Insect galls are plant growths often caused by interactions between plant hormones and growth regulating chemicals produced by some species of wasps, flies, or mites. Galls provide their inhabitants with needed nutrients, proper environment, and protection. They may occur on leaves, bark, flowers, buds, acorns, or roots. Some species form galls on different plant parts in alternating generations. With the exception of some twig and stem galls, most of these growths do not harm trees.
An example is the wool sower gall, caused by a small wasp. It appears on white oak in early summer and resembles clusters of toasted marshmallows. The gall is comprised of a group of smaller, hairy, seed-like structures, each containing a developing wasp.
Usually, there are only a few scattered galls on trees. Weather, predators, parasites, and diseases usually combine to keep gall numbers at low to moderate levels. Most galls have little if any affect on healthy trees. However, horned and gouty oak galls can be very destructive. See Horned Oak Gall (EntFact-457) for more information on them.
- Insecticide sprays of foliage or use of systemic products usually do more harm to natural enemies than to gall makers, so treatments are not recommended.
- Cultural practices that reduce stress and promote general growth and health allow trees to tolerate galls and other arthropods that develop on trees.
- Pruning stem and twig galls when populations are low may provide some control.
By Lee Townsend, Extension Entomologist