A common pest of the tomato plant is the tomato hornworm. That is the caterpillar of the five spotted hawk moth which has a 5 inch wingspan. Very impressive.
The tomato hornworm can devastate a plant. It grows fast and large and needs a lot of food to do that. It can defoliate a whole plant.
Hornworms also like other plants in the nightshade family including potatoes, peppers and eggplant.
The hornworms color allows it to blend in with the plants very well. So if you see your leaves disappearing take a close look for the hornworm. Be careful however because they will bite.
The moths hibernate in the soil and then lay their eggs on the plants in spring. The eggs hatch in a few days and then the caterpillar spends the next month eating your plants until it is full grown. Then it will dig into the ground to spend the winter and emerge in the spring as an adult moth.
The easiest way to get rid of the hornworm is to just pick it off and kill it. If you have an ongoing problem year after year then you probably have a population in your soil. The best way to get rid of that is to turn the soil in the fall to disrupt the pupae underground.
Control includes spreading a bacteria called BT, parasitic wasps and birds.