What Is Good to Use if My Houseplant Has Many Flying Gnats?

Gnatlike Flying Pests

• Several gnatlike pests afflict houseplants. Aphids, whiteflies and fruit flies are often mistaken for gnats. Aphids and whiteflies seldom kill plants but they may disfigure leaves, and they reproduce rapidly, presenting a nuisance indoors. Isolate unaffected plants rather than try moving infested plants and raising clouds of the pests. Insecticidal soaps may require several applications before the bugs give up. Neem oil and plant extracts may be effective. Pesticides labeled for indoor use containing bifenthrin, permethrin, resmethrin or pyrethrins are chemicals of last resort. Fruit flies disappear when the old fruit does.

Fungus Gnats

• Fungus gnats feed on mold and fungus in the soil and may enter the indoors in their larval stage just under the surface of the soil. Larvae appear as tiny maggots in the soil and adult flies fly just above the surface of the soil, looking for a place to lay eggs. Like the aphids and whiteflies, killing an adult generation of fungus gnats does not guarantee an infestation is finished; the next generations are lurking in the soil. Biological agents such as Bacillus thuringiensis var. israelensis or organics such as neem oil are available in soil drenches for fungus gnats. The last resort, of course, is to remove the plant from the soil and flush the plant's roots, replace the old soil with new sterilized potting soil, and toss the old soil on the compost pile.

Quarantine

• The best offense is a good defense. Before bringing plants in, quarantine them in a shady corner of a deck or on any other shady area that sits up off the ground for a week or two. Plan ahead so you complete the quarantine before nights get cold enough for the furnace to begin. Allow the soil to dry out to a depth of 1 or 2 inches before watering. If you notice gnats, use insecticidal soap or a soil drench to let the creatures know that they are not welcome. Quarantine any new houseplants before introducing them into the general population, too.

Houseplant Hygiene

• Fungus gnats and their brethren are native pests that live peacefully in big outdoor or greenhouse environments. In order to keep them from setting up shop on your houseplants, however, practice some uncomplicated hygiene practices to make them feel unwelcome in your plants. When repotting, use only sterilized commercial potting soil that has been dried completely before use. Do not overwater or allow plants to sit in saucers of water, to minimize conditions for fungus or algae growth. Periodically give your houseplants a shower to keep their leaves dust-free and healthy. If you notice pests, treat them immediately; you are the only predator they have.