You may have encountered white, segmented "worms" or grubs when chopping firewood and wondered what they were. The first questions that usually come to mind are: Did they kill my tree? Are they a danger to other healthy trees? In most cases, the answers are no and no.
Compared to the number of insects found in the forest, a relatively small number of bark beetles give all the insects a lousy reputation. Bark beetles primarily live and feed on the sugars just under the bark of live trees. This disrupts the flow of water and nutrients and can play a role in tree mortality. These beetles don't stay around after the tree dies and won't be in your firewood. To learn more about management strategies for preventing bark beetle damage to your live trees, consult Managing Insects and Diseases of Oregon Conifers.
In contrast, the "worms" in your firewood are not worms at all and are actually the larvae of woodboring beetles. Wood-boring beetles tunnel deep into solid wood, which helps with the decomposition process in our forests. With few exceptions, they infest trees already dead or dying from other causes and are not threats to healthy trees. They can live on the dead wood for many years before becoming adults and leaving to find a mate, so we often see them in firewood or other wood material that has been lying around for a long time. There are two families of woodboring beetles in conifers:
- Flatheaded borers
- Roundheaded or longhorned borers
Their appearance gives these insects their name. Flathead wood borer larvae have a flat head compared to the rounded head of the roundheaded wood borers. The flatheaded borer adults tend to be torpedo-shaped with bright metallic coloring, such as the golden buprestid (pictured). Roundheaded borers typically have antennae that are longer than their bodies (hence their other name, longhorned borer).
Adult longhorn borers may be up to 2.5 inches long; flathead borers are generally smaller. Some wood borer larvae can reach large sizes. For example, the ponderous borer's larvae reach nearly 3 inches long. Its chomping action and large mandibles (jaws) are said to have inspired the "teeth" of the modern chainsaw.
Looking at the wood and bark
If you look at a tree that has recently died and pull off a piece of bark, you will often see a winding gallery pattern on the inside of the bark and on the surface of the wood. Woodboring beetles or bark beetles create these galleries.
Borer galleries are continuous and mostly unbranched, while bark beetle galleries are branched, with the main galleries wider than the branches.
Flatheaded borer galleries are packed with fine, circular material whereas roundheaded borer galleries contain more loosely packed, coarser-textured material.
Often, wood borers invade a tree that bark beetles have killed, and the wood borer galleries obscure or "overwrite" the bark beetle galleries. Note that wood borers generally invade the wood itself and can remain in the tree for multiple years, while bark beetles stay immediately under the bark and leave by the next spring or summer.
Assessing the threat
From a wood products standpoint, borers can cause a lot of damage to timber by tunneling into solid wood. They are also food for woodpeckers, who make deeper holes to reach deeply burrowed borers.
Borer larvae can live for many years, and the adults sometimes emerge from framing lumber or other material inside houses, much to the alarm of the people living there. But these beetles do not threaten live trees, thoroughly dried lumber, or finished (painted or varnished) wood. Unlike termites, damage caused by these beetles is generally aesthetic and does not result in structural damage. This resource has more information about structural wood pests.
Some flatheaded borers can detect smoke from long distances. They have learned that flying towards smoke usually leads to fire-killed or weakened trees, i.e., dinner for the beetles and a place to lay their eggs, as they can only lay their eggs in freshly burnt trees, whose defenses have been scorched away. According to a decades-old story, certain species of flatheaded borer would swarm football game attendees at Berkeley's Memorial Stadium during the 1940s. Occasionally, the beetles would land on people and bite exposed skin. It turns out the hanging haze from the thousands of cigarettes smoked on game day was alluring to the beetles.
Harvest and process green material (such as standing madrone) during the fall and winter when the beetles are not in flight to reduce the chance that beetles will invade your firewood.
Exceptions to the rule
Woodboring beetles are large and often spectacular in appearance but, as noted above, generally aren't threats to healthy trees. However, there are a few exceptions to this rule.
The following species are native wood borers that act like bark beetles. They attack live, stressed trees and don't enter the wood:
- The native flatheaded fir borer aggressively feeds on stressed Douglas-fir trees. Trees planted outside of their range or exposed to drought are particularly susceptible to this insect. This insect is responsible for much of the Douglas-fir mortality in southwest Oregon.
- The native bronze birch borer is primarily an aggressive pest of both native and non-native birches in urban forest landscapes. These trees become stressed from overexposure to the sun and lack of water.
The following are damaging non-native woodborers:
- On June 30, 2022, the first West Coast detection of emerald ash borer occurred in Forest Grove, Oregon. Since then, there have been detections in Yamhill, Marion, and Clackamas counties. This pest has proven deadly to all ash species in North America and Europe, including the native Oregon ash (Fraxinus latifolia) in our forested and urban landscapes. This species remains in the wood of ash trees for up to four years and may be found in firewood.
- Mediterranean Oak Borer is an ambrosia beetle. Ambrosia beetles are woodboring beetles that don't eat the wood but carry a fungus into their tunnels to feed their young. The fungus disrupts the movement of water in the host oak tree and causes it to wilt. Over several years new attacks and reinfestation, the tree will lose significant portions of its crown and eventually die.
Don’t move firewood. Deeply burrowed wood borer larvae are hard to see in firewood. They can easily "hitch a ride" to new places only to emerge later as adults. The unintentional transport of damaging invasive species is a major concern. Emerald ash borer, for example, has been spread throughout the eastern US via firewood. By your firewood local! As a general rule, you should try to buy firewood within a 10-mile radius of where you plan to burn it.
