Fungus gnats are a common household pest, usually appearing in homes and offices around potted plants. Although harmless insects that do not bite humans, in large numbers the fungus gnat can be a nuisance.
Fungus gnats are black or brown and are very small, no greater than 1/8 of an inch in length. Clear, thin wings in a pair are on the insect's back, and on the head, thin antennae. Adults have long, thin legs and are often confused with mosquitoes, even though fungus gnats do not bite.
Fungus gnats show up around plants because that is where the adult females tend to lay their eggs. Females produce 200 small, white eggs, and lay them in the soil of houseplants. The gnats pupate in the soil, then after four to seven days, they become adults and remove themselves from the soil. Because their lifespan is only a week's length, they must reproduce quickly, making their numbers grow rapidly.
Fungus gnats do not damage the plants they lay their legs in. The houseplant itself is of little interest to the insects. The fungus gnats choose the soil of houseplants because they prefer moist growing media for their eggs. Fungus gnats are more common in the fall and early winter, when water does not evaporate as quickly from the soil.
Allowing the growing medium to dry before watering again will keep fungus gnats from laying their eggs in the soil. Also, dry soil will prevent any eggs that have been laid in the soil from hatching. Repotting the soil will also deter fungus gnats from propagating in the soil. Soil that has broken down retains more water, making it a better breeding ground for the gnats.
If the gnat problem persists even after several weeks of proper management, insecticides may be required. Pyrethroid-based insecticides are the most effective insecticides. These insecticides can be bought as extended persistence varieties, meaning one application lasts and controls the gnat with one application.