Fungus gnats
Order: Diptera
Family: Keroplatidae, Sciaridae
Description
Fungus gnats include species in the genera Orfelia and Bradysia. Adults are small, dainty flies that resemble mosquitoes and are typically associated with plants grown indoors. They are 3 mm (1/10 inch) long, delicate, black flies with long legs and antennae, and a distinct “Y-shaped” pattern on the forewings. Unlike the common house fly, Musca domestica (Diptera: Muscidae), fungus gnats are weak fliers that move short distances in a darting fashion. The translucent larvae are maggots that measure up to 5.5 mm (~1/5 inch) long. They have a black head capsule and reside in the soil of houseplants and plants grown in the greenhouse. Food may be visible in the gut upon close inspection. The pupae are small and oblong. The eggs of fungus gnats are very small, oval, smooth, and white or semi-transparent.
Fungus gnat larval feeding damage on poinsettia. Image credit: Penn State Department of Plant Pathology; Environmental Microbiology Archives, Penn State University, Bugwood.org
Adult fungus gnat. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org
Fungus gnat larvae. Image credit: Whitney Cranshaw, Colorado State University, Bugwood.org.
Quick Facts
- Fungus gnats are small, delicate bodied flies that commonly develop in growing medium of plants grown indoors such as greenhouses and high tunnels.
- Larvae of fungus gnats feed on algae, fungi, and plant roots in growing medium. Adults do not bite or feed.
- Adults can be monitored with yellow sticky traps, while soil-dwelling larvae can be detected by inserting a slice of potato in the growing medium for one to two days.
- Fungus gnat densities can be reduced by allowing the growing medium to dry between watering. Insecticides and biological control agents can be used to control fungus gnat larvae in growing media.
Life history and habits
During their seven-to-ten-day lifespan, a single female can lay up to 200 eggs in soil media. Fungus gnat larvae usually reside in the top 5-8 cm of the growing medium, depending on moisture level, where they primarily feed on fungi, algae, and decaying plant matter. The larvae can feed on plant roots and leaves resting on the growing medium surface. After about 20 days, the larvae pupate near the surface of the growing medium. Adult fungus gnats are usually seen in growing media, plant foliage, compost, wet piles of mulch, or other areas in the vicinity of an infested plant. However, adult flies are attracted to light and may disperse short distances where they accumulate around window frames. Moist-growing media containing high amounts of peat moss are particularly attractive to adult females. The entire life cycle can be completed in 17 days, depending on temperature. Fungus gnats produce multiple overlapping generations each year.
Injury
Larvae of fungus gnats feed below the soil surface on roots. When densities of larvae are high, feeding injury can lead to stunted growth and increased susceptibility to microbial infection, especially in seedlings and younger plants.
Adult fungus gnats do not bite, but they are a nuisance indoors when their numbers are high. Adult fungus gnats tend to be most noticeable during late fall and winter. Decreased day length and cooler temperatures slow plant growth and water usage. If watering practices are not altered, particularly during fall and winter, the growing medium will remain moist, which improves conditions for fungus gnat development. Furthermore, as the growing medium ages or degrades it tends to retain more moisture, which will attract fungus gnat adults.
Monitoring
While yellow traps alone will not significantly reduce fly densities, they can be used to monitor fungus gnats when deployed in the greenhouse. Often, adults can also be observed flying or resting on surfaces. Yellow sticky cards can be used to trap adult fungus gnats and should be placed under the plant canopy or on the edge of containers. They can also be deployed near seeded plants to help monitor the number of adults.
An effective means of detecting fungus gnat larvae in larger pots is to insert ¼ inch slices of skinned potato into the growing medium. Larvae will migrate to the potato within 24-48 hours, at which point the potato can be removed to determine whether an infestation is present. The threshold at which significant feeding injury can be expected is three to five larvae per potato slice in propagation pots, or 15-20 larvae in production pots.
Cultural control
The most important strategy to minimize fungus gnat problems associated with plants grown indoors is to allow the growing medium to dry between watering, especially the top 1 to 2 inches. Watering should only be done when necessary to maintain plant health and productivity, as allowing the soil surface to dry will kill eggs and larvae. Indoor collections of standing water should be eliminated and plumbing, or irrigation leaks should be addressed.
Examining the soil of new plants can help prevent the introduction of fungus gnats to clean areas. It is also recommended to re-pot plants, particularly when the growing medium is retaining too much moisture. Populations in the greenhouse can be suppressed by reduction of breeding areas. For example, removing decaying plant matter, soil and standing water from underneath benches. Any containers with an abundance of decaying plant matter should be removed such as decayed bulbs and roots, which are a food source for fungus gnat larvae. It is recommended that pasteurized container or potting mixes be used in areas where fungus gnats are problematic, and commercial growers can treat growing media with heat or steam to kill developing flies, algae, and microorganisms. In home gardens, soil can be moistened, placed in a transparent or black plastic bag, and left on a sun-exposed elevated surface for 4-6 weeks. For this approach to work, the pile depth should be limited to 20.5 cm (8 inches). In greenhouse systems, fungus gnat entry can be prevented by storing potting soil indoors or sanitizing via steam or autoclave before potting plants sensitive to fungus gnats. Infested plants should not be introduced to clean areas and should be discarded if possible.
Biological control
Certain natural enemies such as the predatory beetle, Dalotia coriaria (Coleoptera: Staphylinidae), and a soil-dwelling predatory mite, Stratiolaelaps scimitus (Mesostigmata: Laelapidae) are commercially available for controlling fungus gnat infestations. Another biological control option for fungus gnat larvae is applications of insect parasitic nematodes applied as a drench to the growing medium. The nematode species Steinernema feltiae is particularly effective against fungus gnat larvae. It is worth noting that these biocontrol agents are live organisms that should be applied immediately upon arrival to maximize their effectiveness.
Chemical control
When fungus gnat populations are excessive, the microbial insecticide Bacillus thuringiensiss israelensis (Bti) can be effective when applied as a drench to the growing media. Usually, multiple applications are required. Bacillus thuringiensis israelensis is not typically available through retail outlets, but it is used in commercial greenhouses and large interior environments. Products containing Bacillus thuringiensis labeled for caterpillar control are not effective against fungus gnats.
In addition to Bti, formulations of pyrethrins can be effective and display relatively low toxicity toward humans and domestic animals. Pyrethroids can also be effective but are more toxic toward beneficial insects. When possible, it is recommended that plants be moved to well-ventilated outdoor areas for insecticide treatments.
CSU Extension Fact Sheet
Download or view the CSU Extension’s PDF fact sheet for your reference.
CSU Extension Fact Sheet
Download or view the CSU Extension’s PDF fact sheet for your reference.