Issue: August 17, 2002

Fungus gnats

Every year I seem to get gnats in my houseplants and they are driving me crazy. I have tried many "recipes" to get rid of them but to no avail. Do you have suggestions? -R. Brady

You probably have fungus gnats inhabiting the soil of your potted plants. The larvae may feed on the roots of potted plants, stunting the growth of the plants. It is the adult gnat that you are probably seeing. These small insects are a nuisance but do not do as much damage as their larvae.

Cultural practices that reduce the population of fungus gnats include letting the surface soil of potted plants dry between waterings and repotting in a well-drained potting soil that does not stay moist for a long period of time. Potting soils should be pasteurized (treated to kill detrimental insects, diseases, and weed seeds). These practices reduce the population of the larvae in the soil.

To reduce the number of adult fungus gnats in the home, you can use "yellow sticky traps". The gnats are attracted to the yellow color and become stuck in the sticky material removing them as nuisances from the home environment.

A natural toxin produced by Bacillus thuringiensis subsp. israeliensis is effective in controlling fungus gnats. It is sold under the name Gnatrol¨. This product may be used to drench the soil, killing many of the larvae.

A combination of treatments will probably be most successful. When you use a chemical means of controlling fungus gnats, it is important to read, understand, and follow the label directions.

Will angel trumpet poison compost?

I have an Angel's trumpet plant. I know the leaves and flowers are poisonous if eaten. Can these leaves and flowers be put into the compost pile, or will they leave a residual of the poison that can transfer to the plants on which the compost is used?

The poisonous compounds in these plants are organic compounds (carbon-based) that decompose during the composting process. That means they will not be there to be absorbed by plants grown in soil to which the compost has been added.

Some synthetic toxins used in landscapes may not decompose in compost and be of more concern, but the natural products in your plants should not cause concern.

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!