FACT SHEETS>PEST MANAGEMENT>MANAGING FUNGUS GNATS AND SHORE FLIES

Fungus Gnats and Shore Flies

Identification
Fungus gnat larvae (Bradysia spp.) are white and legless, about 1/4 inch long when mature, and have a shiny black head. The adult is mosquito-like in body shape, about 1/8 inch long, with long legs, a clear pair of wings, and long antennae. Fungus gnats are weak fliers and are frequently observed resting on the media in the pot or running over the foliage or other surfaces.

The fungus gnat is sometimes confused with another small dark-bodied fly called the shore fly, Scatella stagnalis. Shore flies have more robust bodies than fungus gnats and their antennae are very short. Their most distinguishing characteristic is the presence of five light-colored spots on each of their dark wings. Shore flies are also stronger, faster fliers than fungus gnats. In the larval stage shore flies can be distinguished by the opaque, tannish-brown color of the body and the absence of a head capsule.

Damage
Fungus gnats and shore flies are attracted to damp locations where fungi are apt to flourish. Fungi are a major part of their diet. Studies have shown that fungus gnats develop more rapidly and have greater survival on fungal diets. In the absence of a fungal food source however, fungus gnats are capable of feeding on healthy plant tissue. Also, they are general feeders and can injure a number of flower crops grown in the greenhouse. Shore flies are not known to feed on healthy plant tissue.

Fungus gnat larvae are observed feeding on roots, fungi and decaying matter, often injuring bulbs, seedlings and plants with succulent stems and roots. The burrowing of larvae in plant tissue promotes decay. Both fungus gnat and shore fly adults and larvae can spread spores of fungal pathogens, such as Pythium species, among plants. Adult flies of both become a nuisance when present in large numbers. Larvae present in infested plants or soil can lead to prolonged emergence of adults.

Life Cycle of Fungus gnat
The life cycle of fungus gnats is shown in Figure 1. A female fungus gnat may lay up to 300 whitish eggs in clusters of 20 to 30 or more on the surface or in the crevices of moist soil or potting media rich in organic matter. Eggs hatch in about six days. Larvae feed for 12-14 days before changing into a pupa, which is formed inside a silken pupal chamber in the soil. The pupal stage may last 5-6 days and adults live up to 10 days. The life cycle from egg to adult requires approximately 4 weeks depending on temperature; development time decreases as temperatures increase, as is true of most insects.

Figure 1. Life cycle of the fungus gnat

Life Cycle of Shore fly
Eggs are laid singly on the surface of algae. Larvae have eight pairs of short legs and a breathing tube with two dark colored openings called spiracles at the posterior end. Pupation occurs at the edge of the algae mats. Breeding takes place in stagnant and strongly saline water found in greenhouses as a result of excessive irrigation and soil leaching. To reduce shore fly numbers, eliminate algae, avoid over watering, and limit fertilizer run-off.

 Management Strategies
* Fungus gnat and shore fly populations may be partially suppressed by sanitation practices that reduce breeding areas.
* Wet areas under benches should be eliminated if possible by controlling the use of water or changing the construction of the floor. Accumulations of soil, media, or decayed plants under benches should be avoided.
* Potting media should be pasteurized before use if possible.
* Moist potting soil high in organic matter that has been left outdoors for long periods may contain fungus gnat larvae. Fungus gnats and shore flies may also be introduced into the greenhouse in the media of infested plants purchased from other greenhouses.
* Keep areas below benches free of spilled potting mix, weeds and other debris in which fungus gnats and shore flies might breed.
* Eliminate standing pools of water on solid benches, on walks, and under benches. This may be accomplished by proper grading and drainage in the greenhouse and by improving watering practices to prevent runoff.
* Eliminate algae as best you can. Several algicides are currently registered for algae control in the greenhouse. Disinfectants can be used as part of pre-crop clean up program and during the cropping cycle for routine algae management. Green-Shield, Physan 20, Triathlon (Quaternary ammonium compounds) can be applied to floors, walls, benches, tools, pots and flats as disinfectants. ZeroTol (Hydogen Dioxide), is a sanitizer also labeled for use on greenhouse surfaces. Read and follow directions on these products.
* Thoroughly inspect all incoming plant material and make early treatments.

It should be noted that in 1985, Dr. Dick Lindquist of Ohio State 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.

It has also been reported that a pulse in adult emergence follows watering of pots that were previously allowed to dry down. This expected peak in adult fungus gnat populations can be used to time sprays targeting adults or subsequent drenches aimed at larvae.

Prevention and early detection are the keys to controlling this insect, thus reducing the introduction and spread of disease in the greenhouse.

Detection and Monitoring
As with any pest, fungus gnat and shore fly control programs are built on prevention and monitoring. To prevent infestations, establish a weekly scouting/monitoring routine for the duration of the crop.

To monitor for larvae, place raw potato chunks with peel removed on the soil surface. Larvae are attracted to the potato chunks, under which they move and congregate. Check the potato chunks daily for larvae. Potato disks cut one inch in diameter and 1/2 to1 inch thick work well. In addition, choose plants on each bench and inspect the soil surface and around the base of the plant including the stem just below the soil line. Record the location and the level of infestation. Badly infested containers of plants should be removed as they serve as a source of infestation.

Adult flies can be monitored with yellow sticky cards placed at the base of the plant at soil line. Weekly inspections of yellow sticky cards can detect the onset of an infestation, and continued recording of the number of adults per card per week can aid in evaluating the efficacy of control efforts.

Space 3" x 5" yellow sticky cards 1-4 per 1,000 sq. ft. throughout the greenhouse. Place yellow cards in a horizontal position just above the soil surface, or lay them on the top of the pots. For early detection, position cards near doorways and vents or among new plants being placed in the house. If time permits, check the cards twice weekly particularly when temperatures warm up in the spring. Research conducted at Cornell University showed that red sticky traps captured more fungus gnat adults than yellow sticky traps. However, yellow sticky traps are commercially available and can be used for a variety of pests in the greenhouse.

Once fungus gnats and/or shore flies begin appearing on sticky cards or larvae are seen under potato chunks, then it is time to make treatment decisions.

Biological Control
Several biological control organisms are available for control of fungus gnats including the microbial insecticide Gnatrol (containing the bacterium Bacillus thuringiensis israelensis), a predaceous mite (Hypoaspis miles) and parasitic nematodes Steinernema carpocapsae and S. feltiae. These nematodes have been applied at a wide range of application rates and have produced variable levels of control in different trials.

None of the biological control products sold for suppression of fungus gnats have been shown to be effective against shore flies. A eucoilid parasitoid (Hexacola sp.), however, often occurs naturally in greenhouse and may slow the growth rate of shore fly populations.

Treatments
Soil treatments (pot drenches and sprenches) at the first sign of insect activity are best used to manage fungus gnats. Direct treatment toward the larval stage. Make sure material is applied to a depth of 1" or more. Materials are most effective when they are retained in the media. Keep this in mind when fertilizers, fungicides and water all need to be applied to plants. For example, if a fungicide and an insecticide is necessary, apply the fungicide first, then water it in with the insecticide. The fungicide will be moved into the root zone where it is needed and the insecticide will stay in the top where it is needed. Some insecticides are labeled for use on greenhouse floors and under benches in addition to treatments to pots. Carefully read and follow all label directions.

Fungus gnats are common pests on plants such as geraniums, poinsettia, begonias and bulbs, especially if soilless mixes high in peat moss or immature compost are used. Insecticides may not affect eggs or pupae, and repeated applications may be necessary.

The following is a partial list of options for managing fungus gnats and/or shoreflies. See label for information on application methods and rates, and plant safety. The asterisk indicates the type of product.

Adept (diflubenzuron), (*insect growth regulator)
For fungus gnat and shore fly larve. Available in 1 oz water soluble packets. Apply 1-2 oz per 100 gal water. Use lower rate as drench.

Astro (permethrin), (*pyrethroid)
For adult fungus gnats. 

Attain TR, Talstar Flowable (bifenthrin), (*pyrethroid)
For adult fungus gnats.

Azatin XL, Ornazin 3% EC (azadirachtin), (*botanical)
When ingested or absorbed by insect larvae, it interrupts the molting process. Apply as a soil drench for fungus gnat and shore fly larvae.

Citation (cyromazine), (*insect growth regulator)
For fungus gnat and shore fly larvae. Available in water soluble packets. One packet per 100 gal. water treats one acre.

Decathlon 20 WP (cyfluthrin), (*pyrethroid)
For adult fungus gnats. 

Distance (pyriproxyfen), (*insect growth regulator)
For fungus gnat and shore fly larvae. 

Duraplex TR (chlorpyrifos & cyfluthrin), (*organophosphate and pyrethroid)
Micro total release for adult stages of fungus gnats and shore flies. Available in 2 oz and 6 oz cans.

DuraGuard ME (chlorpyrifos), (*organophosphate)
For fungus gnat and shore fly larvae and adults. Apply this microencapsulated formulation as a soil surface spray, following label directions. 

Enstar II (s-kinoprene), (*insect growth regulator)
For fungus gnat larvae and adults. An insect growth regulator that delivers an artificially high level of juvenile growth hormone preventing the insect from progressing to the next stage of its life cycle. Apply as a drench. Do not use in irrigation system. Follow label instructions. 

Gnatrol (Bacillus thuringiensis), (*microbial)
For fungus gnat larvae. Gnatrol is not effective against shorefly larvae. Apply a drench or in irrigation system according to label directions. May be applied to soil below benches. Larvae must ingest the bacteria; they then stop feeding and die. It is reported to be toxic to larvae for only 48 hours, so treatments must be repeated. 

Hypoaspis miles (predatory mite), (*natural enemy)
A soil-dwelling predaceous mite that feeds on larvae. Introduce mites at planting or just after. They are well adapted to moist conditions and will survive in greenhouses in a variety of growing media throughout the year. They are supplied in 1 liter containers, generally containing about 10,000 mites. These containers have shaker lids to distribute them over the soil. University of Maryland recommends 10,000 mites per 1000 sq.ft. They are compatible with parasitic nematodes and Gnatrol. 

Marathon 1% Granular, 60 WP (imidacloprid)
For fungus gnat larvae in the soil.

M-Pede (insecticidal soap)
For shore fly adults. Contact insecticide. 

Nemasys, Scanmask, (Steinernema feltiae, parasitic nematodes), (*natural enemy)
For fungus gnat larvae. Best control is achieved with the first application made at planting or shortly thereafter. Two or three subsequent applications at weekly intervals may keep fungus gnat numbers low throughout 10 - 12 week crop. 
It is recommended that you repeat weekly applications three times. Late afternoon/early evening application is preferred. Exposure to strong sunlight can reduce the effectiveness.
Follow label instructions. 

Pyrenone, 1100 Pyrethrum TR (pyrethrins), (*botanical)
For adult fungus gnats. 

1300 Orthene TR (acephate), (*organophosphate)
Micro total release pesticide available in 4 oz and 12 oz cans. One 4 oz can treats 3000 sq. ft. 

Ultra-Fine Oil (paraffinic oil)
Golden Natur’l Spray Oil (soybean oil), (*horticultural oil)
For fungus gnat adults. Contact insecticides.

* Type of product

The use of trade names does not imply endorsement of products named or criticism of those not mentioned. The user bears sole responsibility for correct and legal product use.

References
2005-2006 New England Greenhouse Floriculture Recommendations. Section B.

Freeman R. Fungus gnats. Greenhouse IPM Update. Oct. 1991. 

Lindquist, R. 1993. Consider fungus gnat control early in your crop production programs. Greenhouse Manager. July. 

Lindquist R., McMahon R., Hoitink H., and Fynn P. Fungus Gnat Population Dynamics in Potting Mixes Differing in Suppressiveness to Pythium Root Rot. Ohio Florists' Association, Bulletin No. 748, Feb. 1992. 

Glenister, C. 1993. IPM Laboratories Quarterly. 5(1), IPM Laboratories, Inc., Locke, NY. 

Harris, M. Fungus Gnats: They're More Than Just a Nuisance. Grower Talks. January 1993. 

McLeod M. Monitoring Fungus Gnats and Shore Flies. Suffolk County Agricultural News, Cornell Cooperative Extension. Oct. 1991. 

Ogden M. Try Bacteria to Control Fungus Gnats. Grower Talks. October 1991. 

Revised May 2005 by Tina Smith, UMass Extension Floriculture Program, Dept. Plant, Soil and Insect Sciences

Links to Further Resources on the Web

• The University of California's factsheet on Fungus Gnats, Shore Flies, etc includes information on damage; identification and lifecycle; monitoring; cultural, physical and biological controls and insecticides. Tools for Pest and Weed Management, Useful Resources, and an appendix of Microbial Pesticide Manufacturers and Suppliers.
• Ohio State's brief factsheet on Shore Flies and Fungus Gnats, includes several useful photos.
• Another factsheet from Ohio State, this time on Fungus Gnats only.
• University of Connecticut IPM Program's article on Managing Fungus Gnats and Shore Flies in the Greenhouse, with 5 photos to aid in identification. Special reference is also made to the role played by fungus gnats in the transmission of certain fungus diseases including Pythium, Verticillium, Thielaviopsis, and Botrytis.