Tree bark serves a crucial function, acting as a protective barrier against external threats and facilitating the transport of essential nutrients and water throughout the plant. When this protective layer is compromised, it can lead to serious issues, including stunted growth, susceptibility to disease, and even tree mortality. Damage to tree bark is a common concern, attributed to various culprits from small rodents to larger mammals and insects.
Mammals That Chew Bark
Several mammalian species are known to chew on tree bark, each leaving distinct signs of their activity.
Deer, particularly bucks, can cause damage by rubbing their antlers against tree trunks, resulting in vertical scrapes and shredded bark, often exposing the underlying wood. This antler rubbing typically occurs from early fall to late winter and can be found as high as six feet from the ground. Deer also browse on young trees, tearing twigs and stems, leaving ragged edges due to their lack of upper incisors.
Rabbits and voles frequently target the bark of young trees and shrubs, especially during winter months when other food sources are scarce. Rabbit damage usually appears as gnaw marks on stems and branches within 18 to 20 inches of the ground, or higher if snow provides a ladder, often creating clean, angled cuts on smaller twigs. Voles, resembling small mice, typically chew bark near the ground, often hidden beneath snow, leaving small, side-by-side grooves about ¼ inch wide. This damage frequently girdles the tree, meaning the bark is removed all the way around the circumference, which can kill the tree by interrupting nutrient flow.
Squirrels are also common culprits, stripping bark from trees, particularly in early summer. They remove the outer layer to access the sweet phloem tissue underneath, leaving sections of bark ripped off, often with visible teeth marks. While they can damage branches, they often target younger trees less than 2.4 inches in diameter, and their tooth marks are smaller, around 1.3-1.7 mm wide.
Porcupines feed on young bark and terminal twigs, preferring the nutritious inner bark. Their damage usually appears higher in the tree’s crown but can extend to the trunk, leaving distinct grooved tooth marks about 5 mm wide.
Beavers are known for their ability to fell trees, but they also girdle trees by stripping bark all the way around the trunk. They target trees for food and construction materials, leaving pointed, conical stumps and visible chew marks on the remaining wood. This girdling disrupts the flow of nutrients and water, leading to the eventual death of the tree above the damaged area.
Insects That Damage Bark
Insects also contribute to bark damage, though their signs are typically different from those left by mammals.
Bark beetles, which are small, cylindrical insects, bore through the bark and feed on the cambium layer just beneath it. Their activity often results in small entry holes, reddish-brown sawdust-like frass accumulating in bark crevices, or pitch tubes—small masses of resin mixed with boring dust—on the outside of the bark. Beneath the bark, they create characteristic tunnel patterns, or galleries, where they lay eggs, which can be seen if the outer bark is peeled away.
Tree borers, often the larval stage of beetles or moths, tunnel into trees to feed on the inner wood. Initial signs of borer infestation include sawdust, or frass, on or around the tree, and sap or resin oozing from small holes. As they mature, borers create exit holes in the bark, which can vary in shape depending on the species; for example, emerald ash borers leave D-shaped exit holes. Their feeding can create winding tunnels under the bark, which may cause the outer bark to appear swollen or to peel off.
Cicadas primarily cause damage during their egg-laying process. Female cicadas cut slits into slender twigs and branches, typically 1/8 to 7/16 inches in diameter, to deposit their eggs. These incisions weaken the twigs, causing them to wilt and turn brown, a condition known as “flagging.” The damage appears as a zipper-like pattern along the twigs and can lead to the snapping and hanging of branches. While mature trees usually tolerate this damage, young or newly planted trees can suffer significant injury.
How to Identify the Culprit and Understand Their Motives
Animals consume bark for various reasons. During winter, when other food sources are scarce, mammals like rabbits, voles, and porcupines turn to bark for sustenance, seeking the nutritious inner bark layer. Squirrels may strip bark to access the sweet phloem or to obtain calcium, especially during periods of nutritional need.
Beavers use bark as a food source and wood for building dams and lodges. Some animals, particularly deer, damage bark not for consumption but for other behaviors like antler rubbing to remove velvet or mark territory. Insects, such as bark beetles and borers, feed on the inner bark or wood as part of their life cycle, while cicadas damage twigs for egg-laying.