Several species of Phylloxera, small aphid-like insects, live on hickory and pecan. Chemicals injected as the insects feed stimulate gall production on leaves, leaf petioles, twigs, or nuts. These hollow plant growths provide food, shelter, and protection for the insects. Their impact on the tree can vary depending on how many are present and where they settle to feed.
Galls on petioles can cause premature leaf drop, which has been seen in several areas of the state this year. Usually, light to moderate defoliation does not affect healthy, established trees but several years of significant leaf loss can stunt them and make them susceptible to other stresses. New trees in the landscape are much more vulnerable to injury.
Galls may stunt the growth of infested twigs; ones that are heavily infested may die. Galled nuts may be deformed and drop early.
Galled leaves are dramatic, especially when the galls turn from green to red but are less problematic.
Phylloxera spend the winter in the egg stage inside old galls or in bark crevices of hickory and pecan. The eggs in the spring and the tiny insects crawl to newly expanding foliage. After finding a suitable spot, they settle and begin to feed. This stimulates growth of surrounding tissue, forming the gall.
Some species have one generation each year, others have several. In addition to providing food and a favorable environment, the gall protects the insects from natural enemies and insecticide sprays.
Phylloxera usually don’t cause significant injury to established, thriving landscape trees but high infestations can stress or stunt newly established trees. Application of a dormant oil in the winter may help to reduce potentially damaging infestations.