Fungus gnats are nuisance pests that occur indoors throughout the winter months. Often mistaken for fruit flies, fungus gnats are associated with overwatered houseplants. They may reproduce indoors year-round, but populations tend to increase during this time of year. Plants use less water during the winter and overwatering creates a suitable habitat for fungus gnats.
Adult fungus gnats are small flies measuring up to 1/8-inch. They have grayish-black bodies, dark eyes, and long antennae. They become a nuisance when they flit and fly into faces, dance around computer screens and lights, and appear in every room of the residence. Fungus gnats are not biting flies and are not a health concern for humans or pets. They do invade people’s personal spaces and can quite irritating having these tiny insects fly up noses and around about during meal time.
Fungus gnats undergo complete metamorphosis and the immature form is drastically different from the adult fly. Females lay 100-150 eggs in the upper layers of moist potting soil. Fungus gnat larvae are white, slender, legless maggots with translucent bodies and dark heads. They feed on organic debris, root hairs, and fungus in the saturated substrate. Continuous feeding by larvae may result in spotting, curling, and wilting of the plant. An inexpensive monitoring tool for larvae in soil are raw pieces/chunks of potatoes, which are highly attractive and can indicate the presence and population level in the potted plant. Early detection is the key and the presence of fungus gnat larvae should determine when control measures should be taken.
Yellow sticky traps installed at the soil surface will catch emerging adult flies. These traps are odorless, non-toxic, and made from paper or plastic cards. Yellow sticky traps are designed to attract and trap plant feeding pests such as winged aphids, whiteflies, thrips, and fungus gnats. They are available for purchase online and at garden nurseries. While traps are an effective means of detecting a variety of plant pests, they are only effective for catching those annoying flying adults. Trapping flies is only one piece of an integrated approach to managing fungus gnat in the home.
To eliminate a fungus gnat infestation, the fly life cycle must be broken by removing favorable habitats while simultaneously reducing the number of breeding and egg-laying adults. The easiest, non-chemical approach is to reduce topsoil moisture by less frequent watering to dry out the soil. It may sound drastic, but it may be helpful to stop watering plants for two to three weeks. Another option would be to improve drainage in plant pots by changing the potting medium or changing the pot itself.
If drying out the soil provides too much of a challenge, there are organic and synthetic insecticides labeled for the control of fungus gnats. Biological control products include the microorganisms, Bacillus thuringiensis israelensis strain (Bti) or the beneficial nematode, Steinernema filtiae. Both products are applied to the soil and selective to insect larvae. A synthetic and systemic insecticide option for containerized plants, contains the active ingredient imidacloprid. Employing insecticidal options may be more expensive and time consuming as all affected plants must be treated. Regardless of the product, it is important to read and follow the pesticide label.
Dealing with fungus gnats in the home can be very frustrating, but with a little patience and knowledge they can be identified and managed. Prevention includes regular inspections of the potted plants and less frequent water throughout the winter months.
Keep in mind there are several other small flies. Use this guide to help with identification.
Click here to browse over 140 insect profiles from Nebraska Extension in Douglas-Sarpy Counties.
