CUBECAP™

Insect Note - ENT/rsc-29

By: Michael Waldvogel, Entomology Extension

Fungus gnats can be a serious pest in greenhouses where they can damage the roots of plants. They also feed on fungi and decaying plant material. For more information about fungus gnats as pests of ornamentals, consult Ornamentals and Turf Insect Note 29 Occasionally, fungus gnats become a nuisance indoors in both residential and commercial buildings when adults emerge in large numbers from potted plants or some other chronic source of moisture (and subsequent mold growth). Adults are attracted to lights and are often first noticed at windows or light fixtures.

Identification

Adult fungus gnats are about 1/10 to 1/8 inch long, slender, somewhat mosquito-like, with dark-colored antennae and delicate with long legs. Some species are gray to black in color, while others are orange-to-yellowish in color. Identification can be made by the vein patterns in the single pair of wings, which may be smoke-colored or light in color. One of the most common species, the darkwinged fungus gnat adults has eyes that meet above the base of the antennae. The eggs are extremely small, oval, smooth, shiny white and semi-transparent. Larvae or maggots are legless, thread-like, white, with black heads and 1/4 inch long fully-grown. They are somewhat transparent so food in their guts can be seen through the body wall. The larvae spin silk-like cocoons in which they form pupae.

Life Cycle and Habits

Fungus gnats develop in moist, shaded areas in decaying organic matter such as leaf litter. They complete their life cycle in about four weeks depending on temperature and other environmental conditions. With indoor infestations there will typically be continuous reproduction and overlapping generations with all life stages likely to be found throughout the year. Adults fungus gnats live about 7 to 10 days. The female deposits 100 to 300 eggs in batches of 2 to 30 each in decaying organic matter. Eggs hatch in 4 to 6 days; larvae feed for 12 to 14 days. The pupal stage lasts about 5 to 6 days.

Correcting Fungus Gnat Problems

Non-Chemical Control

The key to solving indoor fungus gnat problems is to find and eliminate the source, i.e., find the area(s) of excess moisture.

Outdoors - If the problem is seasonal, i.e., it declines or totally disappears in the fall/winter then, it is quite likely (but not always) the case that the source is outdoors. The most likely problem spots are landscaped and/or heavily mulched areas or lowlying areas in the yard that remain extremely wet following heavy rainfall or a leaking outdoor water spigot

Indoors -
If the problem is relatively constant regardless of the time of year, then the source is more likely to be originating indoors or at least it is associated directly with the structure rather than an outside (landscaped) source.

Potted plants and other types of interiorscaping are often the culprits. Check plants to see if the soil is excessively wet. Drain any excess water from the dish/container below flower pots. If the weather permits, move the plants outdoors or allow the soil to dry (not to the point where plants will wilt). Then, increase the interval between regular watering and the problem should decline.
Check areas where moisture is commonly found: kitchens, bathrooms, utility rooms (e.g., a leak in the waterline to a clothes washer), as well as crawlspaces (which are more common in residential buildings).
Check other areas where leaks may occur as a result of storm damage or poor maintenance, e.g., roofs and crawlspaces. This is particularly important on commercial buildings with flat membrane-covered (e.g., EPDM) roofs. A leak through a break in the membrane material can cause the underlying insulation to become wet and subsequently moldy - an ideal habitat for fungus gnats. Inspect the roof and ceiling below for signs of leaks.
Try to determine what areas of the building have the highest numbers of gnats. Check window ledges, light fixtures, etc. Fungus gnats are mobile and are more likely to move towards a nearby source of light. Pest control companies use UV light traps to help determine which areas are most heavily infested.
Correcting a moisture problem and cleaning the area should eliminate the fungus gnats fairly quickly.

Insecticides

Adult fungus gnats are killed easily with pyrethrins spray or aerosols labeled for use against "gnats" or "flying insects." However, these chemicals are a very short-term and very temporary solution. Unless you can access and treat the source then, once the chemical dissipates, more flies are likely to appear. Similarly, treating outdoor areas may produce mixed results particularly if you cannot identify the key areas that are infested with fungus gnats. Indoor plants or interiorscaping that is difficult to remove can be treated with a number of pesticides that are listed in the NC Agricultural Chemicals Manual . Among the less hazardous products available to consumers are those containing Bacillus thuringiensis var. israelensis (Gnatrol, Vectobac). These products can be applied as soil drenches to potted plants indoors or to outdoor areas. Outdoor landscaped areas can also be treated with insect parasitic nematodes. You can click here for a list of suppliers of products containing nematodes. Regardless of how "safe" you consider any pesticide or insect control product, always read the label and follow directions and safety precautions.

Pest information and control recommendations presented here were developed for North Carolina and may not be appropriate for other states or regions. Any recommendations for the use of chemicals are included solely as a convenience to the reader and do not imply that insecticides are necessarily the sole or most appropriate method of control. Any mention of brand names or listing of commercial products or services in the publication does not imply endorsements by North Carolina Cooperative Extension nor discrimination against similar products or services. All recommendations for pesticide use were legal at the time of publication, but the status of pesticide registrations and use patterns are subject to change by actions of state and federal regulatory agencies. Individuals who use chemicals are responsible for using these products according to the regulations in their state and to the guidelines on the product label. Before applying any chemical, always obtain current information about its use and read the product label carefully. For assistance, contact the Cooperative Extension Center in your county.

Distributed in furtherance of the acts of Congress of May 8 and June 30, 1914. North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, or disability. In addition, the two Universities welcome all persons without regard to sexual orientation. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating

Fungus gnat image courtesy of D. Morrison, NC Coop. Ext. Service, Scotland Co.
Some information used in this publication was adapted from W. Lyon, Ohio State University

Last updated - May 2004

This text can be found in its original form at:
http://www.ces.ncsu.edu/depts/ent/notes/O&T/flowers/note29/note29.html