Sooner or later, if you grow indoor plants, you'll also grow a litter or two of fungus gnats.
You know these pests. They're about an eighth of an inch long and dark gray. They flit around your houseplants, your fruit bowl and your nose. They seem to come from nowhere.It's no slight to your housekeeping, but these beasties are attracted by -- you guessed it -- fungus. That white fuzzy stuff on top of the potting soil.
Fungus appears when we offer it high humidity and wet soil. Fungus gnats follow. They like the weather conditions you've provided, as well as fresh potting soil, which often contains some fresh compost.
The gnats flock to newly planted pots with their damp soil to lay their 200-plus eggs. The larvae feed on the soil fungi in 10 to 14 days. Conventional wisdom says the adults just hang around making nuisances of themselves. They don't hurt the plants much, but they do like to land on your coffee cup just when you're about to take a sip.
However, new research points to fungus gnats as plant disease carriers. Mary Harris, a doctoral candidate at the University of Georgia in Athens, says she's proved that adult fungus gnats occasionally transmit black root rot disease and Pythium wilt. She suspects they might carry more plant diseases, but so far hasn't been able to pin any down.
Don't overwater your plants if you don't want fungus gnats, Harris says. That goes especially for young plants, which aren't strong enough to withstand an occasional gnat nibble.
Potting mixes make a difference, too, Harris said. Fresh potting mix, with high percentages of compost, is more attractive to fungus gnats.
Bruce Steward, integrated pest management coordinated at Longwood Gardens in Kennet Square, Pa., overcomes that problem with sand. When he pots up bulbs indoors, he adds a half-inch layer of sand to the tops of the bulb pots.
"It really works," Steward said. "The eggs probably dried out, or perhaps the adults couldn't emerge."
For routine fungus gnat control, Steward drenches pots with Gnatrol (Bacillus thuringiensis H-14). He applies it weekly for two to three weeks. The more common Bt strain, israelensis, which is used for mosquito and black fly control, is also effective. But not immediate.
Bacillus thuringiensis only works on larva.
"You've just got to keep at it," Steward said.
Harris agreed that it takes time to control gnats with Bt and suggests using it to prevent infestations, rather than cure them.
In the meantime, make sure you let indoor plants dry out before you water them again.