The term “gnat” is a general description for various small, non-biting flies, but the species most often encountered as household pests are the Fungus Gnat (family Sciaridae) and the Drain Fly (family Psychodidae). Both follow a four-stage life cycle—egg, larva, pupa, and adult—known as complete metamorphosis, though their preferred breeding grounds differ significantly. Understanding the progression through these stages reveals why populations can increase rapidly, as the entire cycle from egg to mature adult can take as little as three to four weeks, depending on the species and environmental conditions.
The Initial Stage: Egg Deposition
The life cycle begins when a female gnat deposits her eggs in a moist, protected location. Female fungus gnats seek out damp soil, particularly that containing high levels of peat moss, decaying organic matter, or fungi. They place their eggs, which are tiny, oval, and translucent white, either singly or in small clusters on the surface or in crevices of the growing medium. A single female can lay between 100 and 300 eggs.
The female drain fly targets areas with standing water and organic sludge, such as the film that builds up inside sink drains, sewer pipes, or condensate pans. Drain fly eggs are small and transparent; a female typically deposits a mass of 15 to 40 eggs at a time, though some species can lay up to 100. Moisture is required for the eggs of both types of gnats to survive and hatch, a process that usually takes only two to six days.
The Growth Stage: Larval Development
The larva stage is the longest and most destructive phase. Fungus gnat larvae are slender, legless, and worm-like, characterized by a translucent, whitish body and a distinct, shiny black head capsule. These organisms primarily feed on fungi, algae, and decaying plant material found in the top two to three inches of the soil.
If the preferred food sources become scarce, fungus gnat larvae will turn their feeding to plant roots, root hairs, and young tender stems, which can stunt the plant’s growth. The larval stage lasts approximately two to three weeks, during which they undergo four molts, or instars, before reaching maturity. Warm temperatures accelerate this development, and if the soil is excessively moist, the movement of large larval populations can leave slime trails on the surface.
Drain fly larvae, often called “drain worms,” have a segmented body and use a breathing siphon at their posterior end to access air while submerged in the organic film of drains. They consume the gelatinous organic sludge, including bacteria and algae, and this stage typically lasts between nine and 15 days, with warmer water temperatures shortening the duration.
The Transformation Stage: Pupation
The gnat enters pupation, where the organism reorganizes its body structure into the adult form. Fungus gnat larvae typically construct a silken thread chamber or pupal case within the soil or growing media where they have been feeding.
The pupa is initially white but darkens as the adult fly develops inside. Pupation is a brief phase, lasting between three and seven days for fungus gnats, and often only one to two days for drain flies. For drain flies, the pupa remains submerged near the water surface, breathing through two small respiratory trumpets before the adult emerges.
The Reproductive Stage: Adult Gnat Lifespan
The winged adult gnat emerges, primarily focused on reproduction. Adult fungus gnats are slender, grayish-black, and weak fliers, often seen running rapidly across the soil surface or hovering near plants. They have a short lifespan, living for only about seven to ten days, and consume very little, mainly just liquids like water or flower nectar.
Within this short window, the adults mate, and the female quickly begins laying her eggs. Adult drain flies, with their hairy, moth-like appearance, have a lifespan of about seven days for females, with males living even less. The entire life cycle, from egg to adult, can be completed in as little as 17 to 30 days for fungus gnats, meaning multiple generations can overlap and lead to a rapid increase in population.