Now that we are in the dead of winter, many people are probably burning firewood and gathering more for the cold weeks ahead. Personally, I enjoy a fire in the fireplace on cold winter nights, but I also have to remind myself that firewood can include unwanted visitors and hitchhikers. As it turns out, firewood can help spread invasive species such as pests and diseases.
By now, I am sure you have heard about the infamous and destructive emerald ash borer, which has killed millions of ash trees throughout the Midwest. EAB is a good recent example of an invasive pest that is spread by firewood and other wood wastes. An older example is Dutch elm disease, which arrived from Europe years ago in elm burl logs and devastated virtually all of our American elms.
How does firewood facilitate the movement of invasive pests and diseases? Many types of insects live in dead or dying trees. One of their jobs is to help in the breakdown and decomposition of the wood, and many use trees as a place to reproduce. In the case of wood-boring insects such as EAB, the immature insects (i.e., larvae) feed in the vascular system of the tree, forming galleries. These galleries prevent the tree from transporting water and nutrients to make food and to move food to where it is needed. Eventually, the tree dies.
Bark beetles are similar, except they tend to mass attack, with the adult boring into the tree. Like borers, the young bark beetle larvae form galleries, where they pick up various types of fungi on their bodies. When the adults exit the tree, they take the fungal spores with them. When adult beetles attack weakened or even healthy trees, they introduce the fungus into the new host. The fungus spreads and plugs up the water conducting vessels (xylem) in the tree and the tree wilts.
Dutch elm disease and oak wilt are a couple of vascular wilt diseases that are spread by bark beetles. Other bark beetles, such as the walnut twig beetle, carry a fungus that causes “thousands of cankers” in black walnut trees — hence the name, thousand cankers disease.
Where is the firewood connection? If we remove trees, whether before they die or afterward, and there are still insect life stages or fungal spores present under the bark, we run the risk of spreading the pest or disease long distances. For example, the adult EAB can only fly a few miles on its own, and probably would never have made it out of the Detroit, Mich., area where it was first found. But due to human movement of firewood and related wood products, it quickly spread to adjoining states and is now as far west as Colorado. The same issue exists with elm and walnut trees if these trees are cut into firewood and then moved around.
How can you prevent long-distance movement of infected or infested firewood? First, if you are going to cut down a tree that has died, regardless of the cause, use the firewood locally; do not transport it to other areas. You may notice signs at entrances to state and federal parks and forests that say: “Do not bring in firewood, only use local firewood.”
Second, try to remove trees during the winter when insects are dormant, and be sure to burn the wood before spring when insects become active again. If that is not possible, then debark the wood and let it dry out. This usually is very destructive to fungi and immature insects. Placing firewood under plastic or a tarp can also raise the temperature high enough to kill pathogens and insects.
Finally, while it might seem nice to share firewood with friends and family members, try to avoid this practice. You never know where that wood may wind up.
As the old saying goes, “Firewood warms you twice.” Enjoy!
Miller is a horticulture professor at Joliet Junior College in Joliet, Ill., and a senior research scientist in entomology at The Morton Arboretum in Lisle, Ill. Email your tree questions to him at firstname.lastname@example.org.