Champion Daylilies

Fungus Gnats

Fungus gnats. These flies can appear in large numbers in or around buildings, prompting complaints, and also can be a problem in greenhouses, nurseries, and interior plantscapes. Fungus gnats infest soil and container media, where larvae feed on organic matter and roots.

Damage

Fungus gnats commonly damage plants. Larvae of these flies feed on roots, thus stunting plant growth. Root damage can occur in interior plant scapes and in houseplants if high populations infest moist, organic-rich soil. Fungus gnat larval damage can be especially serious in greenhouses, nurseries, and sod farms. In addition to larvae chewing on roots, both larvae and adults can spread plant pathogens and may promote disease in commercial crops.

Identification

Fungus gnats (families Mycetophilidae and Sciaridae) adults are sometimes confused with other families of small, including black flies (family Simuliidae), midges (family Chironomidae), and mosquitoes (family Culicidae). If you are unable to determine what kind of fly you have, take samples to your county Cooperative Extension office or university entomology department for identification. Some nurseries and garden supply stores will also help you identify flies. Adult fungus gnats are dark, delicate-looking insects, similar in appearance to mosquitoes. Adult fungus gnats have slender legs with segmented antennae that are longer than their head. Although a few species are up to 1/2-inch long, adults commonly are about 1/16 to 1/8-inch long. Wings are light gray to clear; the common Bradysia species have a Y-shaped wing vein. Fungus gnats are relatively weak fliers and usually are not found flying around indoors. They generally remain near potted plants and often run or rest on growing media, foliage, or litter. Females lay tiny eggs in moist organic debris or potting soil. Larvae have a shiny black head and an elongate, whitish to clear, legless body. They eat organic mulch, leaf mold, grass clippings, compost, root hairs, and fungi. If conditions are especially moist and fungus gnats are abundant, larvae can leave slime trails on the surface of media that look like trails from small snails or slugs.

Life Cycle

Fungus gnats have four stages: egg, larvae (four larval stages or instars), pupa, and adult. Fungus gnats have many generations each year. They occur anytime of year in moist coastal regions and indoors. Adults feed very little, consuming only liquids, such as water or flower nectar. The tiny eggs and oblong pupae occur in damp places where larvae feed.

Management

Physical and cultural methods-primarily screening windows and doors as well as reducing moisture and organic debris-are recommended for managing all of these flies. Biological control agents are also available to control fungus gnats. Insecticides are used in commercial plant production, but are not generally recommended for control around the home. Most of the insects' life span is spent as larvae and pupae in organic matter or soil, so most control methods target the immature stages, not the mobile and short-lived adults.

Monitoring

Visual inspection for adults is usually adequate to determine whether there is a problem. Adults can be observed resting on plants, soil, windows, or walls, or they may be seen in flight. Besides looking for adults, check outside near buildings for excessively moist conditions and organic debris where larvae may be feeding. These are the places to take control actions, as discussed in "Cultural Control." Yellow sticky traps or potato pieces for monitoring may be warranted as discussed in "Management Tools for Professionals" if you suspect that container plants or interior plant scapes are infested with fungus gnats.

Cultural Control

Fungus gnats thrive under moist conditions, especially where there is an abundance of decaying vegetation and fungi; avoid over-watering and provide good drainage. Allow the surface of container soil to dry between watering. Clean up free-standing water and eliminate any plumbing or irrigation system leaks. Moist and decomposing grass clippings, compost, organic fertilizers, and mulches are favorite breeding spots. Avoid using incompletely composted organic matter in potting media unless it is pasteurized first, because it often is infested with fungus gnats. Minimize organic debris around buildings and crops where larvae feed. Avoid fertilizing with excessive amounts of manure, bloodmeal, or similar organic materials.

Physical Control

Keep doors, vents, and windows closed or screened to prevent insects from flying into buildings. Do not bring plants with infested soil indoors. Periodically turn and aerate compost piles where fly larvae feed. Locate compost away from doors and windows and keep it covered. Purchase and use only pasteurized container mix or treat potting soil with heat or steam before using it; this will kill flies as well as the algae and microorganisms they feed on. Store pasteurized potting soil in closed containers to prevent it from becoming infested before use.

Biological Control

Predators, such as rove beetles (family Staphylinidae) and ground beetles (family Carabidae), help control fly larvae outdoors in areas not sprayed with broad-spectrum insecticides. Commercially available Steinernema nematodes, Hypoaspis mites, or the biological insecticide Bacillus thuringiensis subspecies israelensis (Bti) can be applied to control fungus gnat larvae in container media.

Nematodes can provide relatively long-term control of fungus gnat larvae and they can be self-reproducing after several initial applications to establish their populations. Steinernema feltiae is apparently more effective against fungus gnats than other commercially available nematode species. Bti does not reproduce or persist; infestations in media may require repeated applications at about 5-day intervals to provide control. Mix Bti or nematodes with water and apply as a soil drench or spray onto media using conventional spray equipment.

Chemical Control

Insecticides are rarely, if ever, warranted to control these flies around homes. However, if insecticides are required for fungus gnats, consider using Bacillus thuringiensis subsp. israelensis or Steinernema feltiae nematodes to control the larvae in plant containers (see "Biological Control").

If Bti or nematodes are not available and high populations are intolerable, pyrethrins or a pyrethroid can provide temporary, fast-acting control. Pyrethrins have low toxicity to people and pets and are the active ingredients in the botanical pyrethrum, from flowers of certain chrysanthemums. Pyrethroids (e.g., bifenthrin, permethrin) are synthesized from petroleum to be chemically similar to pyrethrins, but often are more effective and persistent, as well as being more toxic to beneficial insects. When using these on houseplants or interior-scape containers, it may be best to move plants outdoors for treatment and wait about a day after application before bringing them back inside.

Fungus gnat larvae

Fungus gnat (left) Shore fly (right)

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