Indoor gardeners are commonly required to battle those little gnats that suddenly seem to crawl from the potting soil and swarm the home. These are called fungus gnats and they’re after the fungus produced by decaying plant material. Adult gnats won't harm the plant, but the larvae can seriously damage feeder roots. The key to their surviving, thriving and breeding is access to a readily available food source and a very moist environment.
Remove the saucer from the bottom of the affected plant’s pot. Set the plant in the kitchen sink and just allow it to drain. Leave it there all day, and even overnight if necessary, until it no longer drips water from the drain holes.
Use yellow paint, markers, or crayon to brightly color both sides of a 3-inch by 5-inch index card. Staple or glue it to a wooden craft stick or plant stake, and coat both sides generously with petroleum jelly. Poke the stake into the plant’s soil. Flying adult gnats will be very attracted to the bright yellow trap, and get themselves stuck to the jelly on the card. Put the trap and its victims in a sealable food storage bag for disposal.
Pour 1/8 to 1/4 inch of liquid attractant into a disposable plastic cup. This bait can be fruit juice, beer, or wine -- any liquid produced by the fermentation of sugar. Add one drop of dishwashing liquid, which will break the surface tension of the liquid and allow the insects to drown in it when they try to consume or land on it. Cover the cup’s entire opening with clear tape and cut a 1/4-inch hole in the middle of it. Place the cup trap right next to your potted plant. Gnats will enter the tiny hole and either drown in the liquid attractant, or get stuck to the tape on the top of the trap if they try to escape. Tape over the opening when you’re ready to discard the trap full of bugs.
Cut a raw potato into 1/2-inch cubes. Place a few of them on your plant’s soil surface. Gnat larvae will emerge from the medium to feed on them within 48 hours. Package the buggy potato pieces in sealable food storage bags for disposal.
Repot your plant if the infestation is severe, or seems to be out of control. Use only newly purchased sterile potting soil free of bark products, and mix it with an equal portion of perlite. Either sterilize the pot or replace it with a new one. Dump the old planting medium and pot into a plastic bag and close it securely to prevent gnats from escaping back into your home. Dispose of the bag in an appropriate receptacle outdoors.