Fungus Gnats and Shore Flies

Fungus gnat larvae are white and legless, about 1/4 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.

The adult fungus gnat is sometimes confused with another small dark-bodied fly called the shore fly, Scatella stagnalis. Shore flies have more robust bodies than fungus gnats and their antennae 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. In the larval stage shore flies can be distinguished by the opaque, tannish-brown color of the body and the absence of a head capsule.

Fungus gnats and shore flies are attracted to damp locations where fungi are apt to flourish. Fungi are a major part of their diet. Studies have shown that fungus gnats develop more rapidly and have greater survival on fungal diets. In the absence of a fungal food source however, fungus gnats are capable of feeding on healthy plant tissue. Fungus gnats are general feeders and can injure a number of flower crops grown in the greenhouse. Adults are primarily a nuisance however, larvae feed on plant roots, fungi and decaying organic matter and tunnel into the crown and stems of plants. The feeding damage creates wounds that allow soil borne pathogens to enter and can kill plants. Fungus gnat larvae may also carry some soil-borne pathogens such as Pythium, Thielaviopsis and Fusarium. Fungus gnats are a common problem on greenhouse crops growing in media that contains a high percentage of peat moss or compost. Larvae present in infested plants or soil can lead to prolonged emergence of adults. Shore flies are not known to feed on healthy plant tissue. Adult and larval stages of shore flies feed primarily on algae or decaying organic matter and breed in moist environments.