Fungus gnats are as common as dirt wherever plants are grown, but like dust balls under the bed we only notice them when they get out of hand. So far as we know, this never happens in the garden, where natural predators and the vagaries of weather and the seasons keep their populations in check. But indoors in hobby greenhouses or on potted plants, fungus gnat numbers can sometimes soar, and a spray from the watering wand or routine sanitation among the pots on the windowsill can stir up a fruit-fly-like explosion of winged insects.
What Triggers Outbreaks?
Although the linkage isn't fully explained yet, researchers agree that high fungus gnat populations are linked to high humidity and soil moisture levels. It may have something to do with the insect's vulnerable egg stage. (Like all flies, fungus gnats have four developmental stages: egg, larva, pupa and adult.) Millie Casey, research assistant at the Ohio State Agricultural Research and Development Center insect lab in Wooster (where she and her colleagues have been studying fungus gnats for several years), says fungus gnat eggs "are very much subject to desiccation. On the other hand, each adult can lay up to 200 eggs, and when the moist conditions that favor egg development are present in a species that goes from egg to adult in only 10 to 14 days, populations are likely to boom."
Another theory is that high humidity and soil moisture encourage the growth of the larval stage. Under normal conditions, larvae feed on soil fungi, just about any soil fungi. Fungus gnats are very closely associated with fungi and highly attracted to them, says Mary Harris, a doctoral candidate working on fungus gnats at the University of Georgia in Athens. (Some people call these insects "fungus-feeding gnats" to call attention to this characteristic.) When moist conditions favoring fungal growth are present, larval food sources can also increase, and with them larval populations.
Fortunately for indoor gardeners whose plants also depend on moist conditions and adequate soil organic matter content, fungus gnats -- and then only the adults -- are usually just a nuisance. A cloud of tiny insects that appears when pots are moved or plants are disturbed may look like a pest problem, but it usually only indicates a living soil.
Some Threat of Disease
When fungus gnat populations explode, however, and larvae exhaust the limited fungal food resources in a greenhouse bench or plant pot, they can turn to feed on plant roots and sometimes stems. Again, under most circumstances this feeding is only a nuisance with no visible plant symptoms. Casey says "If you have a strong, healthy plant with a healthy root system, fungus gnats pose absolutely no problem at all." Young plants, however, are sometimes set back by damage to their already-limited root systems.
Many indoor gardeners can live with even this relatively minor threat to the health of their plants. But now, after reassuring growers for years that the gnats were largely a cosmetic problem, researchers have found the threat doesn't end there. In a recent breakthrough, Harris has shown that both fungus gnat larvae and adults occasionally transmit a variety of diseases. She's proved they spread black root rot and Pythium wilt and suspects they transmit many other fungal pathogens as well. It's akin to discovering dust mites carry the common cold.
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