Fungus Gnats

2010 January 30

On January 10, I caught this tiny gnat when it landed on my computer screen while I was reading my email. I had to freeze it to get it to hold still while I took pictures

This isn’t the first time little gnats landed on my computer screen, they are constantly around the house at a low level and are drawn to lights. Such as a computer screen in an otherwise dimly-lit room.

The source of these flies is pretty obvious – the come from the soil for our potted plants. Back in May, S_ put out some sticky traps on her potted plants, and caught a bunch of them (which didn’t photograph very well. Partly because of the screaming yellow color of the sticky trap making it hard to focus or get the right light intensity, and partly because they were so tiny that focusing was hard in the first place)

I’m inclined to think that these are one of the hundreds of tiny flies known as “fungus gnats”, possibly one of the ones in the family Sciaridae (what I can see of the wing venation looks right, anyway). The one from the computer screen looks like it had a bigger head than the ones on the sticky trap, so it is possible that they are two different (but closely related) species. It is also possible that the males have bigger heads than the females (which is not at all unusual for flies).

As one might guess, fungus gnat larvae eat fungus[1]. They are very common in potted plant soil, since fungus grows well in rich, moist soil like potting mix. At a low level, they are pretty harmless. However, if houseplants are overwatered, the quantity of fungus in the soil will become very high. The large quantity of food will, in turn, make the fungus gnat population explode. At this point, they become a bit of a nuisance around the house. Also, if the gnat larvae become too numerous, they will start eating the root hairs off of some kinds of plants, which obviously isn’t going to be good for the plants.

The countermeasures for fungus gnats are, first, to avoid overwatering your plants in the first place. If the surface layer of the soil becomes dry between waterings, that should be sufficient. If you get a really, really big infestation, they can be pretty much eliminated by applying appropriate pesticides as a soil drench. Probably the best bet is granules of the same Bacillus thuringiensis israelensis that is widely used for mosquito and black fly control. This won’t affect the adults, but it will kill any of the gnat larvae in the soil, and has the added advantage of being pretty much non-toxic to everything except fly larvae.

[1] This is something I’ve written about before: digesting cellulose-rich plant matter is a hard problem, so most animals can’t do it. Digesting fungus, on the other hand, is pretty easy. So, a good way for an animal to digest cellulose is to wait for the fungus to do it first, and then eat the fungus. This essentially moves part of their digestive tracts completely outside of their bodies, meaning that they don’t have to have large fermentation-chamber stomachs or excessively long guts to get their nutrition.

6 Responses
  1. January 30, 2010

    One of the best things I’ve learned from your blog and something I’ve regaled my friends with is how many insects have other living things do most of their digestion for them. This is another great example. The digestive tract of this little guy must be just a few hundred cells long.

  2. January 31, 2010

    Tim, in order to compare and ID these in my own house, I’m wondering about their method of flight: are these gnats really good at hovering and flying really slowly? I used to have a lot of such things when I had orchids, and noticed they were always worse when I overwatered.


  3. January 31, 2010

    Yes, they are pretty good at hovering, and do tend to fly quite slowly. The University of Florida site says ” There is not a similar insect in the production area or interiorscape”, which I take to mean that any time you have tiny flies that are breeding in flowerpot soil, they almost certainly are these. So, I expect that you did, in fact, have fungus gnats.

    Actually, I kind of suspect that practically everybody who keeps houseplants has at least a few fungus gnats.

  4. January 31, 2010

    Thanks for the confirmation.

    They were really annoying – I’m interested in buggy things, but I get almost phobic when they’re flying free anywhere near where I eat or sleep, and at the time I kept my orchids in my kitchen.

    However, I did have a way of getting rid of them – I don’t know if this actually killed them, or (perhaps more likely?) killed their food supply, but what I did when I noticed them was to:

    1) Completely fill/drain the orchid pot (I’d do this and the following steps with the orchid still in the pot).
    2) Dump a bunch of orchid fertilizer directly on the orchid potting material (totaling 1-2 tablespoons’ worth).
    3) Fill up the pot with water, and make sure all the fertilizer was dissolved. Mix it up good.
    4) Let it drain out as normal.

    …then, when it got dry again (every 4-6 days), I’d just water the orchid as normal. For me, after doing that process once (or sometimes twice), the gnats would be gone for at least a couple months. Worked really well, though I do NOT recommend my technique for anyone reading this – that amount of orchid food should be lethal to the plant itself (though my orchids did pretty well. Go figure.)

  5. February 3, 2010

    This is why I love your blog — you can take a tiny, insignificant household pest and make it interesting.

    During the summer we also have good luck getting rid of fungus gnats by just putting the potted plants outside for a while — not really an option when it’s below freezing outside. I’m not sure why it works — maybe the heat just dries out the soil faster. In the winter it’s easy to overwater since a) our house is chilly, leading to less evaporation and b) plants are less active due to shorter days. Letting the soil dry out between waterings usually keeps these guys in check.

  6. Heyokah permalink
    May 30, 2010

    I recently bought a basil plant and got fungus gnats (and some nasty fungus) as well. After doing a bit of research, I found that most ways of dealing with them involve changing the pH of the soil to kill the fungus (and thereby the gnats). Spraying with vinegar (in solution possibly), sprinkling coffee grounds on the soil, etc. Limiting the amount of water to the pot would also help as what you essentially need to do is change the environment that the plants are in – houseplant conditions (constant temp – warm, wet, and not terribly sunny) are great for growing fungus.

    I used a route suggested by orchid growers. Sprinkle cinnamon on the soil before your next watering – apparently the cinnamon is toxic to fungus but not to the plants. Worked for me and I haven’t had any problems since!

    I’ve also noted a salt-and-pepper jumping spider on the window screen next to my plants. I love those little guys and I hope he’s finding enough to eat but I’m glad my pests are gone!

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