Tiny Gnats on Indoor Coleus

Prevention

• Fungus gnat larvae thrive in fungus that develops in moist soil. In dry soil, no food is present to sustain the larvae. Allowing the top 1 to 2 inches of soil to dry between waterings often eliminates the fungus in the soil and reduces the food supply. Isolate the coleus to prevent the gnats from laying eggs in other plants. Badly infested coleus may need to be destroyed to keep the problem from spreading.

Life Cycle

• Although adult fungus gnats live only seven to 10 days, female gnats can lay as many as 200 eggs during that short time. The larvae develop in the soil for 14 to 21 days before pupating, and about a week later, they emerge as adults. To spot the tiny larvae, lay a slice of potato on the soil. After two to four days, you can see the tiny larvae on the underside of the potato.

Non-Chemical Management

• Sticky traps, also known as glueboards, laid on on top of the soil or in the area immediately around the coleus plants are an effective way to catch adult fungus gnats. Although sticky boards may alleviate the nuisance factor, they are only one part of the solution. They won't eliminate the gnats completely because the adult females may have already laid eggs. Insecticidal spray soaps may be useful if application, but only if repeated every two to three days, because the soap kills only on contact.

Biological Controls

• Commercial products containing pyrethrin are fast-acting, and because they are a natural product derived from the pyrethrum daisy, toxicity of the product is low. Similarly, Bacillus thuringiensis, a bacteria that occurs naturally in soil, is often an effective means to control fungus gnats. Read the label, because some types of pyrethrin and bacteria are more effective against fungus gnats.

Insecticides

• Use chemical insecticides only if other means of control don't work. Spray the plant outdoors in the morning when the air is still, and then don't bring the plant indoors until the smell of insecticide is no longer present. Alternatively, look for systemic insecticides in the form of spikes inserted into the soil. Read insecticide labels carefully, and use the products only as directed.