Updated: Sep 13, 2022
Have you ever started seeds, only to end up with fungus gnats days later? I'm sure we all have. But before I get into how I get rid of them, let's learn a little bit about them.
I was being lazy recently and didn't pour boiling water on my soil like I usually do.. next thing you kmow...gnats were everywhere. They're so invasive, and seem like they serve no purpose, but they do. They are food for bigger bugs, and even birds believe it or not. But we don't have birds in our homes, right? The only purpose they serve in our homes is to be annoying. Their life cycle is short, so they make use of that time by procreating and laying a bunch of eggs that hatch in a matter of days. The larvae live in the first 1/4 inch of soil, and can be hard to notice if you aren't paying attention.
They feed on seedling roots and fungus in the soil or seed starting mix. They will even destroy a whole seed. I've had that happen before. For the adults, I leave cups of water near my plants. Since they're attracted to moisture, they land in the cups and they'd die from drowning, but that doesn't stop them all unfortunately.
There are a few methods you can try to kill the larvae.
Peroxide will kill fungus gnat larvae on contact, without doing any harm to your plants. It not only kills the larvae, but it also kills the fungus that they feed on.
Sterilize The Soil
People have complained about brands like Miracle Gro, and how it is infested with fungus gnats. Truth be told, any brand can and will have them. What I typically do, is sterilize the soil before I used it by boiling water, letting it cool a bit, and pouring it into the soil before I used it. The kills any eggs or larvae. I do this is small amounts at a time so that I can make sure dose all of the soil.
Cinnamon or Food Grade DE (Diatomaceous Earth)
Allow the top layer of soil to dry out first, then sprinkle cinnamon or DE on the soil. It has to be dry before using either one because the point is to dehydrate the larvae. Remember, they like moist soil and only live in the first 1/4 inch of soil. Cinnamon is a natural fungicide and will kill the fungus they feed on. DE is a natural, non-toxic, mineralized fossil dust that will dehydrate soft bodied pests on contact. DE is also effective on aphids, caterpillars, pest eggs, snails and slugs.
BT (Bacillus Thuringiensis) or Neem Oil
BT and neem oil will protect seedling roots, but is toxic to gnat larvae. They will feed on the roots and die after ingesting BT.
Since gnats are in the same family as mosquitoes, mosquito bits is effective. It contains BTI (bacillus thuringiensis subspecies israelensis). It is a larvicide that will kill any larvae. I have yet to try this method, but I have seen lots of people say how well it works.
Out of the methods I've tried, I have used boiling water the most. My second choice is peroxide, third is neem oil. I have used cinnamon and DE, but I try not to use them because I don't like having to deal with the messy powders. As gardeners, nothing is worse than seeing our hard work destroyed by pests.