"The major houseplant migration, along with the bananas that are ripening on my counter have caused fungus gnat chaos in my kitchen," states University of Illinois Horticulture Educator Kelly Allsup. Flying gnats have not only populated my home but they have thrived. Another factor that may have contributed to this pest is compost collecting. Without natural pest predators to take care of the problem, I must consider other options.
Fungus gnats are common pests of houseplants. They usually are a nuisance pest as adults, eating very little but laying many eggs in the soil of your favorite houseplants. They spend most of their life as larvae, eating organic matter and young roots. Their ideal environment is wet soil with high organic content.
If you face a similar situation, first tighten up your watering. Let plants dry between watering and throw out any weak and dead plants. If plants are too wet, use a fan to dry out the soil surface. This may also deter the weak-flying adults.
Second, set out yellow sticky cards with adhesive, available on the internet. The yellow coloring attracts flying insects. They can be purchased online and affixed to a Popsicle stick at the edge of the pots. To detect the larvae, place a small cut of potato on the soil surface for a day or two.
Third, add fall leaves to the compost pile in the backyard to reduce the wetness of the rainy fall.
Note, treatments need time to work. An adult female can lay hundreds of eggs in her short life. Three to four weeks are needed to drastically reduce a fungus gnat population. Please read and follow label directions of all organic pesticides.
If the problem persists, use beneficial bacteria called Bacillus thuringiensis that causes the larvae to stop eating. It can be used as a soil drench or dry powder. Be sure to water in dry powder.
Apply diatomaceous earth to the base of the plants. Diatomaceous earth is fossilized aquatic organisms that are not poisonous but cause insects to dry out and die. It remains effective as long as it is kept dry and undisturbed.
Repot the plant in high-quality soil, eliminating existing eggs and larvae.
Visit https://web.extension.illinois.edu/lmw/eb255/entry_6208/ for insect management practices for common houseplant insects. If you would like more information please contact Kelly Allsup, Extension unit educator, Horticulture-Livingston, McLean and Woodford Unit at (309) 663-8306, or email Kelly at email@example.com
Adult Fungus Gnat Picture University of Florida
Larvae on Potato Colorado State University Extension