Galls on trees are caused by insects laying eggs inside or feeding on the branches of leaves of trees and other plants. This usually occurs in the spring. The galls, or tumor-like growths, are produced by the tree in response to chemicals injected into it by an adult or larval gall-making insect. The shape of the gall is determined by the chemicals used by each species of gall-maker. Galls can be round and dense, woolly, fuzzy, veined, bullet-shaped or horned. Over 80% of galls reported in the U.S. grow on different oak species.
What insects cause galls to form?
Most insects that make galls are tiny wasps. Galls can also be caused by mites, insects, nematodes, bacteria or fungi.
Are galls harmful to trees?
Galls can have an ugly appearance. However, most do not seriously affect the health of a plant or tree. Heavy infestations may distort leaves or cause an early leaf drop. There is no need to remove the galls from a tree. The only sure way to prevent galls is to choose plants that are not hosts to gall-making insects and mites.
Sources for pictures and information:
- Gall Making Insects and Mites (pdf)
- Insect of the Month: Gall-Making Insects
- Oak leaf galls
- Galls on Oak Leaves