I hear bugs chewing the wood inside my trees!
Many land managers have contacted me in a panic saying that they could hear bark beetles feeding in their trees. Although bark beetles may be present in those trees what they were likely hearing is wood borer activity. Wood borers tend to be much larger than bark beetles and are, therefore, more likely to be heard chewing away inside trees.
While bark beetles feed solely on a tree’s phloem, wood borers feed on sapwood and heartwood as well as phloem. Native wood borers attack stressed, dying, or dead trees; there are very few native species that actually kill trees.
Wood borers are attracted to volatile gases released by dead or dying trees and lay their eggs under the bark of these trees. Once the larvae hatch, they begin feeding on the inner bark and then tunnel into the wood. The larvae are white, legless grubs and can be quite large. They are valued as a food source by woodpeckers (they make great fishing bait too!) and woodpecker activity is often seen on trees that contain wood borers. The tunnels produced by larval feeding activity have a random pattern and increase in size as the larvae grow.
Frass (beetle poop) is likely to be present within the tunnels. Unlike bark beetle frass, which is fine and reddish in coloration, wood borer frass tends to look more like shredded wheat and is white in color. When wood borers develop into adults, they emerge from trees and leave exit holes that are typically quite a bit larger than those left by bark beetles.
Wood borers play an important ecological role by introducing wood decaying organisms into dead and dying trees which, in turn, helps to speed nutrient cycling. Typically, no management is necessary for native wood borers in a forested setting. Wood borers can damage lumber, but damage is unlikely to occur if the wood has been treated.
Types of Wood Borers
There are three common wood borer families; Cerambycidae, Buprestidae, and Siricidae. The family Cerambycidae, often called longhorned beetles (adults) or roundheaded wood borers (larvae), includes many species. Adults can range in size from ¼ to 2 ½ inches in length. Adults, particularly the males, have long antennae, hence the name longhorned beetle (Figure 1).
The family Buprestidae are commonly known as metallic (adults) or flatheaded (larvae) wood borers. Similar to longhorned beetles, there are many species, and adults may be ¼ to 2 ½ inches in length. Metallic wood borers have small antennae and some are very beautiful, with iridescent or metallic coloration somewhere on the body (Figure 2).
Flatheaded woodborer larvae can be differentiated from roundheaded wood borer larvae in they have a flattened and broadened area beneath the head (thorax) that gives the appearance of a flat head. Rather than the round exit holes left by roundheaded wood borers, flatheaded wood borers leave D-shaped exit holes.
The family Siricidae, often called woodwasps or horntails, are in the order Hymenoptera (i.e., wasps), unlike Cerambycidae and Buprestidae which are in the order Coleoptera (i.e., beetles). Adults may be ½ to 1 ½ inches in length and have a short hornlike process at the end of their bodies. Females have an additional stinger-like ovipositor which is used to oviposit eggs under the bark of trees (Figure 3).
Woodwasp adults can be distinguished from common wasps in that they have thick waists and neither males nor females can actually sting. Woodwasp larvae look similar to roundheaded wood borers but have a small spine at the end of the body. Woodwasps are particularly attracted to fire damaged trees and all except one western species feeds on conifers.
Although native wood borers typically attack stressed, dying and dead trees, several invasive species have been introduced into the United States that are incredibly damaging. The Asian longhorned borer (Anoplophora glabripennis) was introduced into the eastern United States in the early 1990’s. The Asian longhorned beetle feeds on many deciduous species (birch, horse chestnut, poplar, willow, elm and ash), but maples are one of its favorites.
This species has killed thousands of trees in New York and Chicago. Adults are large, 1 to 1 ½ inches long, and have wings that are shining black with irregular splotches of white. The antennae have bands of black and gray and the feet and legs have slate-blue “hairs” (Figure 4). This species can be confused with another invasive, the citrus longhorned beetle (Anoplophora chinensis, Figure 5).
The emerald ash borer (Agrilus planipennis) is an invasive metallic-green wood borer (Figure 6) currently found in 30 states. The emerald ash borer attacks ash trees and has killed hundreds of millions of ash in North America. If you have seen rectangular purple traps hanging in trees alongside the road, these traps are being used to monitor for the emerald ash borer, which is attracted to this particular color. The emerald ash borer may be confused with many native metallic wood borer species, such as Prasinalia cuneata (Figure 7).
The European woodwasp (Sirex noctilio, Figure 8) has been accidently introduced into the eastern U.S. as well. This species attacks and kills living pines. Similar to the emerald ash borer, this species may be easily confused with native species.
Reports from citizens help scientists track the spread of these pests. To report a potential invasive species in Washington state, take a picture if possible, and contact the Washington Invasive Species Council.
By Melissa Fischer, DNR landowner assistance/stewardship forester, email@example.com
- Schiff, N. M., Goulet, H., Smith, D. R., Boudreault, C., Wilson, A. Dan, and Scheffler, Brian E. 2012. Siricidae (Hymenoptera: Symphyta: Siricoidea) of the Western Hemisphere. Canadian Journal of Arthropod Identification No. 21: 305 pp. Published on 6 July, 2012. Available online at: http://www.biology.ualberta.ca/bsc/ejournal/sgsbws_21/sgsbws_21.html.doi: 10.3752/cjai.2012.21