Gnats in your home are annoying and can weaken your plants. Most of the time, gnats enter on potted plants and indoor food plants like tomatoes. If you don't act quickly to clear them up, they will spread themselves from plant to plant until all of your plants are producing generation after generation of gnats. Repellants don't work well. Repelling gnats indoors only sends them somewhere else indoors and spreads the infestations. There are, however, several things you can do to thwart their reproduction.
Gnats enjoy the semi-tropical temperatures found inside the average home, so they really flourish when they are brought inside, usually in the soil of potted plants or in potting soil or mulch added to houseplants. Adult gnats live just a few days, lay eggs in the soil around houseplants, and die. The eggs hatch into tiny maggots that feed off fungus in the soil and the tiny root hairs of the plants. When mature, they grow wings and take to the air, flying around the house and spreading their eggs to other houseplants. Fungus gnats are the most common type of gnat you'll find indoors.
The way we water contributes to the problem. You must adjust your watering to the needs of the plant. The temperature, humidity and air circulation around plants all impact the amount of water your plants need. An improperly watered plant is an unhealthy plant and a prime target for opportunistic insects like gnats. Soil that's too wet grows fungus and attracts gnats.
You should water your plants deeply, but make sure they drain thoroughly. Don't leave standing water in the drain trays under the pot. Gnat larvae grow best in warm, overly damp soil. Allow the top two inches of soil in the pots to dry out between waterings. This will kill the larvae which do not grow well in dry soil.
Yellow sticky traps can be set out to catch the grown-up gnats, and though they work fairly well, they won't stop the problem. Any eggs they lay in the soil, before the trap gets them, will hatch a new generation.
Killing the Larvae
If you spray the soil between waterings with pyrethrin spray, the pyrethrin, a deriviative of chrysanthemum, gets drawn into the dry soil and kills the larvae.
Horticultural oils, distillates of petroleum or mineral oils, act by smothering adult insects. Neem oil sprays, derived from a tropical tree, acts to disrupt the life cycle of insect larvae. These oils can be sprayed on the plants without damaging them.
Bacillus thuringiensis is a type of soil bacteria that controls many types of insects including gnats. Available commercially as Gnatrol or Knock-Out Gnats, applied to your plants, bacillus thuringiens attacks both larvae and adults and breaks up the life cycle of the insect in the soil.
If, after treating the soil and trapping the grownup gnats, you still get new generations of the pesky little bugs, try removing the top two inches of soil and replacing it with new potting soil. Two inches of sand over the top of the potting soil can also provide a physical barrier that prevents the larva from tunneling to the surface through all that gritty silica. If a plant is badly infested with plant larvae and is looking sickly and weak, sometimes it's better to throw out the plant and replace it. A sick plant can be very resistant to treatment efforts and will keep re-infesting your other houseplants until your remove it.
- Colorado State University Extension; Managin g Houseplant Pests; W.S. Cranshaw; Nov. 2006
- Southern Nevada Water Authority: Pest Control
- Rincon-Vitova Insectaries: Greenhouse Tomatoes---Guidelines for Biological Controls
- University of Kentucky Extension: Greenhouse Insect Management; Rick Bessin et al;
- University of Nebraska Lincoln Extension: Insects, Spiders, Mice and More
- Non-Toxic Pest Control Solutions: Gnat Spray for Tomato and Banana Plants
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