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The Problem with Fungus Gnats

One day your plants are doing well. The next day you see tiny flying insects surrounding your plants. What’s happening?

Fungus gnats are tiny black flies, only 1/16 to 1/8 inch long, that are attracted to damp decomposing organic matter in the soil’s top half-inch. 

In small numbers, these gnats seldom do much harm. But when soil is very moist and organically rich, gnat larvae can become numerous. In time, these gnats start chomping on the tiny roots of your plants. They can also spread plant diseases, such as root rot, and stunt plant growth, especially in seedlings. These gnats do not bite people and are seldom an issue outdoors. 

Life cycle: Even though your potting mix may be marked “sterile,” eggs of the fungus gnat may be present. Or, you may have brought them in with the houseplant you just bought. Or, the pests may have come indoors with your plants that vacationed outside for the summer.  Females lay their tiny eggs in the moist organic matter/peat of your potting mix. The larvae live in the soil for two to three weeks, pupate, mature to adulthood, lay up to 100-300 eggs, and live on for another three to five days.  A new generation then begins. In warm temperatures, the life cycle can be as short as 17 days.

Monitoring: One way to identify if you have gnat larvae is to place one-inch chunks of potato slices with cut sides down on the soil. Check slices for the tiny little worms in three to four days, and replace potato slices frequently if you find them. While this is not an effective way to control fungus gnats, it helps you learn if you have gnat larvae and draws the larvae away from the root zones. 

Management: Before potting plants, you can carefully sterilize soil in an oven, but the time and awful smells are usually not worth the effort. 

Be very careful not to overwater your plants. Let the surface dry for the top inch or so before watering again. Adding a half inch to an inch of fine gravel or coarse sand to the surface discourages larvae and egg laying.

Good sanitation is important. Don’t move infested pots to other growing areas in the home; you do not want to spread the problem. Remove dead and dying plant vegetation from the soil surface as it feeds the larvae. 

Adults can be controlled with yellow sticky traps from the local garden center. The yellow color attracts the flying adults. They land and get caught on the sticky surface of the traps. Lay the traps horizontally next to the plants or vertically if they are on a stake. If you are persistent, have enough traps, and change them often you may greatly reduce the number of fungus gnats. 

Other options to control fungus gnats. There are non-toxic, natural biological controls available to home gardeners. A special strain of bacterium, Bacillus thuringiensis subspecies israelensis (Bti) may be purchased at the local garden center (Mosquito Bits, Gnatrol). The Bti products are convenient to use but must be reapplied as a soil drench every five days for continuing control. Bti kills the larval form. For ease of use, place the BTi in your watering can and use it every time you water.  Chemical insecticides are not recommended for household control. If you use any of these controls, follow package directions very carefully. 

If all else fails, you may need to toss the infested pots to avoid the spread of fungus gnats. While losing a plant is tough, it might help limit the spread of fungus gnats to other nearby plants. 

Happy Gardening, 

 Joe Baltrukonis has been a Master Gardener since 2000. 


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