Our oak tree is about 70 feet tall. This spring, it developed dried-out bunches of leaves starting at the ends of the branches. It's rapidly getting worse, with more branches looking like they're dying. What’s going on? Can it be stopped? I’m noticing many oaks have this issue.
Oregon State University and the Oregon Department of Forestry have received many inquiries this year about branch flagging — or branch dieback — in Oregon white oak trees.
This issue involves a relationship between Oregon white oaks, oak twig gall wasps (Bassettia ligni) and western gray squirrels (Sciurus griseus). Gall wasps lay their eggs just under the bark of small oak branches. When the larvae hatch, they release chemicals that cause the tree tissue to swell. This creates a “gall” — a protective structure that surrounds the larvae.
Gall formation can girdle branchlets, leading to dieback. Squirrels often strip bark from those same branchlets to feed on the larvae hidden inside the galls. This debarking can worsen branch dieback.
The Oregon Department of Forestry has an oak pests fact sheet with more information about gall wasp and squirrel damage. You can also find more details in a OSU College of Forestry blog post.
Healthy, vigorous oaks often recover — even after a season with heavy branch flagging caused by gall wasps and squirrels. However, the warm, dry spring we had may be contributing to the unusually high level of damage this year. It’s possible that oak twig gall wasps may become a more significant stressor. Combined with other environmental factors, this could lead to localized oak mortality in some areas.
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