Anyone who has grown houseplants has probably dealt with fungus gnats. Adult fungus gnats are tiny flying pests that can swarm like an annoying black cloud whenever an infested houseplant or outdoor potted plant is moved.
Fungus gnats (Bradysiaspecies) are small, mosquito-like insects that commonly infest the growing medium in which houseplants and outdoor potted plants grow. Often, when outdoor plants are brought indoors for the colder months, fungus gnats harbored in the potting mix are brought indoors, where they can quickly infest the soil of other houseplants.
Fungus gnats can be difficult to control because they thrive in moist potting soil. Fungus gnats lay their eggs in moist potting soil, and the larvae live in the top two to three inches. In the larval stage, the fungus gnat maggots can cause plant damage by eating plant roots. After about two or three weeks, the fungus gnat maggots pupate in the potting soil to become the tiny black adult gnats that fly erratically when a potted plant is moved.
Letting the potting soil dry out between waterings helps control fungus gnats. But sometimes more aggressive measures are needed to control the pests. Fortunately, there are now natural, non-chemical solutions that kill fungus gnat infestations.
Kill fungus gnat larvae with BTI
Because fungus gnats are closely related to mosquitoes, fungus gnat larvae are effectively killed by BTI—the naturally occurring bacterium that is deadly to mosquito larvae. BTI, short for Bacillus thuringiensis israelensis, is deadly to mosquito larvae and fungus gnat larvae but harmless to other living things. The bacterium is nature’s way of keeping fungus gnat and mosquito populations down, but it has no effect on other insects (such as bees and butterflies)—nor does it harm animals, fish, birds, plants, pets or people.
BTI is the active ingredient in top-selling Mosquito Bits®, which is approved for the control of fungus gnats. When the plants are watered, the BTI in the Mosquito Bits® will be released. After subsequent watering, the BTI is washed below the soil surface. Fungus gnat larvae feed on the BTI and die.
Simply mix 1 tablespoon of Mosquito Bits® per quart of water. Soak for 30 minutes and skim off the floating granules. (Granules may be placed in a nylon stocking prior to immersion in water.) Stir or shake the water then apply treated water directly to soil or growing media surface. Compost or discard used granules. This treatment controls fungus gnat larvae present in the soil at the time of application. Repeat weekly for three weeks or use Mosquito Bits® treated water as a part of your normal watering routine. Repeat treatment again if fungus gnats reappear.