Bacillus Thuringiensis

• Bacillus thuringiensis, or Bt, is a bacterium that attacks the lining of a caterpillar's digestive tract so that it can no longer feed. It doesn't kill the insect instantly, but the hornworm stops eating and dies of starvation within a few days. Bt has no effect on humans, pets and wildlife, making it one of the safest insecticides to use against tomato hornworms. Bt degrades quickly when exposed to sunlight, so frequent application is necessary.
  
  

Diatomaceous Earth

• Diatomaceous earth is a fine powdery substance made up of the shells of diatoms. When a tomato hornworm crawls over diatomaceous earth, the razor-sharp shells cut the soft tissue and the hornworm eventually dries out and dies. Diatomaceous earth is harmless to birds, children, pets and earthworms but deadly to soft-bodied insects including many beneficial insects. Apply it to the ground around the plants as well as the leaves and stems. Apply it after a light rain or spray with water to improve adherence. You can also apply it as a liquid mixed with insecticidal soap.
  
  

Other Insecticides

• Insecticidal soap is effective against tomato hornworms, but you must spray it directly on the caterpillar. Other effective insecticides include spinosad, carbaryl, permethrin and bifenthrin. These are contact insecticides with varying lengths of residual effects. If you use one of these insecticides, read the label carefully and make sure it is labeled for use against tomato hornworms and for the type of plant you are treating. Always store insecticides in their original container.
  
  

Handpicking

• Handpicking is the best way to manage tomato hornworms. The caterpillars are large enough to find and remove easily, and you can drop them into a container of soapy water to kill them. If you have trouble spotting them, a light spray with a water hose will make them squirm and raise their heads up off the leaves. Use tongs to remove tomato hornworms if you prefer not to touch them.