Paul Nickerson

Potato Soil Tests: Fungus Gnats

In Pest Management on March 30, 2011 at 10:48 pm

Fungus Gnat Treatments

With the arrival of Spring come beautiful flowers, green grass, and Fungus Gnats! As the temperature warms, it is not uncommon to see small flies buzzing around the surface of the potting soil in your house plants. Small Fungus Gnats lay eggs just below the surface of the soil. Like most other flies, the eggs hatch into larvae which eventually pupate and become adults. The entire cycle takes about 30 days.

While these gnats can be fairly bothersome, they cause no real damage to established plants. The larvae eat organic material in the soil, including small, fibrous roots. Larger plants, with established roots, will not be greatly effected, however, seedlings, and succulents with shallow roots, may show signs of injury.

Potato Test

There is an easy way to test which plants are infected with the larvae. Cut a potato into small pieces and lay them on the surface of the potting soil which you wish to test. Be sure that the inside of the potato is in contact with the soil. Due to the small area in which I have my plants, I placed a few pieces in each pot. After about 48hrs, look at the area of the potato that was touching the soil. You will notice small larva crawling on the potato. Due to the concentrations of starch in the potato, the larvae will move from the soil, into the potato. While this is a great way to determine what plants are infected, discarding and replacing the pieces will continue to remove larvae from the soil. Unfortunately, due to the high populations of larvae, this is not an effective means by which to rid your plants of Fungus Gnats.

Group of Eggs

Once you notice that you have Fungus Gnats, there are a number of actions that can be taken to get rid of them. Fungus Gnat larvae prefer moist soil. As a result, when you determine which plants are infected, allow that plant to thoroughly dry out between waterings, this will kill any larvae, and will dry out any eggs in the soil. Removing the top half inch of potting soil and replacing it with dry sand will inhibit the ability of the gnats to lay their eggs in the soil.

Cider Vinegar Traps

Another effective treatment is to use Apple Cider Vinegar as a trap. Pour small amounts of Cider Vinegar into shallow containers. The fermentation will attract the adult gnats. When the gnats land in the vinegar, it will trap and kill them. With the high number of eggs that each fly can lay, it is much more efficient to kill off adults than try to purge plants of larvae.

While I used the potatoes to search for the larva of Fungus Gnats, the technique can be used to get an idea of what else is living in your soil as well. Here are some of the other fun things I found!

Notice the number of eggs...


Not sure what this little guy is..

  1. Hi again; I love what you’re doing here!

    Are those images of the bottom of the potato and what power was the magnification?

    I’d LOVE to get a cheap microscope to help with gardening as I just finished dealing with a spider mite infestation and what a pain until a good recipe was found; a loop is not optional, but a scope would be idea due to compost tea.

    Find the apple cider vinegar /baking soda/ lemon juice recipe for dealing with those very resistant pests! (This one you must wash off after 20 minutes as it burns them off.)

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: