Adult mites become active as leaf buds open in spring. As mites feed on developing leaves, hormones cause abnormal plant cell growth that encloses the mites inside a gall where they’re protected. Adult mites lay eggs within the gall and die. The eggs hatch and feed, turn into adults, and exit the gall. Then they head for the trunk and branches to overwinter in roughened areas of bark.
Although this sounds scary and appears dramatic, the leaf bumps cause little or no health problems to trees. Since it’s mostly cosmetic, control isn’t usually recommended. Research indicates that there’s no adverse effect to the tree, unless over one-fourth of the leaf surface is grotesquely covered with bumps.
Control is difficult once the galls form because the mites are nestled safely inside, well-protected from insect spray applications. The best time to treat an infested tree is early spring before leaf buds swell, as adults are leaving their overwintering sites on the bark and heading for new leaf growth.
Apply horticultural oil, available at garden centers, to the bark of trunk and branches before spring bud-break. This kills mites as they are heading for new growth after exiting their winter home in bark crevices.