Sooner or later you will have to deal with fungus gnats infesting your houseplants. Fungus gnats are the tiny flying pests that can swarm like an annoying black cloud whenever an infested houseplant or outdoor potted plant is moved.
Fungus gnats are small, mosquito-like insects that commonly infest the soil or growing medium of potted plants. Fungus gnats thrive in moist potting soil, where they lay their eggs. After the eggs hatch, fungus gnat larvae live in the top 1-2 inches of the potting soil. Fungus gnat larvae can cause plant damage by eating plant roots. After about two or three weeks, the fungus gnat larvae pupate in the potting soil. Then they become the tiny black adult gnats that fly erratically when a potted plant is moved.
There are many different treatments available to treat fungus gnats (see reference chart), but I have found this method most effective:
1) Crush or crumble a “mosquito dunk”. Sprinkle powder/chunks on the top of the soil of affected and surrounding plants using approximately ¼ dunk for each plant (depending on the size).
2) Treat plants with a solution of hydrogen peroxide, neem oil or an insecticidal spray.
3) Let plants dry out for 1-2 weeks. If plants do require water “bottom water” only.
Re-treat plants with mosquito dunk powder every 30-60 days. This will ensure that gnats will not be able to reproduce even if they inadvertently get into your house and soil.
Fungus gnats are closely related to mosquitoes, so fungus gnat larvae are effectively killed by mosquito dunks, which contain “BTI”. Bacillus thuringiensisisraelensis is a naturally occurring bacterium that is deadly to mosquito and fungus gnat larvae. BTI has no effect on other insects (such as bees and butterflies). Nor does it harm animals, fish, birds, plants, pets or people.
Sticky traps are effective and can help get rid of annoying adults flying around but be aware that this does not help kill eggs or larvae.
Diatomaceous Earth (often referred to as "DE") is an off white talc-like powder that is the fossilized remains of marine phytoplankton (sea shells). It acts like little razor blades to the gnats essentially cutting them as they go through it. Treatment will take a week or two to work completely. You need to go through a few reproductive cycles to get them all. The drawback to using DE is that it is only effective until you get it wet. So bottom watering or "butt chugging" is recommended.