Like many North Country residents, I typically light my woodstove in November, burning 16-inch logs until April. While carting an armload last winter, I noticed tunnels ranging from the size of a straw to the size of a small pipe in almost all of my red oak logs. What were they? My journey to find out led me on a tour through the interior life of our local trees where there is an ongoing and voracious community of insects.
The damage I found in my firewood occurred throughout the logs and appeared to have been done by the larvae of a beetle. I found entrance holes – tunnels bored horizontally, diagonally, and vertically – and what looked like pupal chambers and exit holes.
By comparing the evidence in my wood to databases and consulting with an entomologist, I identified the culprit as the red oak borer (Enaphalodes rufulus), a native long-horned beetle that lays its eggs in various species of oak, with preference for northern red oak. This beetle spends 80 percent of its life in the dark, chewing wood fiber that is digested by the microbes in its gut.
Red oak borers deposit their eggs in bark cracks, as well as under bark scales and lichen patches. Once the eggs hatch, the larvae chew through the bark into phloem, creating narrow galleries that they occupy throughout their first winter. During their first spring, the larvae make larger, vertical tunnels into the xylem. In these galleries, the larvae pass their second winter. In the second spring, the larvae pupate and emerge as adults in May or June. The entire life cycle takes two years and adults emerge synchronously in odd-numbered years.
The red oaks that became my firewood fell from a forest edge into a meadow in Essex County, New York. We bucked and split them where they fell. Damage from ice or snow may have compromised the trees’ defense systems and advertised their suitability to red oak borers, which can detect stressed trees.
Once infested, the value of the wood is reduced. Exit holes alert both foresters and graders to possible defects. Because red oaks have so much value for wildlife, leaving infested trees standing to appreciate their beauty and usefulness to the ecosystem may be more desirable than selling infested logs for lower than full grade, though oak that is unsuitable for milling because of borer damage still makes outstanding firewood.
Melissa Fierke, associate professor of forest entomology at SUNY College of Environmental Science and Forestry, says that the red oak borer is widespread in the Northeast, but at quite low densities. Thankfully, their population has remained in balance on our woodlot. We still have plenty of standing red oak that does not show evidence of borers, but we’ll never know until we cut them open.