Question
We have a shagbark hickory tree that seems to have an aphid infestation.  It has small green balls about one half inch in size on the leaves and twigs. Some of these are dropping to the ground.  What treatment is recommended or will the tree look after it self. This is the second year we have noticed this problem.  Thank you for any help you might offer

Answer
These are insect galls. In general the small wasp like insect lays its eggs on the twig and secretes a chemical that causes the cell on the twig to swell and grow over the eggs making the gall. The immature insects can often be found in a cell or cells within the developing gall. After a brief period of cell growth, gall development stops. The insect becomes enclosed by the gall and feeds only on gall tissue during its development. Small holes on the outside of the gall indicate that the adult insects have emerged.


Generally galls do not seriously harm the plant. If the tree or ornamental plant is unhealthy or under stress it will be damaged to a greater extent than healthy ones. Fertilize, irrigate, prune and carry out other approved practices to keep plants healthy. If practical, some galls that occur on the stems and limbs may be pruned and destroyed.

After formation, it is impossible to eliminate the galls or the pests with insecticides because they are enclosed and well protected inside the gall. Those that occur on the leaves will drop off with the leaves, but those occurring on the trunk, roots, and stems may persist for several years.

Insecticidal control is usually not practical because: (1) most of the time, injury is not serious; (2) correct timing of application is essential to control the adult pests before the gall is formed; and (3) there are considerations of environmental contamination and expense involved, especially on larger trees.

I would not worry about the galls on the oak limbs. The wasp population will vary greatly and maybe non existent one year and abundant the next. I would fertilize the tree with 10-10-10 fertilizer at the rate of 1 lb per inch of trunk diameter scattered around the tree and water in good. Once a year fertilizer is enough.