Gnats are attracted to light, scent and the moisture of decaying organic material. Adult gnats are gray, dark-gray or black flying insects often seen over the soil surface of wet houseplants. Although small in size, gnats reproduce quickly and swarm infested areas, often becoming a nuisance to homeowners.
"Gnat" is the name commonly used to describe a wide variety of small winged insects; however, in most cases, the flying insects that most people call "gnats" are really fruit flies or fungus gnats. These gnats feed on plants, other insects or blood, and they can be biting or non-biting creatures, depending on the species.
Fungus gnats lay their eggs in plant soil that has been over-watered. When their eggs hatch, young fungus gnats feed on the decomposing organic material in the soil. Plant roots are commonly affected as gnats feed on the roots and root hairs. Although most harmful to small plants, fungus gnats can also destroy large plants, depending on the health of the plant and the size of the gnat infestation.
Fruit flies lay their eggs in extra-ripe or fermenting fruit, and they can be seen hovering near these foods as well as they are attracted to their scent and moisture. When the eggs hatch, the larvae eat the fruit.