Fungus gnats are small, dark, short-lived flies, of the families Sciaridae, Diadocidiidae, Ditomyiidae, Keroplatidae, Bolitophilidae, and Mycetophilidae (order Diptera); they are sometimes placed in the superfamily Mycetophiloidea.
The larvae feed on plant roots and fungi, helping in the decomposition of organic matter. The adults are 2–5 mm long and are important pollinators of plants and carriers of mushroom spores. They carry diseases such as pythium on their feet. They may be quite annoying to humans as they fly into their faces, eyes, and noses.
Some fungus gnats are exceptionally hardy, being able to tolerate cold conditions through their possession of antifreeze proteins.
Typically, overwintering organisms can either avoid freezing or tolerate freezing, but Excechia nugatoria can do both. For E. nugatoria, the production of noncolligative antifreeze proteins (NAPs) protect the head and thorax from freezing, but they do not protect the abdomen.
Freezing of the head and thorax in other insects tends to have adverse effects on neural tissue, so it is not surprising that these protective mechanisms have been observed in certain species, but E. nugatoria is the only insect known to semi-freeze through the winter, which may be an evolutionary advantage according to Sformo: By allowing the abdomen to freeze, evaporative water loss is reduced over the course of the winter.
Fungus gnats are typically harmless to healthy plants - and humans - but can inflict extensive damage to seedlings; their presence can be indicative of more serious problems. In houseplants, the presence of fungus gnats may indicate overwatering; they may be feeding on roots that have sat in drain water too long and are thus rotting, or the gnats may be attracted to fungus growing in saturated topsoil. Consequently, allowing the soil to dry may reduce their numbers. They are sometimes also managed by placing a layer of sterile sand or indoor mulch on top of the soil around plants; by introducing Hypoaspis miles mites or applying the biological larvicide Bacillus thuringiensis (subspecies israelensis) to kill gnat larve; by drenching the soil annually in an insecticidal soap; or by applying detergents and nicotine from tobacco brewed into a toxic tea.