Fungus Gnats Management
Overview
Fungus gnats are pests of greenhouses, nurseries, and interior plantscapes. Although they do not bite or transmit pathogenic organisms to human and their animals, they can be considered a nuisance by their presence in and around structures, especially when they appear in a large number. Adult flies are short-lived insects and may be noticed away from their breeding sources flying around light fixtures or windows; therefore, a successful management plan of fungus gnats can only be accomplished by understanding of their biology and habits in the infested area.
Identification
Adults are mosquito-like, small (1/8 inch long), fragile grayish to black flies with long, slender legs and thread-like antennae. They have one pair of clear or smokey-colored wings with no pattern and few distinct veins (the common Bradysia species have a Y-shaped wing vein). Larvae are clear to creamy-white and can grow to about 1/4 inch long. They have shiny black head capsules.
Biology
- Eggs hatch in 4 to 6 days
- Larvae feed for 12 to 14 days on fungi and decaying organic matter, root hairs and small feeder roots
- The pupal stage is completed in about 5 to 6 days.
- Newly emerged adults live about 7 to 10 days. Females lay up to 100 to 300 eggs in 2 to 30 batches in decaying organic matter on the moist soil surface or in soil cracks.
- The life cycle lasts about four weeks, with continuous reproduction where warm temperatures are available. There are many overlapping generations per year.
Habits
- Fungus gnat's species reproduce outdoors in moist, shaded areas in decaying organic matter such as leaf litter, compost piles, grass clippings, feces, etc. Although the larvae rarely attack healthy plant roots, they may chew at a few roots and reduce the vitality of plants.
- Fungus gnats occasionally become a nuisance pest indoors when adults emerge in large numbers from potted plants or flower boxes containing damp soil rich in humus with microscopic fungi and decaying matter. Indoor plants with over-watering or poor drainage systems create ideal breeding sites of fungus gnats. Additionally, fungus gnats can survive indoors in damp new construction materials and in areas near leaky roofs or water pipes.
- Adults are attracted to lights and are often first noticed at windows.
- Moisture is key to their survival, as eggs and larvae cannot last in dry environments.
Management
Non-chemical Methods:
Find and eliminate the breeding 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 flowerpots. 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.
Monitoring
Place yellow sticky cards in samples of growing media to monitor for any emerged adults. Yellow sticky cards, placed horizontally at the soil surface, can be used to detect adults. Check and change the cards weekly to determine population trends.
Orienting traps horizontal to the ground (facing the soil) is sometimes recommended when monitoring fungus gnats emerging from media. Vertical trap orientation (perpendicular to the soil surface) is more efficient overall if traps are also being used to monitor adults of other kinds of insects. Orient vertical traps so their bottom is even with the top of the plant canopy. Regularly adjust traps upward as plants grow. Hang traps from wires or use clothespins to clip the trap to a stick placed in media. Inspect traps at the same regular interval, once or twice a week. Count and record the number of each type of pest caught. Counting only the insects in a vertical, 1-inch-wide column on both sides of the trap gives results that are representative of the entire trap. Do not reduce trap size to 1-inch vertical strips.
Insecticides
Adult fungus gnats are managed easily with liquid pyrethrins 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 registered and approved residual pesticides. Strictly follow all directions and precautions on the pesticide label. The key thing is to eliminate algae, molds and decaying organic matter wherever they are found. There are some pesticides registered for algae and fungi control under and around container-grown plant benches.
