What Is the Big Green Caterpillar on My Sidewalk? Identifying Giant Silk Moth Caterpillars

It is a classic late-summer surprise. You are walking to the car or weeding along the fence when you notice a caterpillar that stops you in your tracks — plump, brightly colored, and much bigger than the little inchworms of spring, sometimes as long and thick as your finger. By August and September these giants are on the move, and many of the largest belong to a single beautiful group: the giant silk moths. Knowing what you are looking at tells you whether it is harmless (almost always yes) and what it will eventually become.

Why You See Big Caterpillars in Late Summer

Giant silk moths (family Saturniidae) spend the summer eating and growing on the leaves of trees. By late summer the last generation of caterpillars reaches full size and leaves the safety of the canopy to find a place to pupate — spinning a cocoon in leaf litter or burrowing into the soil. That wandering stage is exactly when people find them crossing sidewalks, driveways, and lawns, far from any plant. So if a huge caterpillar seems to be out in the open for no reason, it is usually just looking for somewhere to spend the winter.

The Common Giant Green Caterpillars

A few species turn up again and again. All of these are native, and all but one are completely harmless to handle:

• Luna moth caterpillar: plump and lime green with faint yellow lines down the sides and tiny reddish-orange dots; becomes the famous pale-green, long-tailed luna moth
• Polyphemus moth caterpillar: bright green and accordion-segmented, with small silver or red spots and oblique yellow stripes; feeds on oak, maple, and birch
• Cecropia moth caterpillar: a spectacular blue-green giant lined with rows of colorful knobs (tubercles) — yellow along the back, blue on the sides, and a few orange-red ones near the head; grows into North America’s largest moth
• Imperial moth caterpillar: a big, hairy caterpillar that ranges from green to burgundy-brown, with long white bristles and short horns behind the head

The Scary-Looking One That Is Actually Harmless

If you find something that looks like a small dragon — a fat blue-green caterpillar as long as a hot dog, armed with curved orange-and-black horns — you have met the hickory horned devil, the caterpillar of the regal moth and the largest caterpillar in North America. Despite the menacing spikes, it does not sting and cannot hurt you; the horns are pure bluff. It is a genuinely remarkable find, so admire it, take a photo, and let it continue on its way.

The One You Should Not Touch

There is one important exception. The io moth caterpillar is bright green with a red-and-white stripe running along each side, and its body is covered in clusters of branched, bristly spines. Those spines are venomous and deliver a painful sting on contact, so this is a look-but-do-not-touch caterpillar. As a general rule, any caterpillar covered in spines, bristles, or hairs is best left alone — the smooth, knobbed giant silk moth larvae are the safe ones to admire up close, but when in doubt, do not handle it.

Silk Moth Caterpillar or Hornworm?

The other big green caterpillars of late summer are hornworms — the larvae of sphinx (hawk) moths, including the tomato hornworm that strips garden tomato plants. The quickest tell is the tail: a hornworm has a single stiff horn or spike at its rear end and a smooth body, while giant silk moth caterpillars have knobs, spines, or tubercles spread over the whole body rather than one tail horn.

Not sure which giant you have found crossing the driveway? Snap a photo and let Spotter identify the caterpillar, so you know whether it is a harmless luna in the making, a hickory horned devil worth celebrating, or the one io caterpillar to leave alone.