Thursday, March 13, 2014

Fungus gnats

By Irene Shonle, Gilpin Extension

As I was reading in bed the other day, a tiny black insect kept buzzing around my ears and trying to fly up my nose until I finally became fed up and dispatched it.   Darn those fungus gnats!  They seem to be having spring population boom right now.  Judging from the number of people who have been calling me or commenting on the situation at their house, I’m not alone.   The only thing I can say in their favor is that the flying adults don’t bite, so their annoying qualities only extend to the fact that they like to invade your personal space (and that they mate and give rise to the larvae – that is the life stage that can cause problems).

The first step in figuring out how to solve the issue is to properly diagnose it.  It can be hard to tell the difference between fruit flies and fungus gnats, but since they usually come from different sources (unless you’re worm composting), it’s important to sort this out.  First, note the season. In summer, fruit flies are much more common, but in winter, the fungus gnats usually rule supreme. Fruit flies are relatively large and stocky, with large (often bright red) eyes and a light brown body.  Fungus gnats, in contrast, are smaller and more delicate, resembling a tiny mosquito, and are black.   See the photo below for a photo with both species.
Fungus gnat (left of middle pair) next to fruit fly on a yellow sticky trap

Adult fruit flies hatch from eggs laid in overripe fruit or other fermenting produce, whereas fungus gnats mostly hatch from eggs laid in houseplant growing media.  If you are mostly seeing the critters in bedrooms and areas where there are plants, they are almost certainly fungus gnats. (Go here for more on fruit flies and other household flies)

While the adult fungus gnats are annoying, the larvae are minor to major pests on houseplants. They primarily feed on fungi, algae and decaying plant matter but they will also feed on plant roots.  In high enough numbers, the larvae can stunt the growth of the plants.

Here’s how to deal with them:
First, let plants dry out between watering (just before the point of wilt).  Fungus gnats are very attracted to moist media, and the larvae only live in the top 2-3 inches of soil. Reducing watering reduces the survival of the eggs and larvae.  In the cooler, shorter days of winter, it is especially easy to overwater plants, but most plants will actually be happier and healthier when not overwatered.

Second, repot your plant with fresh growing medium – as it breaks down, the potting soil tends to hold more moisture, which promotes fungus gnat development.  As a bonus, your plant will probably perk up, too

Third, put up some yellow sticky traps around your plants to trap the adults.  Reducing the adult population will reduce the numbers of eggs laid, and can break the cycle.

Finally, if you’re still going crazy, try insecticides. Often, a Bt (Bacillus thuringensis) soil drench will suffice to kill the larvae, but is non-toxic to humans.  Otherwise, you may need to use pyrethroid-based insecticides, with extended persistence for use on houseplants (containing the following active ingredients: bifenthrin, cyfluthrin, permethrin, and lambdacyhalothrin).  Short-persisting contact insecticides such as those containing pyrethrins, soaps, oils, and neem, do not provide sufficient long-term control of fungus gnat adults and require repeat applications at short intervals (couple of days) to exhibit effects.

For more information, go to the following fact sheet:


  1. "The most commonly used strain of Bt (kurstaki strain) will kill only leaf- and needle-feeding caterpillars. In the past decade, Bt strains have been developed that control certain types of fly larvae (israelensis strain, or Bti). These are widely used against larvae of mosquitoes, black flies and fungus gnats." --see fact sheet no. 5.556

    1. Thanks -- I should have pointed out that it is necessary to use the specific strains of Bt -- look for a strain that mentions that it controls fungus gnats on the label; some even have it in their name.

  2. I have to say that I'm a bit relieved that I'm not the only hortie who saw clouds of the buggers this winter. I keep all my houseplants on the verge of death, but I was given an amaryllis bulb as a gift for Christmas. I guess I was a tad heavy handed and kept it too wet, because we had an incredible population of fungus gnats! Fortunately, the bulb is now in bloom and I've dried down the media. Great blog, Irene!