Fungus gnat followup; help, I'm losing my mind!
January 22, 2020 7:37 AM   Subscribe

I cannot find the source of our fungus gnat problem and I cannot get rid of the fungus gnats. I'm losing my mind, catching dozens of them every week, and I just want the problem to go away. Help, please!

My post from late December is here.

Here's the current situation...

Several months ago I started noticing what I first thought were fruit flies. Some investigation revealed they were fungus gnats. I called our pest control service out and the guy put some stuff down into our drains and recommended using some mosquito control pellets in our houseplants (as did several Mefi members).

After a week or two and no change in the amount of gnats, I purchased those yellow sticky traps and put them in our house plants as well as on multiple windows in our house. What I observed was that the ones that caught by far the greatest numbers of gnats were traps on the windows of the first floor of our house. The traps in house plants caught some but not the dozens the window traps caught. Here's an example of the amount above our kitchen sink, in the back of our house. These were up for about 5 or 6 days.

The trap at a front window has even more than this. The traps on house plants have at least half of this amount, and 2nd floor windows have less than a dozen. A few additional details: I've observed a fair number of these gnats dead against the exterior of our house (against white paint). I disposed of a bunch of potted plants in our front porch area, a few weeks ago. The temperature has been up and down but is at it's lowest this week, by far, since the problem started. I'm in Atlanta and it's in the 20s and 30s right now.

Are they most likely coming from outside? Is it an issue with window weather sealing? Would doing a month or two of exterior mosquito control possibly help the problem?

Any and all opinions, thoughts or ideas are welcome. Thank you!
posted by rbf1138 to Home & Garden (8 answers total) 3 users marked this as a favorite
One time I was having a crazy bad gnat problem and eventually discovered that the drip tray under the fridge had a puddle of murky water that had become a breeding ground. Perhaps check under appliances?
posted by DSime at 7:49 AM on January 22

I had a these come into the house last fall when I brought in my houseplants. This happens every year and usually the yellow strips do the job in a matter of days or a few weeks. This year, that was not the case. So I went to the garden store and got a bottle of Thuricide (brand) which is Bacillus Thuringhensis (BT) concentrate, and a good spray bottle. Basically, BT it is a natural soil bacterium, not a chemical spray.

You mix a few tablespoons into a quart of water, and spray it on the soil in every single houseplant. Put extra on the ones that have the most gnats on the yellow strip. Do make sure there's a strip in every plant. I had to reapply about four more times, every week or two, but ultimately this was effective and eliminated the little buggers.

As DSime said, do look for possible sources other than plants. Besides the fridge drip tray, you could have some rotten wood that might harbor them. But mainly they grow in soil.
posted by beagle at 8:02 AM on January 22 [2 favorites]

This systemic in your houseplants will kill them in your houseplants. And then if you still have them- you'll know they're not coming from the houseplants.
posted by Homo neanderthalensis at 8:19 AM on January 22 [2 favorites]

We had that problem when we had mold within a wall - I don't know what to do about that other maybe getting some moisture meters and sticking them into the drywall. It was true, in the end, that they were most concentrated near the actual problem. Is there a sink or plumbing near where you see them most?

You can set little traps with apple cider vinegar + one drop dish soap mixed in a cup covered with saran wrap except for a small hole. Then you can get a sense of the highest density.
posted by lab.beetle at 9:07 AM on January 22 [1 favorite]

I had the same issue. I solved it by plugging the drains when they weren't in use, so that the flies (who likely do come in from the outside, originally) have no opportunity to breed down there.
posted by SPrintF at 4:54 PM on January 22 [1 favorite]

Another vote for BTi. It's a bacteria that is harmless to everyone except fungus gnat larvae. I used a product called gnatrol. As others have said this presumes it's a problem with your houseplants. Takes about 2 weeks.
posted by rouftop at 8:09 PM on January 22

(Hey all, the mosquito pellets she’s using are the same thing as BT.)

How long has it been? I’d give it another few weeks. It’s crazy making, isn’t it?
posted by bluedaisy at 6:34 AM on January 23

Is there any standing water in your yard? They thrive in moist soil, which I'm guessing is ... your entire yard this time of year.

I think the life cycle runs about three weeks, so it can take that long to catch all the living adults and stop the cycle.
posted by bluedaisy at 7:57 AM on January 23

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