There are thousands of species of insects living obscure lives and hoping not to be eaten. Large green caterpillars, no matter what type, have evolved a nearly perfect camouflage. In most gardens, these are most likely to be either the larvae of giant silkworm moths, such as the luna moth, or destructive hornworms. Measuring from 7.5 to 12.5 cm (3 to 5 inches), these caterpillars are the true Goliaths of the garden. Because of their hefty size, they are voracious eaters and can strip bare a garden in short time, often before the unaware gardener knows they are there. Making a positive identification between these two types of caterpillars is easy, thanks to some distinct markings.
Capture green caterpillars in the garden. Look for these large pests on the stems and innermost portion of the leaves.
Examine the body of a caterpillar with a hand lens. Tomato hornworms will have eight white "V" shaped marks on their sides and a black "horn" on the posterior end of the body. Tobacco hornworms have seven white, black-edged slashes on their sides and a red "horn" on their back end. Luna moth larvae have no white slashes on their sides, and, instead, have "knobs" or spiky hairs on their backs. They are also missing the distinctive horn.
Examine the head of the caterpillar. The heads of both species of hornworms will be green. Luna moth larvae have a dark head.