The apple grain aphid is the least damaging. Their feeding causes developing fruit to be malformed, knotty, and reduced in size. From a distance, foliage may appear dusty. Heavy infestations can cause defoliation and reduce fruit size and quality. They may be found clustered near wounds on trunk, branches and roots.
Root infesting forms cause the most serious damage.
Infested roots often have numerous gall-like knots hearing the wooly masses of aphids. Fibrous roots can be completely destroyed, causing stunting of trees and sometimes tree death.