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Gardening Articles: Health :: Houseplants

Fungus Gnats

by Shelly Stiles

African violets thrive in moist soil...and so do fungus gnats.

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.

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