Walnut Trees & Nuisance Insects
- Insects that attack walnut trees can become a nuisance and cause cosmetic damage. They can damage leaves, change the taste of the kernels and make the trees look unsightly. However, few insects can pose a real threat to the life of the trees.
- Oyster shell scales pose a more serious threat to walnut trees compared to most insects that attack the trees. They appear like tiny, gray to dark brown oyster shells that measure less than 1/8 inch in length. They attach closely to the twigs in groups or separately. People often don't notice oyster shell scales until the tops of the walnut trees die back. Eventually, they cover entire branches and trees and kill the whole plants.
- Husk fly adults lay eggs in the husks of walnuts. When the eggs hatch, maggots emerge and feed on the walnuts, producing a substance that penetrates the shells and stains the kernels. These kernels appear unattractive and don't taste good. During winter, these insects pupate in the soil under walnut trees, transforming into adult husk flies when the weather gets warmer. Their size resembles that of common house flies, but they appear yellowish brown with brown bands on their wings.
- Walnut caterpillars feed in clusters and damage the leaves of walnut trees late in the growing season. They often cause little damage because the trees have finished growing when walnut caterpillars appear. They appear reddish brown and measure about 2 inches long. They have black bodies with fine hairs on the surface. They raise their heads and tails when disturbed. Walnut caterpillars pupate in the soil during winter and emerge as moths in spring or summer.
- Fall webworms attack 85 species of trees in the USA, including walnut trees. They create light gray webs with a silken appearance on trees in the late summer and early fall. They use leaves and small branches to form their nests. Fall webworms usually start feeding on leaves late in the growing season and only create nests in limited areas. As such, they usually cause little damage to walnut trees. However, their nests can make the trees look unsightly and they can remove many leaves.
