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What You Should Know
• Fungus gnats require high moisture conditions and decaying material.
• Adults are attracted to light and can be first seen flying near windows and doors.
• Plants damaged by fungus gnats will lack vigor, have poor color, and premature leaf drop.
Fungus gnats are small, long-legged flies in the families Mycetophilidae and Sciaridae (Figs. 1-2). The dark-winged fungus gnat, Bradysia spp. is one of the most common indoor pests. Adults fly over plants, but consume only liquids and are harmless to plants.
They can become a nuisance indoors when adults emerge from potted plants that have damp soil and are rich in humus. Larvae feed mostly on the fungi that grow on decaying material, but may damage roots while burrowing through the soil. The larvae may also feed on portions of the crown and root system.
Management
• Inspect plants carefully before purchasing for signs of infestation. Check house plants before bringing them back inside the house to prevent an infestation.
• Overwatering and poor drainage may result in a fungus gnat infestation. Let the soil dry out completely between waterings to kill the larvae.
• If there are leaks or problems with excessive moisture, correcting the problem is the only effective solution.
• Make sure all old plant material is removed and good sanitation is used. Using sterile soil at home can also help reduce the chance of an infestation.
• Fungus gnat adults may accidentally get inside the house because they are attracted to porch lights. Make sure windows and doors are sealed completely to avoid movement to indoor house plants.
• Yellow sticky traps placed horizontally at the soil surface will capture a large number of females before they lay eggs in the soil (Fig. 4).
• Consider using a bacteria, Bacillus thuringiensis israelensis (Bti) as a liquid drench in the soil to kill the larvae. Gnatrol® is registered for use on fungus gnats in Utah and is a selective product for flies.
Life Cycle and Description
Fungus gnats go through complete metamorphosis (egg, larva, pupa, adult) and reproduce in moist, shaded areas within decaying matter. One generation takes about 4 weeks. Mated females lay 100 to 200 microscopic eggs. Eggs are oval, smooth, shiny white and semi-transparent, and hatch in 4 to 6 days. The larvae are legless and white with a black head (Fig. 3).
The larvae of most species feed in the soil on fungus, decaying organic material and on roots of living plants. They are usually found in the top 2 to 3 cm of the soil. They can be up to 5.5 mm long long and transparent so food in the gut is visible. Larvae feed for about 15 days before spinning a silken cocoon in the soil.
Adults emerge from the pupal case in 5 to 6 days and live for about 7 days. Adult fungus gnats are 2.5-3.5 mm long, grayish to black, slender, mosquito-like, and have long legs, antennae, and one pair of wings. Although weak flyers, adults are attracted to light and may accumulate at windows after emerging from potted plants. Fungus gnats fly away when plants are disturbed.
