Fungus gnats do not damage houseplants.
Fungus gnats are small black insects up to 1/8 inch long associated with houseplants. Gnats do not cause any damage to plants as the larva feed on organic debris and the adults do not feed at all. The larvas are rarely seen and live in the soil but can be detected by laying slices of potato on the soil surface. After a few days, turn over the potato and look for tiny worms with a black head. There are several chemical sprays and biological agents that eliminate fungus gnats. Adult gnats can be trapped with commercially available traps consisting of squares of sticky, yellow card.
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Eliminate nuisance adults with any insecticide spray or aerosol labeled as effective against gnats or flying insects. Sprays based on pyrethrins are effective against adults although they do not kill the larva. Alternatively, spray adults with a product based on insecticidal soap or horticultural oil. Contact insecticides work only if combined with larval control methods, such as reducing the frequency of watering and removing soil debris.
For longer-lasting relief, spray your plants and the surrounding area with a persistent insecticide containing bifenthrin, cyfluthrin, permethrin and lambda-cyhalothrin to control adult gnats for at least three days. These last longer than pyrethrin-based sprays but still do not eliminate the larva in the soil.
Use a product containing imidacloprid to treat fungus gnat larva. It is available for household use as a spray in combination with pyrethroids as well as a soil drench or in slow release formulations.
Ask in your garden center for biological control solutions for fungus gnats. These include parasitic nematodes such as Steinernema feltiae, the bacteria Bacillus thuringiensis var. israelensis and parasitic wasps.
Whiteflies are smaller than fungus gnats at only 1/16 inch and resemble white moths. They feed on the sap of host plants using piercing mouthparts while their larva live on leaf undersides. Treat whitefly with insecticidal sprays based on insecticidal soap, horticultural oils pyrethrins or permethrin. Spray your plant every week for five weeks to completely eliminate whitefly.