Walnut Caterpillar

How to identify it

• Full-grown caterpillars are about 2 inches long, black with long whitish-gray hairs.^[1][2]
• Small larvae are pale yellow-green; middle-sized larvae are dark red with white stripes along the sides.^[1][2]
• When disturbed, they arch their head and tail into the air.^[2]

Where it lives

It occurs throughout the eastern United States and is moderately common on walnut trees in Iowa.^[2]

US map: public records from 34 states since 1990 (GBIF). View the map

Biology and life cycle

Moths emerge from the soil in late spring or early summer and lay eggs in masses of hundreds on the undersides of leaves.^[1][2]

Caterpillars feed in clusters and crawl down to the trunk or a large limb in a group to molt, leaving a hairball of shed skins.^[1][2]

There are two generations a year in North Carolina; they spend the winter as pupae in the soil.^[1][2]

Damage and signs

• Most of the time they feed on walnuts, pecans and hickories; young larvae skeletonize leaves and older ones eat the whole leaf except the stalk.^[1][2]
• Trees completely defoliated two years in a row may be stunted, but late-season defoliation does little harm to healthy, established trees.^[1][2]

Health risk

No health risk is noted by either source.^[1][2]

Control

Traps and physical control

Prune off leaves or twigs with egg masses or young clusters in early summer on small trees, and scrape molting masses off the trunk into a bucket or bag.^[1][2]

Biological control

Birds and other natural enemies usually keep numbers low enough that serious defoliation is rare.^[1][2]

Pesticides

Sprays work best on small caterpillars; NC State names carbaryl and pyrethroids, and keep pyrethroids away from pools, ponds and streams.^[1]

Common questions

What is the hairy gray mass on my walnut tree trunk?

Likely a group of walnut caterpillars that crawled down together to molt.^[1][2]

Will walnut caterpillars kill my tree?

Rarely; late-season defoliation does little harm to healthy trees, though two years of complete defoliation can stunt a tree.^[1][2]

How do I get rid of them without spraying?

Remove egg masses and young clusters on small trees, or scrape the molting mass off the trunk.^[1][2]

Sources

1. NC State Extension, “Walnut Caterpillar (PDIC Factsheet).” content.ces.ncsu.edu/walnut-caterpillar (accessed 2026-10-15)
2. Iowa State University Extension and Outreach (Yard and Garden), “Walnut caterpillar.” yardandgarden.extension.iastate.edu/encyclopedia/walnut-caterpillar (accessed 2026-10-15)
3. iNaturalist, “Datana integerrima taxon page (order and family).” www.inaturalist.org/taxa/218166 (accessed 2026-10-06)