It’s disconcerting when you water a houseplant and see a cloud of tiny bugs. What you’re seeing are most likely fungus gnats—one of the most common indoor pests. These tiny insects (they’re about 1/16 of an inch long) thrive in moist, rich soils; on their dinner menu is mold, fungus, and rotting plant matter.
Damage
Adult fungus gnats don’t cause harm to plants, as they prefer to eat decaying organic matter (i.e., not healthy leaves or stems). They’re just no fun to have living in your house. Fungus gnat larvae, on the other hand, feed on plant roots, and so can cause minor damage (like discoloration and stunted growth) in older plants and major damage—even death—in young plants or seedlings.
Control
To get rid of fungus gnats, you have to close down their café. Let your houseplants’ soil dry out between waterings (though not so severely that the plants suffer). Get rid of decaying stems and leaves. You can also re-pot plants in sterile growing media. Another strategy: A pot of sprouting wheat will attract egg-laying females—after a few days, throw out the contents, and repeat until the gnats are gone. If these measures fail, try using an insecticide containing Bti (Bacillus thuringiensis israelensis), a strain of Bt that kills certain kinds of fly larvae.
—photo by Andrew Darrington