Fungus Gnat Management:
Bastiaan M. Drees, Professor and Extension Entomologist
Department of Entomology
Texas A&M University System
The term, fungus gnat, applies to a number of species in the insect order Diptera, family Sciaridae (dark winged fungus gnat species Bradysia coprophila, impatiens and paupera). Fungus gnat adults are a nuisance to greenhouse operators, interiorscapers as well as consumers. The larval stages can damage healthy roots, stunting or killing young plants even where there is no fungal food source (Lindquist 1994). Prolonged infestations may cause stunted, off-colored plants or foliage (Cole 1985). Damage may actually be more severe to young plants when the potting media or soil has been sterilized. Fungus gnat larvae may also aid in the introduction and spread of plant diseases such as Pythium, Verticillium, Cylindrocladium, Scelerotinia and Theilaviopsis.
Description: Adult fungus gnats are small (1/8 inch long), fragile grayish to black flies with long, slender legs and thread-like antennae. Their wings are clear or smokey-colored with no pattern andfew distinct veins. Larvae are clear to creamy-white and can grow to about 1/4 inch long. They have shiny black head capsules.
Biology: Fungus gnats develop through complete metamorphosis: egg; larva; pupa; and adult. Development occurs in 2 to 4 weeks. Larvae feed primarily on fungi, decaying organic matter and plant roots, particularly in very moist environments. Larval and pupal stages can also, however, survive periods of drought. Fungus gnats normally follow a predictable cycle of population development: The first two generations are the largest, followed by a leveling off or decline in numbers.
Continuous production (ie. adding new plants in fresh growing media) may keep fungus gnat numbers high, because the insects will keep moving to the potting mix containing the fresh media (Lindquist 1994).
Monitoring methods: Yellow sticky cards and potato pieces placed on potting media are good methods for monitoring adult and larval
populations, respectively.
Yellow stick cards are most effective when place horizontally on the potting media surface, although vertically positioned cards hung over the crop canopy are also effective and trap more of other types of insect pests as well. Cards placed under the benches, close to intake vents, near doorways and outside the greenhouse can provide additional information to detect breeding areas. Continuous monitoring (weekly) can provide the following information: 1) first detection of low populations; 2) population density increases or decreases over time; 3) the level and length of suppression resulting from the implementation of suppression tactics.
Potato slices (roughly 1 by 1 by 1/4 to 1/2 inch pieces) placed on the surface of potting media are attractive to larval stages of fungus gnats. The potato slices should be left in place for about 4 hours before counting the number of larvae on and under the slices (Lindquist 1994). Results of this monitoring effort can be used to:
1.detect areas where larvae are developing
2.document the reduction of larval population densities after the implementation of suppression tactics by comparing results from before ant after treatment.
Management: There are no "economic threshold levels" established for managing this pest (Lindquist 1994). Thus, the decision to suppress populations is largely subjective, although regulatory or marketing forces may play an important role in the decision-making process. Fungus gnat control can be an important part of managing some plant diseases.
Integrated pest management of fungus gnats is best when a combination of non-chemical (cultural) methods are used with biological and/or chemical methods. Table 1 provides an inventory of considerations and options which can be incorporated into a fungus gnat management program. Cultural methods are largely preventive and must be maintained throughout the production cycle. Biological methods must be implemented early, when fungus gnat population levels are still low. Insect growth regulator products applied to the potting media are more compatible with biological control. However, once a severe problem has developed, an insecticide program using a combination of treatments for adult as well as larvae fungus gnat control may be necessary.
Literature cited:
Cole, C. L. 1985. Fungus Gnats, L-2041. Texas Agricultural Extension Service, Texas A&M University System, College Station, Texas.
Drees, B. M. 1992. Pest Management Alternatives for Commercial Orna-mental Plants. Texas Association of Nurserymen. Austin, Texas. 140 pp.
Lindquist, R. K. 1994. Integrated management of fungus gnats and shore flies in Proc. 10th Conf. Insect & Disease Management on Ornamentals (Ed. K. Robb), pp. 58-67, Society of American Florists, Alexandria, Virginia.
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