Fungus Gnats

Fungus gnats are a common pest of indoor plants. They generally feed on decaying matter in soil, but while in the larval stage, they also often feed on plant roots and sometimes even soft stems, which can especially damage seedlings. Even if they are not damaging plants, the adults, which resemble fruit flies or small mosquitoes, can be quite a nuisance as they swarm around your plants and your house!

Fungus gnat larvae are small, whitish maggots with black heads that are about half a centimetre at their largest. If you poke around in your potting soil with a dibber or a pencil, you can often see them if they are present. The adults are not strong fliers, and they are often found on top of soil or resting on plant stems and leaves. Fungus gnats can complete an entire life cycle within three weeks, so a small population can quickly explode. On the plus side, they are short-lived, so eradication, if an effective method is found, should not take very long.

Fungus gnat larvae on a dime
Fungus gnats can be prevented, controlled and eradicated. To prevent them, sterilize potting soil (especially if it has been stored outdoors) and isolate newly purchased plants and houseplants that are brought indoors after a summer outside until you are certain they are not infested (at least three weeks). To control them, yellow sticky paper can be used to trap some adults, which are attracted to the colour yellow. These traps can also alert you to their presence before they become too numerous. Plants should also never be overwatered; these flies prefer to live and reproduce in damp soil. A thin layer of sand over the top of the soil can prevent the flies from laying eggs, as it dries out quickly. To completely eradicate fungus gnats, there are several pesticides available. These must be applied several times to kill all stages of the life cycle. If you prefer not to use pesticides, you may wish to either discard affected plants or change the soil, then cover them to prevent the flies from laying new eggs. If you choose this method, be sure there are no other plants nearby that they could choose as a new home.

3 comments:

  1. Thinking of adding some new houseplants to our home. Thanks for the heads up!

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  2. Did you see BugGirl's posting on the Bounce-effect on fungus gnats? It's worth a read:
    http://membracid.wordpress.com/2011/01/23/no-bounce-fabric-softener-will-not-protect-you-from-bugs/

    I've learned to live with my fungus gnats - annoying, but not serious. It's the scale insects and aphids that bug me.

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  3. Dave, I did see the research she cites in her posting. In fact, I tried repelling my fungus gnats with a generic brand dryer sheet that was probably a couple years old. Whatever chemical those Bounce sheets emitted (they think it may have been linalool) that offended the gnats was probably either long gone from my dryer sheets or else it was never there at all. Those little bugs actually had the audacity to land on it! Well, I solved the problem by taking cuttings and using fresh soil, which worked perfectly. It wouldn't be convenient if I'd had more plants there, though.

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