• Topics: Entomology and Nematology  | Ornamentals (IMG -- Vol.1, Ch. IV)  | Fungus Gnats  |  Price, James F  |  Osborne, Lance S  |  Nagle, Curtis A  | McCord, Elzie Jr. 

Management of Fungus Gnats in Ornamentals¹

Fungus gnats can be present in flower and foliage production areas during any part of the year, but hot rainy periods are ideal. Growers can manage fungus gnats successfully and ship healthy potted plants without shipping the insects.

Biology

Fungus gnat adults (Figure 1) are about 1/8 inch long, spindly flies with long legs and long, thread-like antennae. They look more like tiny mosquitoes than common flies. Larvae live in the soil and are difficult to find. If fungus gnats are present, larvae are most likely found in the early morning in the topsoil layer of a thoroughly wet pot. Larvae are translucent gray to white, about 1/4 inch long, worm-like with no legs, and with shiny black heads. There is not a similar insect in the production area or interiorscape.

Figure 1. 

Fungus gnat.

These insects can infest a crop from soil or algae under benches, from contaminated potting soil or by flying short distances into the production area. Fungus gnats almost always are present in growing areas, at least at low densities.

Life Cycle

Females live about a week and lay 30 to 120 eggs singly or in batches of up to 30 on the soil. Eggs hatch in 4 to 7 days (Steffan 1966). Larval development requires about 8 to 20 days, depending upon temperatures. The resting pupal stage lasts about 3 to 5 days and is located near the soil surface.

Adult flies do not damage plants directly, but are objectionable to consumers and cannot be tolerated on potted plants in hospitals, grocery stores, or florist shops. Adults can emerge from immature forms after sale even when none were evident earlier.

Larvae feed on decaying matter and on healthy and diseased roots in the soil medium. They can be particularly damaging to seedlings and other small plants. Damaged roots provide conditions for root diseases, further complicating fungus gnat management and crop health.

Management

Sound crop management denies fungus gnats the conditions necessary for development, reduces need for pesticides, and promotes parasites. Fungus gnat problems may result from over-wet conditions and diseased roots, and should alert growers to poor cultural conditions. Potting media should be stored dry, and pots and production areas must be well drained. Fungus gnats can exist on soil fungi, algae under benches, and on damp mossy benches. Some growers apply hydrated lime to eliminate the fungal food source.

Biological Control

Sometimes naturally occurring parasites can become established and regulate fungus gnat populations. This frequently occurs when broad-spectrum pesticides are not used in the production area. Fungus gnat parasites are fragile appearing wasps, much smaller than fungus gnats and may be seen walking on pot surfaces.

Adult fungus gnats are attracted to yellow sticky traps. These traps can be used to detect their presence or scouts can look for adults moving on pot surfaces. If fungus gnats become a problem within a few weeks of anticipated sales of potted plants, a pesticide control program may be required.

Pesticides

Pesticides for controlling fungus gnats (Table 1) can be applied as drenches to pots or as sprays to foliage, pots, beds, or other soil surfaces as label directions indicate. Many of the products act as larvacides when applied as drenches to pots and soil underneath benches. Treated larvae will die, as will larvae that develop from the eggs already in the soil. However, these materials do not kill adults present at application nor the adults that will develop from pupae present at application. Therefore effects may seem disappointing for the first few days.

Hamlen and Mead (1979) demonstrated that some insecticides applied as sprays to soil surfaces were as effective for fungus gnat control as were drenches of the same materials. Sprays normally should be reapplied once or twice at 10-day to 2-week intervals.

The best growers stress good water management and good root health, and are prepared to use insecticides correctly when fungus gnats are not otherwise controlled. When such practices are followed, fungus gnats should not be a problem.

Table 1 summarizes the chemical control measures that are available to commercial flower producers in Florida and includes microbial insecticides that may be considered components of biological control. Pesticide labels sometimes provide for the product's use on any “ornamental,” “foliage,” or “flower” crop and sometimes for use only on specifically named flower or foliage crops. This summary provides the chemical options available for use in the field, shade house and greenhouse production and interiorscape maintenance settings for any ornamental flower or foliage crop and the additional specific options available for use on certain named flower or foliage crops.

Table 1 is organized into two target crop categories, beginning with a general category “All Flower and Foliage Crops.” This is followed by a short section that refers the reader to product labels to find specifically named flower and foliage crops.

Commercial products available for control of fungus gnats are grouped by active ingredient. Usually, only one example of any particular formulation is given. Other examples or other formulations may exist that are as effective as those mentioned. Notes are provided to qualify some uses and precautionary statements and re-entry intervals are given to aid in the safe use of the pesticides. Hyperlinks to the indicated pesticide labels are not provided in the electronic forms of this summary, but a search at the Websites of CDMS (http://www.cdms.net/manuf/default.asp), C&P Press (http://www.greenbook.net/) or the affiliated manufacturer may provide the desired label.

This summary is only a guide to aid in the selection of pesticides. Care has been given to provide accurate and up-to-date information, but it is possible that, through label changes, author error, etc., improper uses may be indicated. In all cases it is the applicator's responsibility to read and comply with the label that accompanies each pesticide container.

Warnings

All insecticides should be handled with caution and the safety precautions on the container must be followed. Wear protective devices and clothing if they are indicated. Read the entire label, including the small print, before opening the container. Avoid drift of pesticides to adjacent areas or to crops that may be eaten by man or animals. Do not allow pesticides to get into streams or water supplies. Store pesticides in their original labeled containers, out of reach of children, irresponsible people, pets, and store pesticides under lock and key. When containers are empty, rinse with water three times and pour rinsate into the spray tank. Dispose of empty containers promptly and safely.

As an additional precaution, keep the telephone number and address of the nearest County Poison Control Center listed in a convenient location in case of an accidental pesticide poisoning. Also, keep clean copies of labels of all pesticides that are on the premises. In the event of a poisoning, the label of the pesticide involved should be taken to the Poison Control Center or hospital. If a product is labeled for use specifically in Florida (special local need), make sure you get a copy of the supplemental label from your supplier when you purchase the product.

References Cited

Hamlen, R. A. and F. W. Mead. 1979. Fungus gnat larval control in greenhouse plant production. J. Econ. Entomol. 72(2):26971.

Steffan, Wallace A. 1966. A generic revision of the family Sciaridae (Diptera) of American North of Mexico. Univ. of Calif. Pub. in Entomol. 44-16-22.

Tables

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.

U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer-Chancy, Interim Dean.

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