If you have a vegetable garden, one battle you may have to fight is with gnats that are swarming around your plants, breeding in your soil and eating up the roots causing your seedlings to turn brown and die. These soil gnats are actually called fungus gnats and are members of the insect family Sciarid in the insect order Diptera. While fungus gnats may be a nuisance to you while you are working in the garden, they are generally harmless to your vegetable plants. However, it is the gnats in their larval stages that damage healthy roots, spread disease and kill young vegetable seedlings.
Cut 1- by 1-inch raw potato pieces that are about 1/2 inch thick. Lay them randomly in your vegetable garden soil bed. In four hours, look under the potato slices and see where you have fungus gnat larvae (maggots). Fungus gnat larvae have cream-colored bodies that grow up to 1/4 inch long and have shiny black heads. This step will help you identify your problem and know exactly where they are living and breeding.
Remove the mulch and any other debris in your vegetable garden. Also, reduce watering, if possible. Wet soil rich in organic matter is the ideal breeding ground for fungus gnats.
Apply Bacillus thuringiensis var. israelensis, a biological pesticide. Fungus gnats in their larvae stage ingest this pesticide, become paralyzed and die. Thoroughly soak any areas where you saw maggots. Typically, 1 to 8 tsp. of Bacillus thuringiensis var. israelensis is added to every gallon of water, but this depends on the brand and how heavily infested your garden is with fungus gnats.
Use a chemical pesticide that is safe for your vegetable seedlings, such as one that contains pyrethrins. Read the label to verify that the pesticide is safe on your vegetables and use only as directed. Ready-to-spray bottles are easier to use than ones you have to dilute first.
Lay one piece of yellow sticky fly traps every 500 square feet. Every week, look to see if you have gnats or maggots, throw them out as needed and replace. These will help keep the gnats from infesting your vegetable garden again this season.