Question of the Week
Answer: Adult fungus gnats don't damage plants or bite people; their presence is primarily considered a nuisance. Larvae, however, when present in large numbers, can damage roots and stunt plant growth, particularly in seedlings and young plants. Significant root damage and even plant death have been observed in interior plantscapes and in houseplants when high populations were associated with moist, organically rich soil.
Fungus gnats can be managed by decreasing food and habitat. Fungus gnats feed on fungi, decaying organic matter, and plant roots, particularly in very moist environments, so reduction of the fungus food source must be integrated into a program of managing other food sources and breeding habitat (see Management, below). Note that fungus gnats are not the only type of flies that may exist in a potted plant or greenhouse environment. Other families of flies include moth flies (Psychoidea), fruit flies (Drosophilidae), and shore flies (Ephydridae), not to mention various species of predatory flies and wasps (1). Proper management of any pest requires accurate identification of the pest. Once the pest and its life cycle are known, then management strategies can be developed around "weak links" in the life cycle.
Fungus gnats and shore flies are known to transport Pythium spores and innoculum of several important plant-pathogenic fungi around the greenhouse. Larvae feed on decaying organic matter as well as small roots and have recently been shown to act as vectors for pathogenic fungi such as Botrytis and Fusarium (2) as well as Phytophthora (1).
Cultural controls are the easiest and most practical way of managing (and preventing) fungus gnat populations. Soils (or woodchips) that are constantly moist and have undecomposed organic matter make ideal habitats for fungus gnats. Unfortunately, many potted plants or mushroom-rearing facilities—particularly those with bark or wood chip mulch—present ideal environments for these pests. Avoiding overwatering and allowing the soil to dry to the greatest extent possible short of harming the plant is one option for managing this pest. Planting pots should be well drained, and standing water should not be allowed to collect in the catch dish under the pot. Adult populations may be monitored using yellow sticky traps placed (sticky side up) on the soil/mulch surface of the potted plant. Another approach to prevent initial infestations is to pasteurize potting soil prior to planting.
In 1985, Dr. Dick Lindquist of Ohio State University showed that fungus gnat problems are most serious in potting mixes amended with composts lacking in maturity (not completely composted). Microbial activity is excessively high in such mixes, and fungus gnats thrive.
Another consideration is having continuous production cycles, which allows for infestation of new potting material by previous generations of the fungus gnat. It may be worthwhile to try to break the re-infestation cycle by not allowing access to potting soil in order to rid the greenhouse area of adults.
Other management options include use of neem-based formulations, use of non-toxic bacterial formulations, such as Bacillus thuringiensis H-14 (Gnatrol), insect-attacking nematodes (mostly Steinernema feltiae), and predatory mites in the genus Hypoaspis (2, 3).
Hypoaspis miles (Predatory mite)
This predatory mite prefers to feed on first instar fungus gnat larvae and will also feed on thrips pupae. It may also feed on debris and algae. It is important to make releases early in the growing season before fungus gnat larval populations are abundant. Applications can also be directed to the soil beneath greenhouse benches. Avoid applications into the growing media prior to planting because this decreases survival. Applications need to be initiated after planting and the growing medium should be moist but not saturated. Hypoaspis miles is active when growing medium temperatures are greater than 50 degrees F.
Steinernema feltiae (Predatory nematode) (3)
This beneficial nematode attacks fungus gnat larvae. Nematodes are applied as a drench to containers or flats and they can also be applied through drip-irrigation systems providing that filters are removed. Apply nematodes two to three days after inserting cuttings, planting plugs, or starting seeds. To assess the viability of shipments prior to application, place a small quantity of the product in a shallow container with a few drops of tepid water. After a few minutes, look for active nematodes which have a slight 'J' curve at the ends of their bodies.
Repeat applications are usually needed. Growing medium temperatures must be 50 to 80 degrees F with optimum temperatures of 60 to 70 degrees F. Irrigate the growing medium before and after applying nematodes. The nematodes require moisture in order to move within the pores of the growing medium. Apply nematodes in the evening or on cloudy days because they are extremely sensitive to ultraviolet light desiccation. In general, beneficial nematodes are compatible with most pest control materials except for carbamate and organophosphate pesticides.
Coenosia attenuata (Hunter flies)
This is a predatory fly, native to southern Europe, that was first found in a greenhouse in upstate New York, and has subsequently spread across the United States. Hunter fly adults resemble a half-size version (or smaller) of the common house fly adults. In addition to fungus gnat adults, hunter fly adults attack and feed on shore fly, whitefly, and leafminer adults. Adult hunter flies only attack prey that are flying. The soil-dwelling larvae are also predaceous and feed on fungus gnat larvae and other insects in the growing medium.
For information on materials that can be used to control gnat populations, see ATTRA's Biorationals: Ecological Pest Management Database, available at
1) Osborne, L.S. and W. Fooshee. 1998. Fungus gnats. University of Florida, Central Florida Research and Education Center. www.mrec.ifas.ufl.edu/lso/entomol/fungnat/fungnat.htm
2) Pundt, L. Managing Fungus Gnats and Shore Flies in the Greenhouse. University of Connecticut.
3) Smith, Tina. 2012. Fungus Gnats and Shoreflies. University of Massachusetts Amherst, Greenhouse and Floriculture Program. https://extension.umass.edu/floriculture/fact-sheets/fungus-gnats-and-shore-flies
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