Leaf gall mites are much too small to be seen. Each mite is only about one-tenth of a millimeter wide. What most black walnut leaf owners notice is the damage that leaf gall mites leave behind. As they feed on the leaves, they leave behind a trail of raised bumps that resembles a caterpillar's body. This damage is unsightly, but even a significant infestation of leaf gall mites will not damage the health of a black walnut.
Remove all affected leaves and any leaf litter. The gall mite lives on the surface of the leaves. However, take care not to remove more than one-third of your black walnut's leaves no matter how severe the infestation. Bag the leaves and throw them away. Do not compost them.
Spray the twigs and leaf buds of your black walnut with a miticide containing oxythoquinox or carbyl in early spring as the buds are swelling. Gall mites spend the winter in the black walnut's leaf buds and they feed as the leaf develops. Spraying the buds will kill many of the mites before they have a chance to feed.
Repeat steps 1 and 2 the following year. Leaf gall mites are nearly impossible to get rid of, especially in one attempt. But if you treat your black walnut year after year, you can reduce the mite population to a negligible number.