Walnut caterpillars feed on the leaves of walnut trees, but also those of pecan and hickory. A female lays hundreds of tiny white eggs in a cluster on the bottom side of a leaf. In the early stages, the caterpillar is red-orange with a black head and faint yellow stripes down the length of the body. It is already covered in fine, white hairs, but these are not stinging hairs. As is matures, the body becomes black and the hairs grow longer and thicker. When threatened, it raises its head and rear end in an effort to frighten a potential predator. Up to three generations are produced in southern parts of their range.
These caterpillars are often seen in clumps on the trunk of the tree they are feeding on. Most often, they feed on leaves high in the canopy, but they crawl down the tree to molt, leaving a hairy ball of shed skins before returning to foliage to continue feeding. This 'hair ball' can be scraped off the trunk with an obliging stick, even with living caterpillars still in it, and discarded at least 8 m or yards away from the tree. Mature caterpillars eventually dig into the soil to pupate. Late season caterpillars pupate underground through winter.
Walnut caterpillars are heavy feeders, but they are eaten by birds and other predators so they are not usually treated with other control methods. A tree that is mostly defoliated more than two years in a row may suffer in health and nut production, however, and intervention may be merited. Pruning off lower branches that are loaded with caterpillars is a good way to reduce their impact.
The map above showcases (in blue) the states and territories of North America where the Walnut Caterpillar may be found (but is not limited to). This sort of data can be useful in seeing concentrations of a particular species over the continent as well as revealing possible migratory patterns over a species' given lifespan. Some species are naturally confined by environment, weather, mating habits, food resources and the like while others see widespread expansion across most, or all, of North America.*NOTE: States/Territories shown above are a general indicator of areas inhabited by the Walnut Caterpillar. Insects generally go where they please, typically driven by diet, environmental changes, and / or mating habits.