Steamboat Pilot & Today

Steamboat Springs — In Northwest Colorado, what we may call fruit flies buzzing around inside our homes or hopping across the soil of our houseplants this winter are probably 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 over-watering 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 are allowing at least the top inch of the soil of your houseplants to dry out before watering again and ceasing any 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 eliminate hiding places for gnats and other insects.

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

Several insecticides are available to target either flying-stage or larval-stage gnats. Houseplant sprays of pyrethrum, applied at two- to three-day intervals for three to four weeks, kills the adult-stage gnats. Yellow sticky traps, available in garden shops or made at home using strips of yellow construction paper coated with Tanglefoot, can be a helpful tool. Some gnats will get stuck on the traps, giving you an estimate of the size of the population.

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

Whitney Cranshaw's Bacillus thuringiensis Fact Sheet #5.556 (Colorado State University Cooperative Extension, November 1999), 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 -- the insect has to stop eating and starve to death -- so the gardener may think it is not working.

Using both Bt and sticky traps has worked the best for me.

Barb Sanders is a Master Gardener through the Colorado State University Cooperative Extension office in Routt County. Products mentioned in this article are not endorsed by the Master Gardener program but are provided for informational purposes. Questions? Call 879-0825 or e-mail: gardeners@co.routt.co.us.