Squirrels have removed significant amounts of bark from this maple to get at the tasty cambium layer underneath. / Photo Credit: Linda Williams, Wisconsin DNR

By Linda Williams, DNR Forest Health Specialist
Linda.Williams@wisconsin.gov or 920-360-0665

Squirrels can be fun to feed and fun to watch, but when they start damaging or killing your trees, well, that’s not so cute.

It’s the time of year when we start to have some warm days that allow sap to start moving in trees. If there are broken branches or damaged bark, the tree can drip sap. And that can attract squirrels.

You may be wondering why this would be a problem.

Partially girdled branches may leaf out normally but die when hot dry days require more water movement than the damaged branch can handle. Dead branches were fully girdled by squirrels in late winter. / Photo Credit: Linda Williams, Wisconsin DNR

Once gray squirrels find that bit of sap, they will begin to remove the bark from the branches and the main stem so that they can eat the tasty cambium layer just under the bark, where the sweeter sap is flowing. Often, they’re like a kid with a candy jar — they just keep coming back for more. As they repeatedly visit these trees and remove more and more bark, you can see the white wood underneath.

This level of damage, in which all the bark is removed from branches and the main stem, will girdle the tree at that point. In the spring, buds with some stored nutrients may be able to break bud and even start to enlarge, but will rapidly wilt and die because the tree can no longer move food and water to those buds. If the damage is severe, the entire tree can be killed.

If your tree has only some squirrel damage on branches, but not enough to fully girdle the branch, that branch may leaf out normally in the spring. Unfortunately, later in the summer, when it’s hot and dry, and the tree needs to move a lot of water to the leaves, those damaged branches can’t move as much water, and the leaves can wilt and die.

Damage can be localized to a few trees or scattered across much of a county. Over the past 10-plus years, this kind of damage has become more prevalent around the state.

Controlling the population of squirrels is just about the only option for stopping this kind of damage. University of Wisconsin-Madison Extension has a document on squirrels, including some additional control options for urban or yard trees.