Fungus Gnats

Facts

Fungus gnats are a type of short-lived fly that is found throughout the United States. Fungus gnats are some of the tiniest flies, with thin, dark bodies that end in a point and a pair of tiny, clear wings. They have no ability to bite or otherwise harm us, but if the conditions are just right for them their numbers can grow to very high levels.

Appearance / Identification

What Do They Look Like?

fungus gnat
Image credit:UMN.edu
fungus gnat
Image credit: okstate.edu

adult fungus gnat
Image credit: NC State

Size
Adults can reach a length of 1/5 of an inch.

Color
They are generally dark in color and have transparent wings.

Body
Similar to mosquitoes, they have long, thin legs and a long body.

Flying
Fungus gnats are considered poor fliers, and are more likely to be seen walking on plants when undisturbed.

Habitat

Where Do They Live?
They are found frequently around indoor potted plants and greenhouses. Adults are also attracted to light so large groups of fungus gnats may be noticed next to windows. In fungus gnat biology, they inhabit a wide range or commercial and ornamental plants, and can cause significant damage during certain life stages. Interestingly, adult fungus gnats are actually considered to be important pollinators for several species.

Connection to House Plants & Moisture
Inside the home a common place to find perfect conditions for fungus gnats will be house plants, more specifically the soils these plants grow in. We have a tendency to love our plants too much, and commonly over-water our plants, keeping the soil wet continually. The soils usually are high in organic matter, and this combination of moisture and organic matter is a perfect invitation for fungi to grow. It does not necessarily have to be a big mushroom growing up out of the soil, as the gnat larvae are tiny, and will find the minute bits of fungus growth within that medium. Allowing the soils in these pots to dry out on a regular basis is not only a good way to prevent the growth of fungi and fungus gnats, but it may also be better for the plants themselves. Their roots need to “breathe”, and one way we stress or even kill our plants is by keeping their roots immersed constantly in water.

Moisture & Leaks
There may be other possibilities indoors as well, such as areas beneath or around leaking plumbing, perhaps under the kitchen sink or, worse, within walls where pipes and fittings may have begun to leak. It could be carpets or other flooring below windows that are leaking rainwater inside, catch pans under the fridge, mildew growing on or below a window sill that has condensation dripping down off the window continually in the winter. Just think of all the places in your home where water can be, and these all offer the possibility that the water may be escaping onto surfaces and growing the fungi needed by fungus gnats. The control, obviously, is to eliminate that water problem, for it definitely is a “problem” that needs to be corrected.

Life Cycle

Eggs
In fungus gnat biology, females lay between 100-200 eggs in clusters on organic rich soil. Hatching usually occurs within four to six days.

Larvae
Larvae emerge from the egg shells and begin feeding on the plant roots. After two weeks, the larva spins itself into a cocoon and sheds it skin.

Pupae
In another week, a pupa is formed in which the fungus gnat finishes developing under the soil. The adult emerges within a week. More females are born than males which allows for a faster increase in the fungus gnat population. Fungus gnat life cycles occur within 21-40 days depending on temperature and humidity.

Problems Caused by Fungus Gnats

Adults do not carry diseases transmissible to humans and are generally considered more of an annoyance as they only feed on the fungus surrounding the plant. The larvae stage is what is damaging as they eat the roots. This hinders plant growth and allows access for potential diseases to attack. Fungus gnats require high humidity, so over watering may lead to favorable breeding conditions.

How to Get Rid of Them

Knowing the life cycle of fungus gnat biology, it is possible to get rid of these pests.

  • Good sanitation is important in removing fungus gnats.
  • Eliminating algal growth, cleaning up plant debris, and fixing leaks are old good management strategies.
  • Reducing moisture in the soil by less watering and better airflow is also advantageous.

Biological control agents, such as insect-attacking fungus, may have to be introduced.

If you believe the gnats are coming from some hidden locations, such as wall voids, you may want to contract with a licensed termite company to do a home inspection. They are trained to look for the problems caused by excessive moisture conditions, for this also can be a means for wood decay fungi to begin growing, or to attract certain kinds of insects that may damage the wood, such as termites or carpenter ants. If you have a crawl space beneath the home it is common for the soil there to remain damp all the time, again creating the conditions that favor either fungus gnats or possibly decay fungi. A home should be dry, and the presence of a lot of fungus gnats indoors may be telling you that moisture problems are there. These flies could also be coming from the outside, for damp conditions around the exterior also are perfect for their breeding. The adult flies are attracted to lights and may simply be entering each time a door is opened, but if they are indoors it is worth investigating.