E-mail - Flying Insects in Growroom

Doc,
  I have little fruit flies in my garden.  I am using a soil mixture in 3 gallon pots.  I read on your blog about putting sand on top of the soil and I tried putting sand on top of my pots, but it has not gotten rid of them.  I am going to eat my plants so I don’t want to use any chemicals.  What can I do to get rid of these little bugs?

Thanks for the E-mail.  I think you have what I call soil gnats or fungus gnats.  The adults are harmless; they don’t sting or bite you or your plants.  The larvae (like when a caterpillar turns into a butterfly) eat organic matter in the soil and can eat root hairs which would harm the plants and slow growth.  A test to see if your soil is infested with soil gnat larvae is to look very closely (use a magnifying lens if you have one) you will see tiny little white ‘worms’ or larvae.  Another test is to put a piece of raw potato on top of your soil.  In  just one day you should see little black headed larvae under or even on the potato chowing down.  Since you put sand on top of the soil (perlite or any substance free of organic matter and that dries out will work) you might not be able to do this.  The adults live a week or so, so you will still see them flying for that long.  If it has been over a week, or you see MORE adults you still have a problem.  What might happen is that the soil gnats could be laying eggs using the drainage holes.  Pick up the pots.  You should notice some soil gnats flying from ON the plant, but what you want to look for is soil gnats UNDER the pots.  If you see a bunch of gnats under your pots, this is why putting sand on top did not work, they just found another place to lay their eggs in your grow area.  

You should clean out your trays of any plant matter.  You can then fill the trays ½ full with a 1-4 ratio of hydrogen peroxide to water.  One part 3% hydrogen peroxide, and four parts tap water.  This will work better if the plant/soil is dry.  Make sure the mixture covers the drainage holes for a few minutes.  The mixture should kill most of the larvae but won’t harm your plants if you make the 1-4 ratio correctly and only let the plants sit in the solution a few minutes.

Last, you should get some yellow stickytraps from HTGSupply.com to monitor the soil gnat population.  If you could harvest all your plants at once and clean up everything with the hydrogen peroxide (or bleach for the pots and trays) you should try to wait a week to make sure all the adults in your house are dead.  If you have house plants, you may want to treat them with an insecticide otherwise soil gnats in your house plants will find their way to your indoor garden again.

Let me know if this works

Good Growing,

Dr. E.R. Myers

Click here for other options including insecticides to deal with indoor pests.