What caused the galls to form on this pecan leaf?
These galls were caused by an insect called the pecan leaf phylloxera (Phylloxera notabilis). These are tiny aphid-like insects that feed on immature pecan tree foliage. Due to their size, the insects themselves are rarely seen. The feeding damage causes rapid growth of the affected tissues. As the abnormal growth develops, it encloses the phylloxera and creates the galls. Phylloxera are not likely to cause significant harm to the host trees; therefore, treatment is rarely needed. Read more about pecan leaf phylloxera here.