Tiny Flies in Plant Soil
- An adult fungus gnat measures 1/8 inch long and has a long-legged black body similar to that of a mosquito. The tip of each wing has a distinctive Y-shaped vein. The legless white larva has a black head and measures 1/4 inch long. Fungus gnats do not fly very well and usually sit or crawl on the soil or host plant. An adult shore fly, which is a little larger than a fungus gnat, has gray wings with light spots. The 1/8-inch-long white larva lacks legs or a distinctive head segment. Shore fly adults, larvae and brown pupae live near algae in moist soil.
- The tiny eggs of fungus gnats and shore flies are often not visible to the naked eye. A female fungus gnat lays about 300 single or clustered eggs in moist, rich potting media. The eggs hatch six days later, and the larvae feed on soil fungi for about two weeks. Inside a cocoon, a larva transforms into a pupa, and seven days later an adult fungus gnat emerges and lives for about two weeks. A female shore fly lays about 300 single eggs in moist soil. One day later, the larvae hatch from the eggs and pupate after feeding on algae for four to six days. After five days, adults emerge and live up to three weeks.
- Fungus gnat larvae feed mostly on soil fungi, but they can damage plants when they feed on and strip plant roots. According to the University of Vermont Extension, flowering plants such as carnations, chrysanthemums, geraniums and poinsettias are especially susceptible to damage from fungus gnats. A host plant will develop yellow leaves, leaf drop, stunted growth and faded color. Shore flies feed on soil-borne algae and do not feed on plant tissue, but they often deposit unsightly black droppings on plants. Both fungus gnats and shore flies can spread fungal root diseases among plants.
- Sanitation can reduce fungus gnat and shore fly populations. Use pasteurized potting media, and do not use media high in organic matter if it has been left outdoors. To deprive shore flies of algae, disinfect potting benches and wait until the soil is dry before watering. The Colorado State University Extension states that some insecticides, including pyrethrin and bifenthrin, control adult fungus gnats. Bacillus thuringiensis bacteria and growth regulators such as azadirachtin and kenoprene control fungus gnat larvae. Fungus gnats and shore flies are vulnerable to natural predators, including rove beetles and hunter flies.
