The Gall(s) of Some Trees

Galls are irregular plant growths that are stimulated by the reaction between plant hormones and powerful growth regulating chemicals produced by some insects and a few mites. Galls may occur on leaves, bark, flowers, buds, acorns, or roots. Their inner walls provide a high quality food source for the inhabitant, as well as protection from insecticide sprays. Some galls provide shelter from natural enemies.

Gall makers must attack at a very specific time in order to be successful. Otherwise, they may not be able to stimulate the plant to produce the tissue that forms the gall. Generally, initiation of leaf galls occurs during the “bud break” stage or as new leaves begin to unfold.

Gnarled Oak Leaf Galls

Gnarled oak leaf galls, caused by the larvae of a small fly/midge, are appearing on pin oaks. The thickened distorted growth of infested leaves resembles chemical injury but occurs on randomly scattered leaves rather than affecting large contiguous areas that would be expected from chemical drift.

Opening the gall reveals the small maggots that live there for a few weeks before dropping to pupate in the soil. Cars parked under galled trees may be covered with what appears to be wriggling rice grains, which are the maggots that have “rained” down from above.

Figure 1. Gnarled oak leaf midge gall (Photo: M. Stanton, Kenton County Extension agent for Horticulture)

Figure 2. Gnarled oak leaf galls are full of small white maggots (Photo: Lee Townsend, UK)


Insect Gall Management

Once the symptom or gall appears, the causative animal is protected within the structure. This means that remedial actions, other than pruning in some cases, are not effective. Preventive action is necessary to attempt to reduce the infestation the following season, which may be of limited value. Fortunately, most galls, especially those on leaves and leaf structures, do not harm tree health.


By Lee Townsend, Extension Entomologist


Posted in Landscape Trees & Shrubs