There is nothing more vexing than to have tomato plants full of unripe fruit one day only to come back out the very next day and find denuded stems and chunked out fruit... This time of year I cannot wait to eat that first red tomato right off the vine only to realize that I've been visited by tomato hornworms!
Tomato hornworms can be huge! Last year since I found ones that were 5 inches long! Typically they tend to be about 3-4" long. A prominent "horn" on their rear gives them their name. They are sneaky green caterpillars with diagonal lines on their sides that blend in beautifully with the rest of the leaves. They not only eat the tomato foliage but they're cheeky enough to take giant bites out of green fruit. The other tell-tale sign that you have hornworms will be large droppings that accumulate on the ground beneath the affected plants. Since they're not fond of direct sunlight, they tend to feed on the interior of the plant during the day and are more easily spotted when they move to the outside of the plant at dawn and dusk.
I'm all about the circle of life, but this pest tests my goodwill. Tomato hornworms are the larvae of one of two large moths -- the Hawkmoth and the Sphinx moth and they overwinter in the soil in the pupal stage. Adult moths appear in late spring and lay single, pearl colored eggs on the undersides of plant leaves that hatch in about a week. The ravenous larvae feed on foliage for about a month before they enter the soil and pupate. They can be difficult to spot as their coloring conveniently (for them) matches the plant. However, just look on the undersides of leaf-tipped branches. I'll betcha' a nickel they're there.
Most times I just hand-pick and drop them into a bucket of water or when infestation is severe, I use a neem oil based insecticide.
Beneficial insects are also helpful in managing this pest. Two that you can unleash are Braconid wasps which will kill these caterpillars by implanting rice-like eggs on their backs and Trichogramma wasps parasitize the eggs.
A good companion plant to use to repel the worms are marigolds.
If not kept in check, this nuisance will defoliate tomatoes, potatoes, eggplants, and peppers. Be sure to turn up the soil after harvest to destroy any pupae that may still be lurking.