Black walnut trees can attract several types of worms that feast on the tree's foliage. The worms are usually moth species. While the worms can cause serious foliage damage, causing the tree to look less than beautiful, the worms cause little other damage to the tree.
Webworms form large webs in which they live and eat on the end of the branches of black walnut trees. The worms, which are actually caterpillars, usually appear in late summer or early fall. Another worm, the tent caterpillar, builds smaller webs and lives outside the web while it forages on leaves.
Because web and tree worms start foraging on late summer leaves, they do little damage to the tree, since the leaves are about to drop off in the fall anyway. The caterpillars do leave unsightly branches with chewed-on leaves, but the worms cause little more than cosmetic damage.
To help control worms, consider pruning infested branches from the tree. Destroy the branches so the worms don't infest other plants or trees in the yard. Additional control includes keeping the trees in healthy shape to keep them from becoming weak and susceptible to insects.