Fungus Gnats

ENT-59
Date: 
04/16/2015
David J. Shetlar and Jennifer E. Andon, Department of Entomology

Fungus gnats and dark-winged fungus gnats are found throughout the United States and occasionally become a nuisance indoors when adults emerge in large numbers from potted plants, from flower boxes containing damp soil rich in humus, or from heavily mulched flower beds. Adults are attracted to lights and are often first noticed at windows or flying around potted plants. The larvae feed primarily on fungi that grow in soil high in organic matter as well as wet mulch. The larvae can also injure the roots of bedding and potted plants. They are general feeders, but are often found around African violets, carnations, cyclamens, geraniums, poinsettias and foliage plants. Damage to plant roots may promote root diseases. Plant damage symptoms may appear as sudden wilting, loss of vigor, poor growth, foliage yellowing and early leaf drop. While some fungus gnats may become serious pests in greenhouse facilities and mushroom-growing facilities, they are considered harmless to humans and animals.

Identification

Adult fungus gnats are about 2–5 millimeters long, gray to black, slender, mosquitolike flies with long legs and antennae, and one pair of wings. The adults are considered to be weak fliers, and they often appear to dance about, taking short flights. Eggs are white, oval, smooth, shiny, semitransparent, and barely visible to the naked eye. Larvae are legless and threadlike, approximately ¼- to ⅜-inch long. They have white transparent bodies and shiny black head capsules. Pupae occur in silk-lined cocoons in the soil.

Dark-winged fungus gnat. Dark-winged fungus gnat.
 

Life Cycle and Habits

 
  Fungus gnat larvae feeding on decaying thatch in a lawn.

Fungus gnats reproduce in moist, shaded areas in decaying organic matter such as leaf litter. The life cycle takes about four weeks under normal spring and summer weather conditions, but it is in continuous reproduction in homes or greenhouses where warm temperatures are maintained. Generations often overlap, with all life stages present during the breeding season. Adults live about 7 to 10 days and are typically weak fliers but are able to run rapidly. A female may lay up to 100 eggs (primarily females), singly or in bunches, over her lifetime. Eggs hatch in four to six days, with the developing larvae feeding for approximately 12 to 14 days. The pupal stage lasts about five to six days.

Management

Prevention

Inspect plants carefully before purchasing for signs of insect infestation. Always use sterile potting soil to prevent introduction of fungus gnats. Overwatering, water leaks and poor drainage may encourage infestations of fungus gnats. Allowing the soil to dry as much as possible without injury to the plants is effective in killing many maggots. Houseplants taken outside during warm weather may become infested with insects before being brought back indoors. Inspect plants carefully and discard if heavily infested and you are unwilling to treat the plants. Remove all old plant material and debris in and around the home, and keep mulch layers less than 3 inches in depth.

Nonchemical Control

Moisture management and minimizing the accumulation of mulch and other types of plant debris should be the primary approaches to fungus gnat control, both in potted plants and in the landscape. Potted plants and flower beds should be allowed to dry periodically, even to the wilting point of the plants! When watering landscapes, use the "deep but infrequent" technique, rather than the "frequent but light" technique. Using bark-based mulches and keeping them to layers that are less than 3 inches in depth help reduce the development of decay fungi. Woodchip-type mulches often promote the growth of fungal layers. Peat amendments to potting mixes and flower bed soils seem to be more attractive to fungus gnat females intent on laying eggs than soils mixed with commercial composts. Where the soil remains moist because of excessive runoff, swimming pools or poor drainage, formulations of a bacterial-based insecticide should be considered. Look for products based on Bacillus thuringiensis subsp. israeliensis. Insect parasitic nematodes are also effective if irrigation is available after the nematodes have been applied.

Insecticides

Generally, use caution when considering the use of insecticides to control fungus gnats within a home or building. Inside, insecticides are not usually necessary for control, as exclusion, sanitation, and potted plant management are effective approaches. However, for an ongoing problem, adult fungus gnats can be easily knocked down and killed with aerosols labeled for control of gnats or flying insects. If it appears that the fungus gnats are entering the house or building from the outside, use of a perimeter treatment strategy may be advised. This entails using a spray, granule or powder applied as a 3- to 6-foot wide band around the foundation of the building. The intent of this treatment should be to control the fungus gnat larvae that may be breeding in the mulch or organic matter directly adjacent to the building. Always read the pesticide label and follow directions and safety precautions. 


This fact sheet is a revision of HYG-2114.

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