Many commercial greenhouses have battled fungus gnats at one time or another. Left unmanaged, the insect can lead to crop losses. Fortunately, a great deal is known about practices that can be used to minimize fungus gnat populations. An especially useful source of information is the New England Greenhouse Update, maintained by University of Massachusetts and University of Connecticut Extension Floriculture Programs, see www.negreenhouseupdate.info. Much of the information below is adapted from that site.
Larvae do the damage
Although fungus gnat adults are easier to see, it’s the juvenile stage, or larvae, that can feed on tender young roots and stems of seedlings and cuttings. Feeding not only harms the plant directly, but may also provide an entryway for pathogens, leading to disease. Many plants are prone to damage by fungus gnat larvae, including African violets, poinsettias, carnations, Easter lilies, geraniums, cyclamens, bedding plants and foliage plants. Symptoms first appear as a loss in plant vigor; as damage progresses, plants may fade in color, begin to wilt and finally lose foliage. Roots with feeding damage often show brown scars.
Fungus gnat larvae are white and legless, about .25 inch long when mature and have a shiny black head. The adult is mosquito-like in body shape, about 1/8 inch long, with long legs, a clear pair of wings and long antennae. Fungus gnats are weak fliers and are frequently observed resting on the media in the pot or running over the foliage or other surfaces.
Yellow sticky cards are most effective for monitoring fungus gnat adults if placed horizontally on the soil surface, although vertical cards positioned at the top of the crop canopy are also effective, and they will trap more of other types of insect pests as well. Cards placed under the benches and near potting areas, etc. can help detect breeding areas.
The fungus gnat adult can be confused with another small, dark-bodied fly called the shore fly, which has a more robust body and antennae that are very short. Their most distinguishing characteristic is the presence of five light-colored spots on each of their dark wings. Shore flies are also stronger, faster fliers than fungus gnats. Shore flies are not known to damage plants.
Life cycle of fungus gnats
A female fungus gnat may lay up to 300 eggs on or in moist soil. Eggs hatch in about six days. Larvae feed for about two weeks before changing into a pupa, which takes five to six days to become an adult, which lives up to 10 days. The life cycle from egg to adult takes about four weeks, depending on temperature.
Handle potting mix to minimize fungus gnats
Adult fungus gnats are attracted to soil mixes with high microbial activity, or with high amounts of peat moss or hardwood bark. Avoid using mixes with immature composts, those less than one year old. No potting mix is completely immune to fungus gnat infestations. How the media is handled and stored may be more important than the type of mix used. If the mix is allowed to become moist, it may support more fungus gnat activity. Tears or openings in the bags enable resident fungus gnat populations to gain entry into the media bags. Store the media under cover and in sealed bags so it stays dry.
If potting mix becomes moist, it can attract fungus gnats, even before it’s used for planting. Be sure to store mix under cover and/or in tightly sealed bags with no holes or rips.
Fungus gnats prefer abundant moisture. Poor drainage, puddles and water leaks may lead to an increase in their population, in part because these conditions allow the fungi and algae they feed on to flourish.
Monitor for adults and larvae
Monitoring is a key part of any insect management program. Inspect incoming plugs for fungus gnat larvae or their damage. Place yellow sticky cards in samples of growing media to monitor for emerged adults. Check and change the cards weekly to monitor population trends.
Use potato plugs at least 1 inch in diameter placed on the soil surface to monitor for larvae. Place the plugs in contact with the media to ensure they do not dry out. Clearly mark the pots where you placed the potato plugs so you can easily find them. After 48 hours, check the growing media under the plug for larvae as well as the surface of the potato itself.
Fungus gnats thrive in moist environments, so take care not to overwater flats or pots to the point that excess water collects under benches.
In addition to the above cultural practices, pest control materials may be needed to control fungus gnat populations. Insect growth regulators, biological controls or synthetic insecticides labeled for fungus gnats may be applied to the growing medium. Most materials are targeted at fungus gnat larvae and do not affect eggs or pupae, so repeat applications will likely be needed.
The soilborne bacterium Bacillus thuringiensis (B.t.) israelensis (sold as Gnatrol) should be used before fungus gnat larval populations are high, since the bacterium must be ingested in order to be effective. Applications are more effective on the young larvae than on more mature larvae. B.t. should be applied until fungus gnat populations start to decline, since eggs keep hatching into new larvae. Apply a drench or in irrigation system according to label directions. It is reported to be toxic to larvae for only 48 hours, so treatments must be repeated.
Hypoaspis miles is a mite that feeds on young fungus gnat larvae and will also feed on thrips pupae as well as organic debris and algae. It is available from greenhouse IPM suppliers. Be sure to release predatory mites early in the growing season before fungus gnat larval populations are abundant. Avoid applications into the growing media prior to planting because this decreases survival. Applications need to be initiated after planting, and the growing medium should be moist but not saturated. Applications can also be directed to the soil beneath greenhouse benches. Hypoaspis mites are active when growing medium temperatures are greater than 50 degrees.
Steinernema feltiae is an insect-killing nematode that attacks fungus gnat larvae. NemaShield and Nemasys are two commercially available products containing S. feltiae. The nematodes are applied as a drench to containers or flats or they can be applied through drip irrigation, but filters must be removed. Apply nematodes two to three days after inserting cuttings, planting plugs or starting seeds. Nematodes must be alive in order to be effective. To assess their viability prior to application, place a small quantity of the product in a shallow container with a few drops of tepid water. After a few minutes, look for active nematodes, which have a slight “J” curve at the ends of their bodies.
Repeat applications are usually needed. Growing medium temperatures must be 50 to 80 degrees, with optimum temperatures of 60 to 70 degrees. Irrigate the growing medium before and after applying nematodes. The nematodes require moisture in order to move within the pores of the growing medium. Apply nematodes immediately after receiving them, in the evening or on cloudy days because they are extremely sensitive to ultraviolet light.
The author is vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office. Comment or question? Visit www.farmingforumsite.com and join in the discussions.