Woodpeckers have been seen across the region chipping away at the bark of young bur oak. They are drilling into the bark in search of these small larvae of the gall wasp Callirhytis flavipes. During the winter the small, white larvae are found within the inner bark of the branches and twigs of mature oak trees and the trunks of young trees. The gall wasps emerge in the spring as adults and move to the newly expanded leaves where they insert eggs into the midrib, the central vein of the leaf. Once the eggs hatch, the larvae form a gall on the vein and live out their short lives within this structure. Adults emerge later in the season and lay eggs on the twigs and branches.
Tree Injury & Other Issues
The galls formed by this gall wasp are not particularly harmful to the tree, no more than the many other galls that form on oaks. What makes this gall wasp a problem is the woodpeckers that feed on the larvae during the winter. The woodpeckers can shred most of the bark from young trees, enough that the trees are killed by this injury. The trees that are not killed by the woodpecker activity, often have the tops killed back enough that the trees become misshaped and of little value as a windbreak tree.
Management of the problem is difficult. Some people have tried protecting their small oaks with Tanglefoot Bird Repellent® on the trunk. This is a sticky material that comes in a caulking tube that can be smeared on the trunk to discourage woodpeckers. This is a very time-consuming task and must be repeated every year. Insecticides to kill the gall wasps have not been completely evaluated yet. The timing for insecticide sprays is critical and the gall wasps are flying for an extended time period in the spring and late summer. Injecting insecticides to kill the larvae as they feed have not proved successful yet for Callirhytis. Not all trees are infested by the gall wasps. It is very common to find several bur oaks growing near one another and only one tree infested by the wasps. The bark on the infested trees appears to be less furrowed than the uninjured tree but this is difficult to evaluate as the woodpeckers have often removed so much bark it is hard to tell the origin texture.