Indiana is home to dozens of caterpillar species, from garden-destroying hornworms to the striking monarch larvae that depend on the state’s native milkweed. Whether you’ve spotted something munching your tomatoes, building webs in your trees, or crawling across your deck, here’s what you’re likely looking at and whether it’s a problem.
Monarch Caterpillars and Milkweed
The monarch is Indiana’s most recognized butterfly, and its caterpillar is entirely dependent on milkweed. Female monarchs lay eggs on the underside of milkweed leaves, which act as shade, rain shelter, and camouflage from predators. Once the larvae hatch, they eat day and night, consuming as much as their own body weight on the first day alone. Over roughly two weeks of feeding, a monarch caterpillar gains about 2,700 times its hatching weight.
The milkweed diet also doubles as chemical armor. Milkweed is toxic, and monarch caterpillars store those poisons in their bodies, making them unappetizing to birds and other predators. The caterpillars are easy to identify: bold white, yellow, and black bands on a plump body, typically found clinging to milkweed stems in fields, roadsides, and gardens. If you find one, leave it alone. Monarchs need all the help they can get, and planting native milkweed species is one of the simplest ways to support them.
Hornworms in the Vegetable Garden
If you grow tomatoes in Indiana, you’ll eventually meet a hornworm. These fat, bright green caterpillars can reach four inches long and will strip stems and leaves overnight. Despite the names, it’s actually the tobacco hornworm that shows up more often on Indiana tomato plants than the tomato hornworm.
Telling them apart is straightforward once you know what to look for. The tobacco hornworm has seven white diagonal stripes per side, each edged in black, and a red horn at its rear end. The tomato hornworm has eight V-shaped chevron marks per side with no black edging and a dark blue horn. Both do serious damage, but handpicking them off plants is effective for home gardens. If you spot one covered in small white cocoons, leave it be. Those cocoons belong to parasitic wasps that are already doing pest control for you.
Eastern Tent Caterpillars vs. Fall Webworms
Silky webs in your trees can mean one of two species, and the time of year tells you which. Eastern tent caterpillars appear in spring, while fall webworms show up in summer and fall.
Eastern tent caterpillars overwinter as egg masses glued to twigs, hatching in April. Within two days of emerging, the larvae start weaving a thick, dense tent in the forks and crotches of branches. They don’t actually feed inside the tent. Instead, they use it as a shelter at night and during rain, venturing out along branches to eat leaves during the day. The full-grown larvae are about two inches long, black with white and blue markings and a white stripe running down the back.
Fall webworms take a different approach. Their webs are loose, messy, and enclose entire branch tips. The larvae feed inside the web, expanding it as they grow. Adults emerge from the soil in June and July as nearly pure white moths, and their caterpillars hatch in mid-July, feeding for a month or more before dropping to the ground to pupate. Full-grown fall webworm larvae are about an inch and a half long, hairy, and range from yellow with dark spots to dull bluish black.
Neither species typically kills healthy trees, though heavy infestations look alarming. You can prune out webs on small trees or simply wait for the caterpillars to finish their cycle.
Spongy Moth: Indiana’s Invasive Threat
The spongy moth (formerly called the gypsy moth) is one of North America’s most damaging invasive forest pests, and it has spread into Indiana from the Northeast and upper Midwest. Its caterpillars feed on hundreds of tree and shrub species but strongly prefer oaks. When populations spike, the caterpillars can completely strip a tree of its leaves. Several years of this back-to-back defoliation, combined with drought or other stresses, often kills the tree.
Indiana’s Department of Natural Resources actively manages spongy moth through monitoring and targeted treatments, including mating disruption programs that use organic products to prevent moths from reproducing in affected areas. Treatments typically happen in late June. If you suspect a spongy moth infestation on your property, the DNR’s Division of Entomology and Plant Pathology can be reached at 866-NO-EXOTIC (866-663-9684) or DEPP@dnr.IN.gov.
Stinging Caterpillars to Avoid
Several caterpillar species found in Indiana carry venomous spines or irritating hairs that cause real pain on contact. The most notable include the saddleback caterpillar, io moth caterpillar, puss caterpillar (also called the flannel moth caterpillar), buck moth caterpillar, and spiny oak slug caterpillar. These species look different from one another, but they share one trait: touching them hurts.
Contact with venomous spines can cause immediate burning pain, redness, swelling, welts, and blistering at the sting site. Some people also develop headaches and nausea. More serious allergic reactions, including shortness of breath or difficulty swallowing, can develop if you touch your eyes or mouth after handling a stinging caterpillar.
The saddleback is one of the easiest to recognize, with a brown body, bright green “saddle” marking on its back, and prominent spiny horns at both ends. Io moth caterpillars are pale green with red and white lateral stripes, covered in clusters of branching green spines that deliver a sharp sting. Puss caterpillars look deceptively soft and fuzzy, like a tuft of fur, but their hair conceals venomous spines underneath. The general rule: if a caterpillar is hairy, spiny, or brightly colored, don’t pick it up.
What to Do After a Caterpillar Sting
If you brush against a stinging caterpillar, the first step is removing any spines still embedded in your skin. Press a strip of adhesive tape firmly over the affected area, then peel it off. This pulls out the tiny hairs and spines that are too small to see or grab with tweezers. Repeat with fresh tape a few times to get as many as possible.
After that, wash the area thoroughly with soap and water. Ice packs help reduce swelling and pain. Over-the-counter antihistamines and hydrocortisone cream can ease itching and inflammation. Remove and wash any clothing that contacted the caterpillar, since loose spines can cling to fabric and cause repeated irritation. Most stings resolve within a few hours to a couple of days, but widespread blistering, difficulty breathing, or swelling around the face and throat warrants emergency medical attention.