Fungus Gnats  

by Lance S. Osborne, Ph.D. and W. Chris Fooshee, M.S.
University of Florida, Mid-Florida Research & Education Center
Apopka, FL 32703


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Foliage growers and retailers frequently become concerned about tiny "black flies" which can become abundant under greenhouse and indoor plant growing conditions. These flies often are fungus gnats and represent a potential thereat to plants. A key (Fly ID Key) for identification of flies, including fungus gnats is available. Adult fungus gnats are slender with comparatively long legs and antennae. They are grayish-black and about 1/8 inch long (2.5mm). The adult (Figure 1) is a weak flier and is most visible near or on the soil surface or under leaves. Small yellowish-white eggs (Figure 2) are deposited on the soil surface. The immature legless larva (Figure 3) has a white to clear body with a shiny-black head and when fully grown is about 1/4 inch (5.5mm). This stage lives in the soil. The larva develops into a pupa  (Figure 4) in the soil, which is white but darkens as it matures. The larvae have been associated with feeding on and decay of plant roots and lower stem tissues (Figure 5). Highly organic soils of high moisture content appear to enhance infestations, especially in the presence of decaying plant tissue. Feeding may be particularly injurious to seedlings, rooted  cuttings (Figure 6) or young plants. Symptoms first become apparent when plants begin to wilt or develop an unhealthy appearance. Dark-winged fungus gnat adults are usually noticed before injury caused by the larvae is apparent. 




Fungus gnats are prevalent during most of the year, but they develop significant populations in winter and spring or when the weather has been cloudy and overcast for a number of days. All stages can be found in moist, protected areas. Developmental periods depend on temperature. In general, the larvae hatch from eggs about 4 days after being laid. In about 10-140 days the larvae begin to pupate. The pupal stage lasts 3-4 days. The adult emerges, mates and beings to lay from 100-150 eggs during their 5-7 day life. Adults are weak fliers; however, they can run rapidly on the soil surface or remain motionless. 



Fungus gnats have always been considered a pest in greenhouses and interiorscapes. For this reason the University of Florida has a publication entitled:Management of Fungus Gnats in Greenhouse Ornamentals (SS-ENY-912 [Extension Entomology Report #74] by Jim Price, Don Short and Lance Osborne. This report is updated annually. It describes fungus gnat biology and gives a list of the registered pesticides for their management. Keep in mind that fungus gnats are not the only flies that can be found in significant numbers in greenhouses or interior landscapes. We also find flies in the families Psychoidae (moth flies) (Figure 7),  Drosophilidae (vinegar or fruit flies) (Figure 8),Ephydridae (shore flies)(Figure 9), and an amazing array of predatory flies and parasitic wasps (Figure 10). The newly hatched larvae will begin feeding on organic material such as dead leaves. They will also attack the root hairs and roots and eventually work their way up the plant and into the stem. Fungus gnat larvae also transmit plant diseases such as Pythium spp., Phytophthora spp., and Fusarium spp. 


High numbers of adult flies result in many nuisance complaints by the public and workers. Some office workers have even complained that these flies were biting people. Fungus gnats do not bite people; they don't have the mouthparts that would enable them to bite. 

Detection of fungus gnats is relatively easy. Larvae can often be found under decaying plant material on the soil surface. Their presence can be readily detected by placing a small piece of potato or carrot (Figure 11) on the surface for a couple days and then observing it with the aid of a hand lens. Larvae also produce thin "webs" on the soil surface which become obvious early in the morning when droplets of moisture collect on them. Adults "flit" around on the soil surface or fly in close proximity to the plant. They are attracted to yellow sticky traps. 

  Control of fungus gnats can be difficult but not impossible. Proper water management is crucial in controlling both fungus gnats and shore flies. When soil stays moist, the environment becomes very conducive for development of fungus gnat populations. Secondly, many interior plant maintenance contracts require pots be top dressed with bark nuggets; this material, placed on a moist soil surface, can also contribute significantly to a gnat problem.  
  Most controls are directed at the larval stages, with the most common being chemical drenches to infested soils. In addition to the many older materials that have been recommended for controlling fungus gnats, new chemicals are available. Many of these new material are insect growth regulators. Because these materials interfere with the development of the immature stages they don't have much activity on the adults. If a rapid reduction in adult activity is required, they will not be effective.  

Biological control is another option that has had success in a number of Florida greenhouses. Under these conditions insect pathogenic nematodes (Figure 12) are the control agent of choice. Nematodes are becoming widely accepted as a valuable alternative to pesticides. Beneficial nematodes do not attack plants. The infective stages are applied to the soil using standard spray equipment or through the irrigation system. The infective stages are then able to find and kill the fungus gnat larvae. As with any biological control agent, the end user must check the product for quality. In this case, you must inspect the material and determine if the nematodes are alive.   
  Some success has been obtained with a predatory mite called Hypoaspis miles. This predator lives in the soil and feeds on young stages of fungus gnats, some thrips and springtails. One generation takes about 10 days. The use of this mite is not recommended when populations are high or as a method to eliminate the population completely. When used properly, they keep populations low. The utility of using biological control on plants in situations other than commercial nurseries has not been critically evaluated but some successes have been reported.