Oak leaves in Illinois are showing small, irregular white spots. These are generally caused by piercing–sucking insects living on the leaf underside and removing the sap, including the green chlorophyll, from groups of cells. The genus Erythroneura contains many species of small leafhoppers that specialize on oaks. Adults are barely 1/8 inch long, are wedge-shaped, and fly readily. Many species are green, but others are whitish. They may have red or other colored markings on the back. The nymphs are also greenish or whitish, depending on the species, are wingless, and are smaller than the adults. Both nymphs and adults move sideways, typically crawling to the opposite side of the leaf or stem when disturbed.
Lacebugs are also numerous on oak and cause white spots by sucking the sap out of the leaves. Adults are about 1/8 inch long and blackish. Their lacelike, white wings have brown markings and lie flat across the back. The nymphs are blackish and smaller. Lacewing eggs are tiny, black, and laid in small, loose clusters on the leaf underside. Both nymphs and adults produce black, tarlike spots of feces that are very obvious on the leaf underside. In fact, if a leaf has white spots above and black spots underneath, the damage was almost certainly caused by lacebugs. If there are white spots above and no spots underneath, leafhoppers are probably to blame.
Oak skeletonizer windowfeeds on oak. Small, light-colored caterpillars eat through the lower leaf surface but leave the upper surface intact. These areas are whitish because the green tissue underneath has been removed. None of these insects normally cause enough damage to warrant control, but knowing what caused the damage is reassuring to the client.