Giant Green Caterpillar in the Adirondacks: Hickory Horned Devil or Something Else?

When a “Huge Worm” Isn’t What You Expect

A 3–4 inch greenish-grey caterpillar with a rear horn and four “Christmas tree” horns on its head was found in upstate New York. The initial ID—hickory horned devil (Citheronia regalis)—matches many features: up to 4.5 inches, dark green body, black prolegs, four long curved horns on anterior segments.

But the Finder Disagrees

Follow-up details: green head, serrated “Christmas tree” projections (green with white tips), single dorsal line of similar bumps, pointed prolegs (not suction-cup). This doesn’t perfectly match the hickory horned devil.

Why Caterpillar ID Is Tricky

• Thousands of species; many look similar.
• Color and morphology change with instar.
• Regional variations exist.
• Verbal descriptions are inherently ambiguous.

Best Path to Certain ID

1. Photograph the caterpillar clearly (top, side, head).
2. Include a size reference (coin, ruler).
3. Email to jimhyland@yahoo.com or upload to BugGuide.net, iNaturalist, or a university extension service.
4. Note host plant—caterpillars are often host-specific.

Likely Candidates for NY

• Hickory horned devil (walnut, hickory, sumac, etc.)
• Luna moth caterpillar (green, smooth, no horns)
• Polyphemus moth caterpillar (green with silver spots)
• Various sphinx/hawk moth larvae (hornworms)

Until a photo confirms the species, avoid handling—some caterpillars have urticating spines. Most are harmless and ecologically valuable.