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Integrated Pest Management Program

Department of Plant Science and Landscape Architecture, Department of Extension

Fact Sheets > Greenhouse > Insect and Mite Pests

Fungus Gnats

fig. 1 - adult fungus gnat

Fungus gnats (Bradysia spp.) commonly develop in the moist environments common in the greenhouse, especially in propagation houses and in newly planted plant material.  

IDENTIFICATION

Adult fungus gnats are small (1/8 inch long), mosquito-like insects, with long legs and antennae. (figure1) Their two wings are delicate and clear with a Y-shaped vein in the wing pattern. (figure 3) Adults are weak flyers and tend to fly in a zig-zag pattern. They may be observed resting on the growing media surface or moving across lower leaves.  Adult females are attracted to fungi so might be fig.2  shorefly adultobserved near plants with Botrytis sporulation. Females lay their eggs nearby so the  developing larvae have access to a fungal food source. Fungus gnat larvae are small, (approximately ¼ of an inch long when mature), translucent to white in color with a distinctive black head capsule (Figure 4).

Both fungus gnats and shore flies are common in the greenhouse. However, it is important to distinguish between the two, because management strategies differ.  Shore fly adults (approximately 1/8 of an inch long), resemble a small housefly with stockier bodies, plus shorter legs and antennae than fungus gnats.  Shore flies also have five distinct white spots which fungus gnats do not have.  Shore fly larvae are white, wedge-shaped and do not have a distinctive head capsule. Larvae may be found near algae, a primary food source. They do not feed on plants.

fig 3. y-shaped wing vein

FUNGUS GNAT DAMAGE

Fungus gnat larvae feed on fungi and decaying organic matter, but so feed upon plant roots. This larval feeding is most damaging to seedlings, and young plants. Larvae also feed on the developing callus of direct stuck cuttings, delaying rooting. Fungus gnat larvae are general feeders. Plants with succulent stems, such as begonias, geraniums, sedum, coleus and poinsettias, are especially prone to injury and can suffer serious losses.  As the young feeder roots and stems are damaged,  plants wilt and leaves turn yellow and drop. In laboratory studies, adult fungus gnats carried spores of  Botrytis, Verticillium, Fusarium and Thielaviopsis as they moved from plant to plant. Spores have also been found in their droppings. It is unclear how important this disease transmission is in commercial greenhouses.

fig. 4 fungus gnat larvae

LIFE CYCLE

The fungus gnat's life cycle from egg to adult may be completed in 21 to 28 days depending on temperature. Eggs are laid in cracks and crevices in the media surface and hatch in four to six days. Fungus gnat larvae feed and develop for about two weeks at 72oF . Pupation occurs in the soil. After four to five days, adults emerge. Overlapping and continuous generations make control difficult.

SCOUTING

Monitoring is especially crucial if you are planning on targeting biological controls or insect growth regulators against the fungus gnat larvae. Inspect incoming plugs for fungus gnat larvae or their damage.  Place yellow sticky cards in samples of growing media to monitor for any emerged adults.  fig. 5 potato plug

Yellow sticky cards, placed horizontally at the soil surface, can be used to detect fungus gnat adults. Check and change the cards weekly to detect early fungus gnat infestations. Use potato plugs (at least one inch in diameter) placed on the soil surface to monitor for fungus gnat larvae (Figures 5 and 6). When using potato plugs, place the plug so there is contact with the media to ensure that the potato plug does not dry out. To look for larvae, first check the growing media under the plug and then the surface of the potato itself.  Check the potato plugs after 48 hours for the presence of larvae.  Be sure to mark the locations where you placed the potato plugs, so you can easily find them! If not removed, potato chunks can "melt out," sprout or be fed upon by mice.  For smaller cuttings or plugs, potato slices, resembling a “French fry” can be placed in the growing media.

CULTURAL CONTROLS

Adults are attracted to newly planted crops, making it important to thoroughly clean the greenhouse before introducing new crops. Dry, level, weed-free, well-drained floors help eliminate breeding areas. Keeping compost piles away from the greenhouse and cleaning up any spilled media on the floor also helps eliminate breeding areas. Avoid overwatering and allowing excess moisture to accumulate underneath greenhouse benches.  Remove plant debris, weeds, and old growing media from inside and outside the greenhouse.

Inspect incoming plugs for fungus gnat larvae or their feeding damage.  Recent studies have shown that fungus gnats may be introduced into a greenhouse from soilless media or rooted plant plugs. 

Adults are attracted to mixes with high microbial activity, or with high amounts of peat moss or hardwood bark. Avoid using mixes with immature composts less than one year old. However, no potting mix is completely immune to fungus gnat infestations.  Adult females prefer to lay their eggs in protected, humid crevices in the media. How the media is handled and stored may be more important than the type of growing media used. If the growing media is stored outside and stays moist, it may support more fungus gnat activity. Tears or openings in the bags enable resident, native fungus gnat populations to gain entry into the media bags. Store the media so that it stays dry.

Covering the growing media with a layer of coarse sand or diatomaceous earth does not help prevent egg laying by the adult females. Diatomaceous earth absorbs moisture from the growing media so that cracks develop where larvae can pupate and females can lay eggs.

BIOLOGICAL CONTROL

Commercially available natural enemies include the soil dwelling predatory mite, Hypoaspis miles, (= Stratiolaelaps scimitus), the entomopathogenic nematode, Steinernema feltiae, and the rove beetle, Atheta coriaria. All should be used preventively and applied to moist growing media.

Steinernema feltiae are beneficial, insect killing nematodes that are applied as a drench treatment against fungus gnat larvae.  After entering the target insect through various openings, the nematodes multiply within the host and release a bacterium whose toxin kills the larvae.  These beneficial nematodes reproduce within the fungus gnat larvae; exit the dead body and search for new hosts to infect. Fungus gnat larvae are killed in one to two days.  (See Beneficial Nematodes: An Easy Way to Begin Using Biological Control in the Greenhouse for specific application tips).

A small, soil-dwelling predatory mite, Hypoaspis miles, feeds on fungus gnat larvae as well as thrips pupae and shore fly larvae. It is shipped in a vermiculite/peat carrier with all stages of the predatory mites. The vermiculite/peat carrier can be distributed over the media surface, especially when pots are placed close together.  These predatory mites are best used when fungus gnat populations are low.  

The rove beetle is a generalist predator that feeds upon fungus gnat and shore fly larvae in the growing media. Adults are slender, dark brown or black and covering with hairs and have very short wing covers. Adults are nocturnal so are best released in the evening. Both adults and larvae tend to hide in cracks and crevices of the growing media.

Bacillus thuringiensis var. israelensis, sold under the trade name of Gnatrol WDG, is most effective against the young first instar larvae. The bacteria must be ingested by the larva, after which a toxic protein crystal is released into the insect's gut. Larvae stop feeding and die. Gnatrol WDG is only toxic to larvae for two days. Repeat applications, i.e. two or three applications at high rates, may be needed to provide effective control.

Chemical Controls

Insect growth regulators, microbials, and other pest control materials may be applied to the growing media to manage fungus gnat larvae.  Repeat applications may be needed.  For well-established populations, applications of an adulticide may also be of benefit.  For more information see the latest edition the New England Floricultural Crop Pest Management and Growth Regulations Guide: A Management Guide for Insects, Diseases, Weeds and Growth Regulators for more specific guidelines. Available from Northeast Greenhouse Conference and Expo and the UConn CANR Communications Resource Center.

REFERENCES:

  • Cabrera, A, R. Cloyd and E. Zaborski. 2003. Effect of Monitoring Technique in Determining the Presence of Fungus Gnat (Diptera: Sciaridae), Larvae in Growing Media. Journal of Agricultural Urban Entomology 20(1): 41-47.
  • Cloyd, R. and E. Zaborski. 2003. Fungus Gnats, Bradysia spp. (Diptera: Sciaridae) and other Arthropods in Commercial Bagged Soilless Growing Media and Rooted Plant Plugs. Journal of Economic Entomology. 97(2): 503-510.
  • Cloyd, R., A. Dickinson, and K. Kemp.  2007. Effect of Diatomaceous Earth and Trichoderma harzianum T-22 (Rifai Strain KRL-AG2) on the Fungus Gnat Bradysia sp. Nr. Coprophila (Diptera: Sciaridae).  Journal of Economic Entomology 100 (4): 1353-1359.
  • Lindquist, R. 1998. Fungus gnat and shore fly management. Proceedings of the 14th Society of American Florists Insect and Disease Management on Ornamentals.
  •  L. Stack. (ed.) 2013-2014. New England Greenhouse Floricultural Recommendations - A Management Guide for Insects, Diseases, Weeds and Growth Regulators.
  • Meers, T. and R. Cloyd. 2005. Egg-Laying Preference of Female Fungus Gnat Bradysia sp. nr. coprophila (Diptera: Sciaridae) on Three Different Soilless Substrates. Journal of Economic Entomology. 98(6): 1937-1942.

Leanne Pundt, Extension Educator , University of Connecticut 1999.
Revised January 2006.  January 2013

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