How to Kill Gnats in Houseplant Soil

Reduce chances of gnat infestations altogether by avoiding overwatering plants. Allow the top 1 to 2 inches of the growing medium to get dry between watering, as this will affect the survival of any eggs or larvae present in the soil.

Apply insecticides with the active ingredient cyfluthrin, bifenthrin, lambdacyhalothrin or permethrin, as recommended by the Colorado State University Extension site. These are pyrethroid products that provide extended control.

Spray the insecticide directly on the surface of the houseplant soil to kill the new adults as they emerge from the soil.

Obtain yellow sticky cards sold especially for gnats and place under the plant canopy and on the edges of the plant pots. The traps attract the adults and a reduction in the population of the female gnats will help to greatly reduce eggs and larvae in the soil.

Use the microbial insecticide Bacillus thuringiensissubsp. Israelensis, or Bti, as a soil drench to kill the gnat larvae in the soil. This is effective for larvae control since the soil surface applied insecticides do not penetrate deep enough to affect the larvae.

Introduce the insect parasitic nematodes of the Steinermema feltiae species into the soil as a biological control option for killing gnats. The nematodes are roundworm like microorganisms that penetrate the body of gnat larvae and feed on the internal body content, killing the prey in three to four days.