible concern. While this is possible, I'm not sure the mulch would have stayed moist enough for the past several months to sustain such a seemingly significant nuisance problem. Since the development is still under construction, it is possible that there is some sort of pile of ground up organics (i.e. trees and shrubs removed during construction) somewhere nearby that is contributing to the issue. The other possibility is that there is an issue at one of the nearby mushroom houses that caused this problem. While I cannot conclusively say that's the source of your issue, I did contact the Chester Co Conservation District (CCCD - The CCCD has staff that work with the agriculture community (including mushroom farms) on best management practices. I made the appropriate staff person (Adam Mowery) at CCCD aware of the issue at your development and, hopefully, he will make some visits to mushroom farms nearby in an effort to see if there are any operations that may be having an issue that can be resolved. I cannot promise that CCCD will be able to make any visits in the near future nor can I promise that those visits would defintely end your problems.

Fortunately for you, the cold weather should put an end to your problem provided the source of the infestations is actually from somewhere outside the home.

If your problem persists, I would recommend you contact a pest control company. It may be helpful to work with a company that has an entomologist on staff, although this is not necessary, especially if the company has much experience dealing with Phorid flies. You can always ask for references.

As I discussed while I was there, regular pesticide applications might be no more desirable than the hundreds of little fly bodies littering your windowsills. One thing you can try is to hang some fly paper traps near the window to collect the flies as they try to escape the home via the window. While the fly paper may be unsightly, it's a non-toxic alternative that can be removed if guests are coming over."

What do you think?

Thank you for helping!!

dajonx 09-30-2011 09:03 AM

Here are additional pictures so you can have an idea of what my backyard looks like. Perhaps there's something that I'm missing?

Color of the siding:

Back of the townhouse:


PAbugman 09-30-2011 03:49 PM

Looks like you got a good ID on the flies. Since they are invading your house from the outside, there is not much you personally can do without having control over the location of their breeding source. These guys all breed in moist, decaying organic matter such as a mushroom farm, malfunctioning septic systems, nearby municipal sewer treatment, etc. Continue trying to find the source and bring pressure to bear on them using the people that you are working with. Treating the exterior of siding with Bifenthrin will kill a lot of flies, but it may not be a noticeable reduction.

bsteimel 12-01-2014 10:08 AM


I seem to be also having a problem with these gnats. Do you still live in Kennett Square? Did you ever find a solution? I'm in the process of replacing the weather stripping in the house. I did spray some simply pesticide but I'm not surprised it didn't really work. I really use it for ants and other bugs like that. I tried apple cider vinegar that I've used for fruit flies and they were not attracted to it. That was my first inclination that these were not the usual fruit flies. I was hoping they would be gone when we got some colder weather but anytime it gets above freezing they are right back. I live by the High School in close proximity to 2 Mushroom farms.

P.S. Sorry to revive an old thread but I can't send Private messages yet

dajonx 12-01-2014 10:17 AM

Hi Bsteimel,

Knock on wood, I have not had any gnat problems in the past two years and I do still live in Kennett Square. I know the homeowner's association used a different landscaper which makes me think that the gnats must've came from the mulch they were using. (Perhaps gnat eggs in the mulch which hatched later?) If we do notice any gnats flying around, we use a sticky trap that helps a LOT. This is what we use:

You can also get refill packs:

I hope that helps! Let me know if you have any other questions.

Gustavas 12-01-2014 11:42 AM

once they get in they can breed pretty quick. I would make sure to empty your trash regularly, clean the kitchen really well, and make sure you don't leave any water for them to live on.

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