Buck Moth Caterpillar

The buck moth caterpillar is the larval stage of Hemileuca maia, a day-flying member of the giant silkworm moth family (Saturniidae) native to eastern and central North America. It is best known for feeding gregariously on oak trees and for the branched, bristling spines that cover its body, which have made it one of the more conspicuous and talked-about caterpillars of spring in oak-dominated landscapes.

Early instars stay clustered together on host foliage, then disperse somewhat as they mature, eventually wandering down trunks and across the ground in search of pupation sites. In urban and suburban settings with mature oak trees, mass movements of late-stage larvae across sidewalks and streets are a familiar seasonal sight.

• Body dark brown to black, sometimes with a reddish or purplish cast, covered in branching, spiny tubercles arranged in rows
• A pale, cream-to-white stripe often runs along each side of the body
• Spines are stiff, multi-branched, and give the caterpillar a bristly, almost cactus-like appearance
• Head capsule dark and shiny
• Mature larvae reach roughly 2 to 2.5 inches (50-64 mm) in length
• Early instars are smaller, darker, and feed in tight clusters on leaf undersides before dispersing

Buck moth caterpillars are found wherever their host oaks grow, including open oak woodlands, sandy pine-oak barrens, coastal scrub, and residential or urban areas planted with oak trees, across much of the eastern and central United States. Larvae are present from early spring into early summer, feeding on developing oak leaves before the canopy fully matures.

Young larvae feed in tight, synchronized groups on oak foliage, which helps them overwhelm a leaf's defenses and may offer some protection from predators. As they grow, the caterpillars spread out and feed more independently, stripping sections of oak canopy in heavily populated years. Mature larvae descend to the ground to pupate in leaf litter or loose soil. In defense, the caterpillar relies on its dense coat of branched spines rather than fleeing, and it can sting in defense if handled or brushed against. As oak specialists, buck moth larvae are an important seasonal food source for birds and other predators that have learned to avoid or tolerate the spines, and they contribute to the natural thinning of oak foliage.

Adult buck moths are unusual among Saturniidae in flying by day, typically in autumn, when females lay clusters of eggs in rings around oak twigs. The eggs overwinter on the twig and hatch the following spring as new oak leaves emerge. Larvae pass through several instars over the spring, feeding on oak foliage, before dropping to the ground in late spring or early summer to pupate in the soil or leaf litter. The species overwinters as eggs, and adults emerge in fall to complete one generation per year in most of its range.