Description
Fungus gnats are found in greenhouse crops, seedling nurseries, interior plantscapes and mushroom farms. They are small flies in the family Sciaridae. Adults are 2-5mm in length with long bead-like antennae, long legs and clear wings with a prominent Y-shaped vein. Larvae are legless and white or translucent with a dark head capsule. Adults are poor flyers and tend to run along the soil surface when disturbed. Before using biological control, it is important to correctly identify fungus gnats because they can be mistaken for shore flies (see table below) or moth flies, which are gray colored and triangular in profile.
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Fungus Gnats
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Shore Flies
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Life Cycle
A complete life cycle takes about 5 weeks at 68ºF. There are usually continuous overlapping generations in a greenhouse. Females lay eggs near the soil surface and may lay 100-200 eggs over their lifetime. The eggs hatch in 4-6 days. Larvae feed for 12-14 days on decaying plant material, algae, soil fungi, fine root hairs and tender lower stems. The larvae pupate in the soil and adults emerge after 5-6 days.
Monitoring Tips
- Use yellow sticky traps, placed 12 inches above the soil surface
- Count the number of fungus gnats on traps weekly
- Replace traps every 3-4 weeks, particularly in warm weather
- Traps are generally used at a rate of one trap per 5,000 sq. ft. to monitor for fungus gnats
Biological Controls
There are three compatible controls for fungus gnats which are often used simultaneously.
Stratiolaelaps scimitus - This soil dwelling predatory mite feeds on fungus gnat larvae. The best way to use Stratiolaelaps is to establish it in the greenhouse before fungus gnats appear. Apply to seedlings in flats or cubes at the stat of the growing seasons, and then again when planting out. Supplement Stratiolaelaps with other biocontrols below if fungus gnat populations are high. In new plantings and where sticky trap counts are below 20 gnats/trap/week, introduce Stratiolaelaps at the low preventative rate, 10/sq. ft. If trap counts are higher and increasing, supplement with nematodes or Bt.
Entomopathogenic Nematodes - Steinernema carpocapsae, S. feltiae and Heterorhabditis spp. are beneficial nematodes that control fungus gnats and other insects. They can be applied to the soil through conventional sprayers or through irrigation systems. Nematodes are effective against high populations of fungus gnats but unlike Stratiolaelpas, will not reproduce or remain in the growing media and must be reapplied on a consistent basis.
- Apply nematodes when yellow sticky traps average over 50-75 fungus gnat adults per trap, weekly or while fungus gnat populations appear to be increasing.
- If root diseases are a problem in the crop, apply nematodes sooner to reduce the risk of disease transmission by fungus gnat larvae.
- Three applications, 7-10 days apart, are usually required. Nematode products vary, so always follow product recommendations for rates and timing. Calculate application on the actual growing area, not greenhouse area.
- If using nematodes on a sponge, apply 1 million nematodes/1,000 sq. ft.
Bacillus thuringiensis var. israeliensis (Bti) - A strain of bacteria that infects fungus gnat larvae is also used for control (Gnatrol). It is applied in water to the soil or growing media after fungus gnats are established. Always follow instructions on the product label.
Other Measures
- Treat soil under benches with hydrated lime (1.5 lb/gal water or 0.18 kg lime/liter) as a slurry, applied to the soil surface.
- Cover floors with ground-cover fabric to eliminate breeding sites
- Improve drainage and modify watering schedules to eliminate wet spots that produce algae and become breeding sites
- Where possible, pasteurize soil media to destroy fungus gnat eggs (this does not necessarily control the pupae as they are resistant to adverse conditions).
- Increase the density of yellow sticky traps in propagation areas to catch adults.
- Use an algaecide to control algal slime and make the soil surface less attractive to fungus gnats