Eight to 10 generations can occur during a normal growing season. ERM populations usually peak in July. Females begin laying overwintering eggs in early to mid-August. The mites injure the tree by feeding on leaves, destroying chlorophyll, and increasing respiration. This is accomplished by the insertion of the mite's mouth parts into the leaf cells to withdraw the contents.
All motile stages feed on the foliage.
The lower leaf surface is preferred, but both leaf surfaces are attacked when populations are high. All apple varieties are susceptible to attack, but the mites appear to increase faster and to higher densities on 'Delicious' and 'Yorking'. A characteristic brown foliage, starting as a subtle cast to the green leaf, but in severe cases becomes bronze, results from heavy mite feeding. The most serious injury occurs in late spring and early summer when trees are producing fruit buds for the following season. Mites feeding on leaves also reduce their ability to manufacture enough photosynthates for desirable sizing of fruit.