The presence of fruit fly-like flies in the indoor garden are often indicative of fungus gnats. Unlike fruit flies, whose larvae feed on decaying fruit, fungus gnat larvae congregate in houseplant soil.
Fungus gnats (Bradysia) are pests that resemble small mosquitoes, but don't bite. The adults are 1/8-inch-long with spindly legs and antennae. Fungus gnat larvae, which develop in houseplant soil, are white with shiny black heads and are about 1/4-inch long.
Fungus gnat larvae damage houseplants when they feed on roots and root hairs. According to the Purdue Plant and Pest Diagnostic Laboratory, larvae can also penetrate stems and disrupt nutrient and water uptake, and both adults and larvae transmit disease.
Fungus gnat larvae generally live and feed in the top 2 to 3 inches of moist soil. Within two to three weeks, they become adult fungus gnats. Female fungus gnats then lay eggs in moist soil and repeat the cycle.
Prevent fungus gnat larvae from living and transforming into adults by letting the top 2 to 3 inches of soil dry. Dry soil also discourages adults from laying more eggs. Adults can be caught on yellow sticky traps known as gnat sticks, and certain insecticides kill adult gnats.