Often, as you are chopping firewood for personal or commercial use, you may have found some small worms crawling around in the wood. They can be very small or they can be as large as your thumb. Seeing these creatures might cause a moment of panic in an unsuspecting person, who doesn’t know what they are or why they have taken up residence inside of their tree.
The good news is these worms are generally harmless. They are actually grubs, or insect larvae. There are a variety of different insects that burrow inside of trees and lay eggs there. Once the eggs hatch, the larvae grow inside of the wood until they pupate and mature into full grown insects.
A common species of wood boring insect is the Pacific flatheaded borer. This is a beetle that has been a huge pest in the United States in Canada, seeking out weakened or stressed trees and burrowing into them. If the tree is small enough, it might only take one borer to kill the whole tree.
Pacific flatheaded borers usually burrow into the side of the tree that is facing the sun, leaving a dark colored depression in the bark. While most wood boring insects, such as termites, throw out a powdery excrement called “frass,” Pacific flatheaded borers tend not to create very much of it. This makes them a bit harder to detect than other insects, but you can usually notice their presence by cracked bark or the aforementioned dark wounds in the wood. Eventually, the larvae will mature into a flat, brown beetle that ranges in size from about one quarter to one half inch. They usually have grey markings on their backs and a brilliant green under their wing covers.
Pacific flatheaded borers tend to hatch between mid-June and mid-August, leaving grubs to feed between the bark and sapwood as they grow. Like many boring insects, they prefer sick and stressed trees. If there is a healthy amount of sap in the tree, the Pacific flatheaded borer grub may not survive.
Another common pest among the wood boring beetles is the golden buprestid beetle. These beetles are a bane for not only tree owners but homeowners as well, as they prefer to burrow into dead or dying wood, and may continue to live inside of wood that has already been harvested to build homes. Like other species of wood boring beetles, they lay their eggs in weakened or dying wood and the larvae will feed until they mature into full grown beetles themselves. This particular species can stay in its larval stage for years, so you might notice larvae inside of firewood made from lumber that was chopped years earlier.
The damage the golden buprestid beetle leaves behind looks similar to, and can be mistaken for, the damage left by carpenter ants. They leave small fecal pellets behind them, and create small tunnels with rounded bases. When the beetle matures, it becomes an iridescent green with gold at the borders of its wings.
The California root borer is very common on the west coast of the United States. This particular beetle burrows into the roots of trees and lays its eggs there. The larvae are usually a cream or brownish color and they tend to burrow upward through the roots. Eventually, they will pupate near the soil surface and mature into a reddish brown beetle, which crawls up through the soil by early August.
The new house borer is a large black beetle that infests soft lumber. The beetle gets its name because the larvae take two years to mature. In this time, the wood can be harvested and used to build a house, startling unsuspecting homeowners as large beetles begin emerging in their new home! They will not, however, continue to lay eggs inside the home once they emerge; like the other wood boring insect species, they prefer sick or stressed trees and not wood that is already dead.
These are just a few examples of wood boring insects that might leave grubs inside the lumber you use as firewood. While they are capable of being huge pests, chances are they are not responsible for any lasting damage to your tree, provided you have made sure that your tree is healthy. They also cannot spread to other trees while they are in larval form; only the fully mature beetles are capable of doing this. As long as you are frequently inspecting your trees for signs of stress or damage, and occasionally having a tree service professional check on the health of your tree, wood boring insects should not be a major problem for you.
These grubs are also an important part of the ecosystem; they serve as a food source to woodpeckers, as well as other insect species. Despite their general harmlessness, you do still want to take care when dealing with these species. Buying local firewood will help to ensure that pest species are not brought across state lines, where they could potentially do damage once they mature. Harvesting firewood during winter when wood boring insects are dormant is also a good way to avoid finding these grubs in your lumber.