Phylloxera are small, aphid-like insects that overwinter in the rough bark of pecan trees. In early spring, just when the first new leaf bursts from expanding buds, phylloxerans crawl out onto the emerging leaves and start to feed. As leaves expand, phylloxerans secrete a substance that causes pecan leaves to grow a galls around each insect.
If phylloxera galls become so numerous they seriously distort leaves, you will see early defoliation of infested leaflets starting in July. To control phylloxera, an insecticide should be applied early in the spring at leaf burst before galls start to form.