Missouri is home to over a dozen green caterpillar species, ranging from tiny garden pests on your tomato plants to spectacular 5-inch giants on hickory trees. Most are completely harmless, a few can sting, and only a handful cause real damage to gardens or landscapes. Identifying what you’ve found comes down to size, markings, what plant it was on, and the time of year.
Large Green Caterpillars on Trees
If you’ve found a big, striking green caterpillar on a hardwood tree, you’re most likely looking at one of Missouri’s giant silk moth or sphinx moth larvae. These are the species that make people stop and take photos.
The hickory horned devil is the largest caterpillar in Missouri and one of the largest in North America. Full-grown larvae reach 4 to 5 inches long, about as big around as your thumb, with a dull green body armed with black spines along its back and four curved, black-and-yellow horns behind an orange head. Despite looking terrifying, it’s completely harmless. You’ll find it on hickory, walnut, sumac, persimmon, sweetgum, ash, and sycamore from July through October. It becomes the regal moth, a large orange and gray moth with a 6-inch wingspan.
The cecropia moth caterpillar is another showstopper at 3 to 4 inches long, with a pea-green body dotted with colorful raised bumps (called tubercles) in blue, yellow, and red. It feeds on apple, maple, walnut, willow, elm, poplar, ash, plum, box elder, cherry, and lilac from late May through August. The adult cecropia moth is North America’s largest native moth.
The polyphemus moth caterpillar is bright green with a brown head and reaches nearly 3 inches. It feeds on a wide range of hardwoods including hickory, maple, oak, ash, walnut, sycamore, and birch. The adult moth is known for the large eyespots on its hindwings.
The imperial moth caterpillar varies from light green to dark green to reddish brown and appears July through October on oaks, sweetgum, maple, hickory, sassafras, elm, and sycamore. None of these large silk moth caterpillars sting or cause meaningful damage to healthy trees.
Medium Green Caterpillars
Several medium-sized species (roughly 1.5 to 2.5 inches) are common across Missouri yards and woodlands.
The greenstriped mapleworm has a pale green body with a distinctive red head and grows to about 1.5 to 2 inches. As the name suggests, it feeds on maples and sometimes oaks growing near maples. It’s active from late spring through late fall and can occasionally defoliate ornamental maples, though healthy trees usually recover.
The smalleyed sphinx caterpillar is pale to bright green with an unusually grainy skin texture, reaching about 2.5 inches. It prefers cherry trees and is present from May through September.
Green Caterpillars That Become Butterflies
Two of Missouri’s most familiar butterflies start as green caterpillars, so if you’re finding one on a garden shrub or small tree, it may be worth leaving alone.
The tiger swallowtail caterpillar is about 1.5 inches long, green with a darker shade on top and lighter coloring near the legs. It has two prominent false eyespots that make it look like a tiny snake. You’ll find it on tulip tree, birch, ash, basswood, cherry, apple, willow, and magnolia from May through October.
The spicebush swallowtail caterpillar also has dramatic false eyespots and a green body (sometimes bright orange in later stages). Its primary host plants are spicebush, sassafras, and magnolias, and it’s active May through October. Both swallowtail species are beneficial pollinators as adults and pose no threat to your plants in meaningful numbers.
Green Caterpillars in the Vegetable Garden
The green caterpillars eating your vegetables are an entirely different group from the tree-dwelling species above. The most common garden culprits in Missouri are tomato hornworms, cabbage loopers, and imported cabbageworms.
Tomato hornworms are large (up to 4 inches), bright green with white diagonal stripes and a prominent horn on the rear end. They feed on tomatoes, peppers, and other plants in the nightshade family, and a single caterpillar can strip a tomato plant surprisingly fast. They’re also found on corn, soybeans, and grain sorghum. Cabbage loopers and imported cabbageworms are smaller, pale green, and target anything in the cabbage family: broccoli, kale, cauliflower, Brussels sprouts, and cabbage itself. Loopers move with a distinctive inchworm motion, arching their bodies as they crawl.
The One to Avoid: Io Moth Caterpillar
Of all the green caterpillars in Missouri, the Io moth caterpillar is the one you should not touch. It’s pale green with white and reddish-brown stripes running along its sides, and its entire body is covered in clusters of branching green spines. Those spines deliver a painful sting on contact, causing immediate burning, redness, and sometimes blistering that can last for hours.
Io moth caterpillars are present from July through October on an unusually wide range of plants: maple, sassafras, oak, hickory, elm, apple, beech, cherry, black locust, mulberry, dogwood, sycamore, and even corn and grasses. They often feed in groups when young, making them easier to spot but also easier to accidentally brush against.
If you do get stung, don’t rub the area. Press a piece of tape (any kind) sticky-side down onto the skin and peel it off to remove the tiny spines. Wash the area, apply hydrocortisone cream, and take an antihistamine if itching is significant. Check whether your tetanus shots are current.
How to Identify What You Found
Start with three questions: how big is it, what plant was it on, and what time of year is it? A 4-inch green caterpillar on a hickory tree in August is almost certainly a hickory horned devil. A 1.5-inch green caterpillar with a red head on a maple is a greenstriped mapleworm. A fat green caterpillar with white stripes on your tomato plant is a hornworm.
Look at the skin texture and markings. Smooth skin with eyespots points to a swallowtail butterfly larva. A grainy texture on cherry suggests a smalleyed sphinx. Colorful bumps on a large caterpillar on maple or apple suggests a cecropia moth. Branching spines with stripes means Io moth, and you should keep your hands away.
Managing Caterpillars That Cause Damage
Most green caterpillars on trees do not need any intervention. Even when species like the greenstriped mapleworm defoliate a branch or section, healthy mature trees recover without lasting harm. The large silk moth caterpillars (cecropia, polyphemus, imperial, hickory horned devil) are solitary feeders that rarely cause noticeable damage and are worth protecting as part of Missouri’s native ecosystem.
Garden pests are a different story. For hornworms, spray your tomato plants with water and watch for the caterpillars to thrash around, making them easier to spot and handpick. Drop them in a bucket of soapy water. One important exception: if you see a hornworm covered in small white cocoons on its back, leave it alone. Those cocoons belong to parasitic wasps that have already killed the caterpillar, and the emerging wasps will help control future hornworm populations.
For cabbage loopers and cabbageworms, a spray containing Bt (a naturally occurring soil bacterium that kills caterpillars but is safe for people, pets, and beneficial insects) is the standard approach. Spray all leaf surfaces, especially the undersides where caterpillars feed, every two weeks until the problem clears up. Reapply after rain. For early-season prevention, covering young plants with floating row cover fabric keeps the adult moths from laying eggs in the first place.
Handpicking works well for any species if you catch the infestation early. Check for dark-colored droppings (called frass) on leaves or the ground below, which is often the first sign caterpillars are present even when the caterpillars themselves are camouflaged. Remove rolled or folded leaves too, as some species shelter inside them during the day. Natural predators including fireflies, ground beetles, soldier beetles, stink bugs, and tachinid flies all prey on caterpillars. Birds like chickadees, bluebirds, flycatchers, and barn swallows will also help keep numbers down if you encourage them with feeders and nesting boxes.