Getting Rid of Gnats on Seedlings

A FEW FUNGUS GNATS ARE LIVING ON THE VEGETABLE SEEDLINGS that I started indoors earlier this month. At least I think they’re fungus gnats. They don’t fly or act like fruit flies, so I think there’s a good chance that they are gnats. I’ve been able to hand kill three or four of them, which you could never do with a healthy fruit fly because they are too fast.

Fortunately, there aren’t very many of them. The most I’ve seen is three at one time, and they pretty much stay to three of the seedlings.

They wouldn’t really bother me, but unfortunately it seems that the larvae feed on small seedling roots, so I need to get rid of them before the problem gets bigger. My research has turned up four methods for eliminating or reducing the number of gnats. I’ll start with the method that I’ve selected. If your infestation is bad enough, you may want to use multiple methods to deal with the problem.

Here’s how I’m getting rid of gnats on my vegetable seedlings:

1. Sticky traps. Like fly paper, sticky cards* trap the gnats because they land on it and are stuck to the paper. For larger infestations this method may not be effective enough. It doesn’t kill enough of them before they can lay eggs. But in my situation, I should be able to get the job done with an easy, although less effective, solution.

Here’s a video showing how to use sticky traps. The last part of the video has some good information on how to use the traps, so make sure you stick with it until the end because that’s where the real learning takes place.

If you decide to go with this method, you can purchase them from Amazon at Sticky Traps. If you use this link, I make a few cents on the purchase at no extra cost to you.

2. Gnatrol. This is a highly selective, biological larvicide that is typically used in greenhouses to control fungus gnat larvae. The active ingredient is Bacillus thuringiensis israelensis (Bti). Once the larvae eat the insecticide, it paralyzes them so that they stop eating and die within 24 hours. It is listed on the Organic Materials Review Institute (OMRI) as approved for organic growing.

If you use this Amazon link to purchase Gnatrol, I receive a small amount of money, which helps me continue to write this blog.

3. Liquid dishwashing soap. This method calls for adding 3 drops of dishwashing liquid to 16 ounces of water and spraying it on the surface of the seed starting mix and on the seedlings themselves. It works because the gnats breath through their skin and the dishwashing liquid suffocates them. If you tap the seedling pot, more gnats will come to the top to see what’s going on, allowing you to spray more. You should spray once a week, because the gnats hatch every seven to ten days.

If you use this method, you should test the solution on one leaf of a seedling before spraying it on the plants. If it is too strong, which is a possibility, it will damage the leaf. Wait a while before spraying more on the plant. This will give the soap a chance to do damage (if it’s going to). You can dilute the solution more if the leaf is damaged and repeat the test. After all, three drops is not a very accurate measure.

If you have a great method for controlling fungus gnats in your seedlings, let us know by commenting below.

Related articles that might interest you:

1. Do You Really Need to Buy Beneficial Insects?
2. Controlling Squash Borer
3. Growing Apple Trees Organically

*If you use this link to purchase, I receive a small amount of money from Amazon, which helps me continue to publish Suburban Hobby Farmer.


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7 Responses to “Getting Rid of Gnats on Seedlings”

  1. March 22, 2011 at 11:35 pm #

    Timely. I came home from a week of vacation to find a small infestation in my seedlings. Monitoring right now, trying to decide what to try. Thanks.

    • March 23, 2011 at 7:01 am #

      Dave — Thanks for stopping by. Another option for fungus gnats is neem oil*. Some people believe that is the best solution. Let us know how it turns out.


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