Fungus gnats are small flies that infest soil, potting mix, other container media, and other sources of organic decomposition. Their larvae primarily feed on fungi and organic matter in soil, but also chew roots and can be a problem in greenhouses, nurseries, potted plants and interior plantscapes. Adult fungus gnats may emerge from houseplants indoors and become a nuisance. (J.A. Bethke, S. H. Dreistadt, 2013)
Adult fungus gnats (Bradysia spp.) are small (approx. 1/8 inch long), blackish- grey, gnat-like flies with long gangly legs, many-segmented antennae which are longer than their heads, and have a single pair of greyish transparent wings with a Y- shaped vein near each wing tip. They are weak flyers and may be found running/flying near soil level. Cornell Cooperative Extension. Fungus Gnat Biology and Control. J.P. Sanderson
Mated adult females deposit up to 200 eggs singly or in clusters in crevices or cracks on the surface of the growing media. Adult females prefer to lay eggs where fungus is growing. The offspring of a given female will either be all males or all females. At 70° -75° F, these whitish-yellow eggs hatch in 3-6 days. The four larval instars then feed for about 2 weeks and usually pupate near the soil surface within a thread chamber. After 3-7 days in the pupal stage, adults emerge and live for up to 8 days. They can develop from egg to adult in 3-4 weeks. This life cycle is dependent on temperature and as temperatures decrease, the length of their developmental time increases. Cornell Cooperative Extension. Fungus Gnat Biology and Control. J.P. Sanderson
Recommended controls for Fungus Gnats
Each control will have its own set of parameters that will be best suited for individual environments. Certain controls may only be available for commercial application.
Commercial farmers are required to reference their own state laws to ascertain if the recommended controls fall within compliance of their states regulatory guidelines.
Stratiolaelaps scimitus (formerly known as Hypoaspis miles) www.entofilm.com
MECHANICAL AND PHYSICAL CONTROLS
Keep production areas free of weeds and algal scum, which can serve as breeding sites for fungus gnat populations. Maintaining overwatered conditions and using either incompletely composted organic matter or manure in potting media provides ideal conditions for fungus gnats. Commercial sources of peat may be infested with fungus gnats and should be steamed before use when growing crops sensitive to fungus gnats.
Monitoring and Treatment Decisions
Yellow sticky cards placed in greenhouses will capture adult fungus gnats. For more information, see MONITORING WITH STICKY TRAPS. Small emergence traps can also be used to determine precisely where adults are emerging. Larval populations can be monitored with cubes or slices of potatoes pressed just into the soil. Fungus gnat larvae can be readily seen feeding on the potato pieces.
Diatomaceous earth has shown be an effective control when applied and mixed in the top 2"-3" of the soil. Make sure to wear proper respiratory protection when applying diatomaceous earth