Gardening: Managing fungus gnats on plants |

Gardening: Managing fungus gnats on plants

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Three products for controlling fungus gnats are Knock-out Gnats! available from Gardens Alive, Gnatrol and Vectobac, biological larvicides, and EPA approved for use on fungus gnats.

For more information about this winter problem, visit, and refer to two fact sheets: Flies in the Home No. 5.502 and Bacillus thuringiensis No. 5.556.

— Here in northern Colorado, what we may call fruit flies flying around inside our homes or hopping across the soil of our houseplants in winter probably are fungus gnats. They could have entered your home with outdoor plants that have been brought inside for the winter or on dirty tools.

They are nurtured by watering too mcuh and the fungus created by dead and decaying leaves of the plants. They feed on the soil fungi but do little or no damage the roots of the plants.

Two simple control methods include allowing at least the top inch of the soil of your houseplants to dry out before watering again, and ceasing use of fish emulsion fertilizer.

Clean each plant of dead leaves and remove any fallen leaves from the soil surface to discourage the growth of fungus and hiding places for not only gnats but other insect pests, as well.

Placing a small bowl of sweetened water or mouthwash near the plants will attract the gnats, causing them to drown in the liquid.

Several insecticides are available to target either the flying stage or the larval stage. Houseplant sprays of pyrethrum, applied at two- to three-day intervals for three to four weeks, treats the adult-stage gnats. Yellow sticky traps may be used for monitoring as some gnats will get stuck, giving you an estimate of the size of the population. You may find these in garden shops or make your own by using yellow construction paper strips, which you coat with Tanglefoot.

The soil furthermore can be drenched with a product containing Bacillus thuringiensis var. israelensis, a human- and pet-safe bacterium, to prevent the growth of the larvae.

Whitney Cranshaw’s Bacillus thuringiensis Fact Sheet No. 5.556 (Colorado State University Cooperative Extension 11/99), states that Bt is a bacterium that is found in soils worldwide. Strains have been developed to kill specific insects by paralyzing the digestive system. The strains are insect-specific and harmless to insects that are beneficial to plants, such as honeybees.

Bt is nontoxic to humans and mammals and is safe to use on food crops up to harvest time. A disadvantage of Bt is that it takes a while to work, as the insect has to stop eating and starve to death, so the gardener may think the product is not working. Use this larvicide once a week for three weeks.

This last method, along with sticky traps has worked the best for me.

Barb Sanders is a master gardener through the Routt County Extension Office. Products mentioned in this article are not endorsed by the Master Gardener program but are provided for informational purposes. Call 970-879-0825.

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