Little flies flying around houseplants are fungus gnats(Bradysia species), slender, 1/8-inch long, grayish to black insects each with one pair of clear wings. Adult females lay 75 to 200 eggs, which hatch in one week. Fungus gnat larvae are whitish with black shiny heads. Most concerns involve nuisance problems with the adults. They are weak fliers and tend to move with a quirky, skittish motion. Although present year-round they most commonly are observed in midwinter, sometimes collecting in corners around windows. The adult gnats are harmless, do not bite and live only a few days. The larvae live in moist, organically rich soil--often the top layer of houseplant soil--and they may become more plentiful in soil that’s over watered. The larvae rarely do much damage to plants although they may nibble a few roots, reducing the vitality of a plant. To control fungus gnats, the conditions of the breeding area need to be corrected to reduce the amount of fungi where the gnats breed. The first step is to examine watering habits. The soil must be allowed to dry thoroughly between waterings to eliminate decomposing plant material, which will kill the larvae and leave the adults without a place to lay their eggs. One way to reduce a severe infestation of fungus gnats is to scrape off the top layer of soil around the plant, strategically place yellow sticky traps near the plant and allow the top layer of soil to dry thoroughly. Yellow sticky traps can be purchased at garden centers then placed face up on the potting mix or hung on the string usually attached to the paper. For the greenhouse grower and homeowner, these traps can be used to help identify insects in order to verify the most effective approach for control as well as rid the area of the fungus gnat adults. Sometimes fungus gnats can spread disease from plant to plant, so control is warranted. Pyrethrins or pyrethroids will control the adult flies, but these chemicals will need to be sprayed every few days before fungus gnat populations are reduced. Larval control is more effective using a Bacillius thuriengensis product such as Gnatrol, which should be available in a nursery or garden center. This product will need to be applied to the soil as a drench in accordance with label directions. For more information on fungus gnats, see the fact sheets Flies in the Home, Managing Houseplant Pests, and the PlantTalk script, Fungus Gnats.

Browse related Faqs by tag: horticulture, insects, houseplants, colorado, fungus