Squirrels blamed for damage to Olympia oak trees

OLYMPIA, Wash. — Saving the imperiled Garry oak trees found on the prairies and oak woodlands of the South Sound may require a new attitude toward non-native eastern gray squirrels, according to Sound arborist Ray Gleason.

Gleason, owner of Cascade Tree Experts, has found several examples of Garry oaks in the Black Lake area south of Tumwater filled with dead, small-diameter branches.

The culprits appear to be eastern gray squirrels, flocking to the trees to eat gall wasp larvae known to infest oak trees.

The squirrels strip the bark off the branches in search of the larvae. In some cases, they’re doing enough damage to threaten the long-term health of the native oak trees, Gleason said.

Gleason wants to get the following message out to the public, especially those who feed eastern gray squirrels in their backyards:

“If you want to save the oak trees, you need to stop feeding and encouraging the squirrels,” he said.

The Garry oak, also known as the Oregon white oak, ranges from southwestern British Columbia into California, but the prairie habitat it prefers has suffered major losses due to development, farming and invasive species.

It’s not clear if the oak tree damage done by the nonnative squirrels is an isolated incident or more widespread, noted Chris Looney, an entomologist with the state Department of Agriculture.

Looney said a call went out last week to scientists and conservation groups working on Garry oak preservation efforts in the Northwest to see if anyone else has seen signs of oak trees filled with dead limbs, especially near the tops of trees.

“Is it super localized or has this damage been going on undetected?” Looney asked.

Gleason suspects that it is a fairly new phenomenon tied to exploding populations of eastern gray squirrels.

“If you want to save the oaks, you need to control the squirrels,” Gleason suggested.

Gleason found the squirrel damage when he was called by a Black Lake homeowner in July to remove a bunch of dead branches from their decades-old oak trees.

“I saw this damage that I’ve never seen before,” Gleason said, pointing to a branch less than one inch in diameter, devoid of bark. He said he traced the problem to squirrels scraping and chewing the bark off the limbs to feed on the wasp larvae under the bark.

Several species of gall wasps inhabit the native oak trees, Looney said. During their egg-laying cycle or early larval stage, the wasps secrete chemicals that interact with chemicals in the plant tissue to form tumor-like structures called galls.

In most cases, the oak trees can tolerate the parasitic wasps, Looney noted.

But the squirrels may be too much for the trees to handle, Gleason said.

“In 20 years, these trees could be in serious trouble,” he said. “The squirrels are topping the trees.”

Information from: The Olympian, www.theolympian.com