Most cavities in a tree are caused by some type of living organism, either an animal or a pathogen of some sort. Some cavities will only be found in dead trees or a portion of a live tree with dead wood. Other cavities can be in otherwise completely healthy trees. Let’s take a look at some of the different causes of tree cavities and holes.

Insect exit holes

There are many different species of insect that bore into trees and feed on the wood during their larval stage. Some beetles will introduce a fungus into the tree so they can feed on that. Those larvae will eventually pupate and turn into adults and need to get out of the tree. They do that by chewing their way out, leaving holes in the tree. The shape and size of the hole can help identify the insect. D-shaped holes are made by metallic wood boring beetles (family Buprestidae) like the Emerald Ash Borer.  Large round holes are often from long-horned beetles (Family Cerambycidae) like the pine sawyer. The exit holes from adults emerging are generally not what damage the tree, instead, it is the feeding galleries. The larvae will snake through the wood and feed. Some species restrict their feeding to the inner bark and outermost layer of wood. Other species can bore down deep into the tree. Most of our native wood-boring insects only really feed on stressed or declining trees that have had their defenses weakened. Some of the exotic invasive insects can attack healthy trees.

Woodpeckers

Woodpeckers will often target wood-boring insect larvae or other insects that live under the bark or in the wood of a tree. To get at those tasty meals, the woodpeckers will excavate cavities in the tree. If the insects are just under the bark, these cavities will be small. If the insects are deeper in the wood, these cavities can be quite large. Cavities from woodpecker feeding are irregular in shape and ragged along the edges. Woodpeckers will also make cavities to nest in. These are generally much larger and regular in shape. We have seven species of woodpecker that are regularly found in Illinois: downy (the smallest, coming in at around 5-6 inches in length), hairy, red-bellied, red-headed, northern flicker, pileated (the largest, getting up to nearly 20 inches in length) and the yellow-bellied sapsucker, which we will discuss next.

Sapsucker feeding

The one exception to woodpecker feeding is the yellow-bellied sapsuckers. These are a species of woodpecker that migrates south into Illinois for the winter. Unlike the other woodpeckers, sapsuckers drill shallow holes, called sap wells, into tree bark and feed on the tree sap. These wells are often created in either a straight line or a distinct grid pattern. The cavities are nor ragged-edged. Instead, they are tiny and round but sometimes, the sapsucker enlarge the wells into a smooth-edged square 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.

Target cankers

Target cankers are conical-shaped cavities in trees with concentric rings. These are formed by a fungus in the Neonectria group. A tiny wound is formed by the fungus, causing the tree to form a ring of thick callous tissue to try and seal over the wound. Each time the tree does this, the callous tissue is killed by the fungus, creating a wider and raised wound ridge. As this goes on and on, the cavities grow wider and taller with noticeable rings of tissue. Sometimes the diameter of the canker is much larger than the tree itself! Target cankers typically will not kill a tree but can seriously deform and weaken a trunk. These are most often found on sassafras and maple trees but can be found on others. Usually, trees can resist the formation of target cankers but weakened or stressed trees are most susceptible. There are many other types of cankers that can be formed in a tree as well that have different shapes.

Wood-decaying fungi and other decomposers

Heart rot and other types of fungi can decompose the heartwood of a tree and sometimes other portions of the tree. Tree branches that die and break off may lead to a cavity forming as the wood of that dead limb rots, but the live wood around it remains. Trees will form callous tissue that can slowly close cavities and seal off the site where a limb was lost, but sometimes the cavity is extensive in size and does not close easily. Other times animals, like squirrels, will purposefully chew the edges of the cavity to keep it open and available as a den site.

Basal damage

Basal damage simply describes the location of the wound or cavity, which would be at the base of the tree. Cavities formed here can be started from mechanical damage from repeated mowers hitting the tree, damage from logging where an adjacent was felled into that tree, fire wounding the bark, etc. Sometimes basal damage is facilitated by root rot or other pathogens.

Regardless of what has made a cavity in your tree, they often end up serving as valuable habitat for wildlife. 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 or dead snags that have cavities is one of the best ways to improve wildlife habitat on your land.

MEET THE AUTHOR
Chris Evans is an Extension forestry and research specialist and interim state coordinator for the Extension Master Naturalist Program. He has been with the Extension for over five years. Chris has a bachelor’s degree in wildlife biology from Murray State University and a master’s degree in forest biology from Iowa State University. He is interested in forest health and management, native plant restoration, and invasive species management.