Issue: December 3, 2005

Fungus gnats again

Question:

I have zillions of small gnats infesting six pots of indoor plants. The gnats don't seem to be hurting the plants or anything else, but they are mainly just annoying and seem to be proliferating at an alarming rate.

Becki H.
Albuquerque, NM

Answer:

When we begin heating our homes in winter, we often notice insects that have come indoors on our houseplants or perhaps have been living on our indoor plants unnoticed.

Without seeing your little visitors, I think you have what are called fungus gnats. Their juvenile (larval) stage lives in the potting soil and feed on fungus and plant roots (doing minimal damage). As adults, they emerge and fly around inside, especially near lights, in front of the television/computer, near windows, and sometimes into your food. They are not harmful but are very annoying.

Treatment options include reduced watering of the houseplants. If the soil dries, the gnat larvae may die. Most houseplants can tolerate some drying of at least the upper layers of soil. There is a product called "Gnatrol" (TM) that can be applied to the soil as a drench treatment. This is a biological toxin (produced by bacteria) and considered organic. Be sure to read, understand, and follow the label directions. This product is very safe when used according to the directions.

You can also employ trapping techniques using yellow sticky traps. These may be purchased, or you can make them from yellow surveyors tape or yellow plastic butter tubs, etc., coated with vegetable oil, Vaseline, or other sticky material. Put these traps in a window or other well-lighted location. The adult gnats are attracted to the yellow color and get stuck on the trap. This removes them from the home environment and reduces their ability to reproduce. (They die on the trap.) After you catch a lot of gnats, just discard the whole trap or wipe the insects off and reapply the sticky material mentioned above, and you are ready to catch more.

In my experience, these insects often appear after I change to a new potting soil. Some potting soils seem to have the eggs and larvae of the gnats already present. Putting the moist potting soil into a sealed bag in a sunny window for a few days may allow sufficient heating to kill most of the insect eggs. You can also pasteurize the soil by heating moist soil in the oven to a temperature of 140 degrees F. If possible, use a portable oven on a porch or in the garage to avoid unpleasant odors in your home. After the soil cools, keep the potting soil sealed to prevent reinfestation by insects until you need it to repot plants.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!