Down the Garden Path: Tree cavities – more than meets the eye

Winter provides us with a different perspective on the trees all around us, especially those deciduous trees which now stand without leaves. You may more easily notice the “imperfections,” such as callouses or holes. According to University of Illinois Extension Educator Christopher Evans, most cavities in a tree are caused by some type of living organism, either an animal or pathogen. Some cavities will only be found in dead trees or a portion of living tree with dead wood. Others can be found in otherwise completely healthy trees. Regardless of what has made a cavity in your tree, they often end up serving as valuable habitat for wildlife.

Let’s investigate some of the common causes of tree cavities or holes.

Insects

There are many different species of insects that bore into trees and feed on the wood in their larval stage. The larvae will eventually pupate and turn into an adult and they need to get out of the tree. They do that by chewing their way out and leaving holes in the tree. The exit holes from adults emerging are generally not what damage the tree. Instead it is the feeding galleries where larvae will burrow through the wood and feed. Most of our native wood-boring insects only feed on distressed or declining trees, but some of the invasive insects can attack healthy trees.

Birds

Woodpeckers will often target wood-boring insect larvae and other insects that live under the bark or in the wood. To get to those tasty meals, the woodpeckers will excavate cavities into the tree. Cavities from woodpeckers feeding are irregular in shape and ragged around the edges. They also will make places to nest in. These are generally larger and regular in shape.

The one exception to woodpecker feeding is the yellow-bellied sapsucker, which will drill shallow holes, called sap wells, into tree bark to feed on the tree sap. These wells are often created in either a straight line or a distinct grid pattern, and the network of cavities can get quite extensive. Generally, light to moderate feeding is not that impactful to the trees, but heavy feeding can be damaging to the tree. Look for sapsuckers in the winter where they feed on a variety of tree species, including maple, cherry, oak, pine and birch.

Fungi

Target cankers are conical shaped cavities in trees with concentric rings. A tiny wound is formed by the fungus causing the tree to form a thick ring of callous tissue to try to seal over the wound. Each time the tree does this, the callous tissue is killed off by the fungus, creating a wider and raised edge. This goes on and on, and the cavities grow wider and taller. Target cankers will not typically kill the tree but seriously deform and weaken the trunk. These are most often found on sassafras and maple trees.

Heart rot and other types of fungi can decompose the heartwood of a tree and sometimes other portions too. Trees will form callous tissue that will slowly close cavities and seal off the site.

Wildlife

Sometimes, animals like squirrels, will purposely chew the edge of a cavity to keep it open and available as a den site. Many birds, such as the yellow prothonotary warbler or Eastern screech owl, nest in cavities, as do many mammals. Tree frogs, lizards and even snakes will utilize tree cavities as resting or foraging sites. Bats often rear their young in tree cavities or under loose or peeling bark. Maintaining trees that have cavities is one of the best ways to improve wildlife habitat on your property.

For more information, check out University of Illinois Extension Educator Christopher Evans’ blog at extension.illinois.edu/blogs/naturalist-news.