Found this enormous green worm with huge red devil horns on my walnut tree. It looks like a dragon and is terrifyingly big. What kind of monster is this?

Imagine strolling through your backyard on a crisp fall day, only to be stopped in your tracks by an enormous green caterpillar dangling from your walnut tree. Its size is daunting, its color vibrant, and its bright red horns seem to scream danger. At first glance, it might appear to be something out of a science fiction movie—a creature lost in time or from another world.

However, this seemingly monstrous insect is a natural part of the ecosystem. This article seeks to unravel the mystery behind this intimidating yet fascinating creature, shedding light on its true identity, its purpose, and what to do if you encounter one.

The hickory horned devil is the largest caterpillar found in North America, reaching lengths of up to 5.5 inches (14 cm). Its striking appearance is characterized by its vibrant green body and long, spiky red horns that give it a fearsome dragon-like appearance. Despite its intimidating look, this caterpillar is harmless to humans and is the larval stage of the regal moth, also known as Citheronia regalis.

The hickory horned devil's red horns are its most distinctive feature and serve as a defense mechanism. These horns, while menacing, are entirely harmless. They are not venomous, nor do they sting. Instead, they are meant to deter predators by giving the caterpillar a more formidable appearance.

Aside from its size and horns, the hickory horned devil can be identified by its bright green body, which may have blue or yellowish tinges. Its body is segmented, and it has small black spots along its sides. During its final instar, it may turn to a turquoise or bluish color before it pupates.

After spending a few weeks in its caterpillar form, the hickory horned devil will burrow into the ground to pupate and eventually emerge as the regal moth. This moth is notable for its large size, with a wingspan that can reach up to 6 inches (15 cm). Its wings are orange with yellow and white markings, and its body is robust and furry.

Despite its alarming appearance, the hickory horned devil poses no threat to humans or pets. It does not bite or sting, and its horns are not harmful. It is safe to observe these creatures up close, although handling should be done gently to avoid stressing the caterpillar.

The hickory horned devil is not a significant threat to trees or gardens. While they feed on the leaves of various hardwood trees, including walnuts, hickories, and persimmons, their numbers are not usually large enough to cause noticeable damage. They are solitary feeders and rarely occur in numbers that would be detrimental to plant health.

Hickory horned devils are native to the southeastern United States but can be found as far north as New York and as far west as Missouri. They are typically seen in late summer to early fall when they descend from the trees to find a site to pupate.

If you find a hickory horned devil on your tree, the best course of action is to leave it be. These caterpillars are part of a healthy ecosystem and will soon transform into moths. If you need to move it, do so gently and relocate it to a similar tree or shrub where it can continue its life cycle.

While the hickory horned devil is quite distinctive, it can sometimes be confused with other large caterpillars, such as the cecropia moth caterpillar or the luna moth caterpillar. The key differences are the horn structures and the specific color patterns. Hickory horned devils have the most pronounced red horns, whereas the others do not.

Hickory horned devils play an important role in their ecosystem by serving as prey for birds and other wildlife. Their presence indicates a healthy balance of flora and fauna. As adults, regal moths are important pollinators, contributing to the biodiversity of their habitats.

Generally, it's best to leave the hickory horned devil alone unless it's in immediate danger, such as in a heavily trafficked area. Relocation should be done with care and only if necessary. If you find an unusually large number of these caterpillars or if they are causing significant damage, it may be worth consulting with a local extension office or entomologist for advice.