The problem with fungus gnats is their larvae.
The adults are just annoying and make your garden feel like a jungle. The larvae live in the medium and feed on your roots ultimately hindering growth. This can also cause root rot as the roots are damaged, giving harmful bacteria a place to live. Adult Gnats can lay up to 200 eggs in the growing medium during their 10 day life span so immediate action is needed to prevent infestation. Fungus gnat larvae rarely kill plants but the damage they inflict will dramatically reduce yield and overall vigor of your plants.
The first step
You need to do is place yellow sticky traps laying sticky side up on all your buckets. Gnats go in and out of the medium and if you cover most of it with sticky traps they will get caught before they ever make it. This will also give you an idea of the level of infestation which is important when trying to see if your prevention methods are working.
There are several ways to deal with these garden burglars.
I will list these options and also give you one way that, I believe, works the best. All these methods, except for pyrethrum bombs to kill the adults, involve drenching the medium since that is where the larvae live. There is a product called Gnatrol on the market that is a bacteria which attacks the larvae. Use this by adding 6oz. per 100 gallons to feed water. This won’t hurt the plants as it is just bacteria. You can also use the method described in the root aphid section of this article. This involves pyrethrum drenches or dunks with Take Down or Evergreen.
Now I have tried all of these techniques
I find that they don’t work as well as a simple top water drench with Azamax once a week. This tactic proved to be the best. By monitoring the yellow sticky traps, I found that with each passing week the population of adults decreased. Once you have them under control, they are no longer a problem.
The important thing to remember in pest control, that I can’t stress enough, is prevention. Prevention, prevention, prevention! Always make sure your grow room or greenhouse is clean.
Monthly pyrethrum bombs when there are no problems is a good idea to ward off any potential issues. I also suggest weekly foliar applications of Azamax, until about the fourth week of bloom, in case any critters do get in. Don’t get lazy and frequently check your garden. The sooner you find something, the sooner you can treat it. Once things get out of hand you are in for a battle which is costly both in time and money.” Nip it in the bud” so to speak. Get these little thieves before they get you and your wallet.
Good Luck and happy gardening,