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Gardening Articles: Care :: Pests & Problems

Fungus Gnats (page 2 of 2)

by Shelly Stiles


Water management is crucial to controlling fungus gnat buildups. Harris says to absolutely avoid overwatering, particularly of young plants. This is especially good advice for the winter months when air circulation, light levels and growth rates -- and therefore evapotranspiration -- are lower.

Potting mixes also seem to make a difference. Harris says that fresh potting mixes seem more attractive to fungus gnats than older mixes (another reason to be vigilant over your young plants, which are usually potted up in fresh materials). Dr. Richard Lindquist, professor of entomology at Ohio State in Wooster, and Casey found that potting mixes containing higher percentages of compost, especially compost less than six months old, were more likely to breed larger numbers of fungus gnats.

But you don't have to reserve your compost for the garden. Bruce Steward, integrated pest management coordinator at Longwood Gardens in Kennett Square, Pennsylvania, says they grow their bulbs "in a soil type the gnats like." Simply take advantage of the fungus gnat larvae's preference for the upper soil layers and add a half-inch layer of sand to the tops of their bulb pots. "It really works," says Steward. "The eggs probably dried out, or perhaps the adults couldn't emerge."

For routine fungus gnat control, Steward drenches pots and flats with Gnatrol (Bacillus thuringiensis H-14), applying it weekly for two or three weeks. (The B. t. israelensis strain, usually marketed for mosquito and black fly control, is also effective against fungus gnats.) Results aren't immediate. Bt doesn't kill egg-laying adults, so "you've just got to keep at it," Steward says. Harris agrees that it takes time to get control with Bt, and suggests using it to prevent infestations rather than cure them. "A rule of thumb with biologicals," she says, "is you can't wait until you get an infestation. All these organisms seem to work best when pest populations are lower." She recommends watering new transplants with a B.t. israelensis or Gnatrol solution.

Many commercial greenhouse growers use synthetic growth regulators to control fungus gnats. Azadirachtin, an extract from seeds of the neem tree, seems to work in the same way, and is available to home gardeners. Like Bt, it's usually applied as a soil drench. It appears to have little impact on beneficial organisms.

Beneficials are a focus of Stanton Gill's five-year integrated pest management research program at the University of Maryland's Central Maryland Research and Education Center in Ellicott City. His greenhouse trials have shown that a predatory mite, Hypoaspis miles, and a nematode, Steinernema feltiae, offer good control of fungus gnat larvae. The combination of mite and nematode seems to work very well, says Mary Harris. Beneficials can't always be counted on for the long run, because once they've consumed their pest hosts, they'll die or depart, leaving the plant unprotected. But when fungus gnat food resources are exhausted, Hypoaspis mites will turn to feed on the nematodes -- and be present when fungus gnat populations rise again. Harris stresses that only the nematode Steinernema feltiae really works against fungus gnats. Indoor gardeners who use the more common species S. carpocapsae will be disappointed with the results, according to Harris.

You can, as Harris recommends, use Bt and other beneficials as preventive measures before pest levels build up. Or, like Longwood's Bruce Steward and many commercial growers, you can wait until monitoring indicates it's time to act.

Steward uses yellow sticky cards to monitor fungus gnats. Stanton Gill monitors the larvae with a one-inch-diameter potato ball laid on the surface of the growing bench or pot, a technique suggested by Mary Harris. It works as long as the greenhouse mice don't pack the potato pieces away.

Photography by National Gardening Association

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