The term “gnat” is a broad, non-scientific label applied to numerous tiny, winged insects, most belonging to the order Diptera, the true flies. Whether gnats are considered “bad” depends on the specific species and the context of the infestation. Their impact ranges from being a harmless nuisance to causing significant plant damage or delivering painful bites.
Which Insects Are Called Gnats
The term “gnat” is a general description commonly used for several distinct families of small flies, each with unique behaviors and habitats. The most frequent indoor pests are Fungus Gnats, slender, dark-bodied insects about 1/8 inch long, often mistaken for mosquitoes. They are weak fliers and typically hover near potted plants, where the females lay their eggs in the top two inches of moist, organically rich soil.
Another common indoor fly often misidentified as a gnat is the Drain Fly, also known as the moth fly. These flies have a fuzzy, moth-like appearance and are found near sinks, showers, and other areas where a film of organic sludge accumulates in drains. They do not bite people or plants, but their presence signals a sanitation issue in the plumbing system.
Outdoors, the most problematic species are Biting Gnats, including Biting Midges (or “no-see-ums”) and Black Flies (or buffalo gnats). Biting Midges are minute, typically less than 1/16 of an inch, and are notorious for their painful bites, often found near wetlands or coastal areas. Black Flies are slightly larger, possessing a characteristic hump on their back, and their larvae develop in fast-flowing streams and rivers.
Health Risks From Gnats
The health risks associated with gnats vary dramatically depending on whether the species is a biting or non-biting type. The most common household species, such as Fungus Gnats and Drain Flies, do not possess the mouthparts necessary to pierce skin and are generally harmless to humans. Their primary health impact is an indirect one, acting as a nuisance and, in the case of Drain Flies, potentially carrying bacteria from the unsanitary organic film in drains to surfaces like food preparation areas.
However, biting species, such as Biting Midges and Black Flies, pose a direct threat through their feeding behavior. Female biting gnats require a blood meal to produce eggs and use scissor-like jaws to cut into the skin. The saliva injected during the bite often causes an intensely itchy, red welt on exposed skin.
Disease transmission to humans is rare, but bites can lead to secondary bacterial infections if scratched excessively. Some individuals may experience an allergic reaction to the proteins in the gnat’s saliva, resulting in swelling or a rash. Eye Gnats are a separate species attracted to bodily fluids and have been implicated in the transmission of acute conjunctivitis, or “pink eye,” to humans and livestock.
Damage to Property and Plants
Fungus Gnats are responsible for the most significant damage to plants, particularly in indoor settings, greenhouses, and nurseries. While the adult flies are harmless, the larvae feed primarily on fungi and decaying organic matter in the soil. When populations are high, the translucent, black-headed larvae will aggressively feed on plant root hairs and tunnel into plant roots.
This root damage can lead to symptoms like sudden wilting, yellowing leaves, and stunted growth, especially in seedlings and young plants. The feeding wounds also create entry points for soil-borne plant pathogens, such as Fusarium and Pythium, which can cause root rot and lead to the plant’s decline. Managing this damage often involves letting the top layer of soil dry out between waterings to eliminate the moist conditions the larvae require to survive.
Drain Flies do not cause direct damage to property or plants but indicate a structural issue within the plumbing. Their larvae develop in the gelatinous organic film that lines drains and sewer pipes. A persistent drain fly infestation suggests a buildup of sludge that requires thorough cleaning to remove the larval food source. Outdoor gnats, such as midges, can also be a nuisance by swarming in immense numbers and leaving a film of dead insects on vehicles and outdoor furniture.