Spring Brings Out Pine Engraver Bark Beetles

Pine engraver frass
Pine engraver frass seen on trunk of infested treed. Photo: Brytten Steed, USDA Forest Service, Bugwood.org

As temperatures begin to warm up, the pine engraver bark beetle will once again rear its ugly head. The pine engraver is a very small (1/8 to 3/16 inches long) bark beetle that attacks small diameter pine trees (2-8 inches DBH*) or the tops of large pine trees.

Upon emergence in the spring, the pine engraver typically infests fresh slash, wind throw, or snow-damaged trees. The adults mate and build “galleries” under the bark and the female begins laying eggs. Within 4-14 days, the eggs will hatch into larvae and feed on the phloem (living, inner-most layer of bark). After 10-20 days, the larvae will pupate and emerge as adults 10 days later. Overall, it takes about 40-55 days for the pine engraver to complete development from egg to adult. The new generation of adults produced will begin fresh attacks following emergence. If slash is available, this will be preferentially infested. If no slash is available, the beetles will begin attacking nearby live, standing trees. By mid- to late-August another generation will be completed in this material. The generation that emerges in August typically seeks out places to hibernate for the winter, but sometimes makes a feeding attack prior to hibernation, where trees are attacked and the phloem eaten, but no brood is produced.

Signs of pine engraver infestation include reddish-colored frass (refuse and excrement) on the outside of the bark, Y or H-shaped galleries under the bark and/or woodpecker damage on the surface of the bole. The pine engraver is often associated with the western pine beetle and the red turpentine beetle.

Typical y-shaped pine engraver gallery.
Typical y-shaped pine engraver gallery. Photo: USDA Forest Service Region 6 Pacific Northwest Archive, USDA Forest Service, Bugwood.org

The pine engraver can outbreak, and outbreaks can include hundreds of trees. Outbreaks typically occur as a result of drought, overcrowding, and/or the creation of slash, windthrow, or snow damage. Thinning dense pine stands can help mitigate the potential of a pine engraver outbreak by increasing the availability of water, sunlight, and nutrients for the residual trees. This enhances their vigor and allows for more defensive capabilities, such as increased resin flow.

The timing of slash creation is crucial. It is best not to create slash from January through July. Slash created during this time period does not have enough time to dry out prior to the several flights that the pine engraver may carry out each year. Slash created in fall or early winter will usually dry by spring and will be unsuitable habitat. If slash must be created during high risk months, there are several disposal options that may help reduce the risk of infestations that lead to outbreaks.

Pine engraver-infested trees with slash pile circled in red

Following the creation of slash, the best way to mitigate outbreak potential is by burning slash piles. This of course cannot always be accomplished due to dry, fire-weather conditions. Another option is called the “green chain” where you continuously make slash throughout the entire pine engraver flight period so that the beetles continue to infest the slash (which they prefer) rather than the standing, live trees. At the end of the season these piles can be burned. A third option would be “pile high and deep”, in which case you would create a very large slash pile (10-20 feet in width, length, and height). This method can be effective because the first generation of pine engraver emerging from the outer portion of the slash pile will likely continue moving deeper into the pile where branches are still moist and viable. Again, at the end of the season these piles can be burned. Although these methods have been shown to be effective in some cases, please remember, bark beetles can be unpredictable and may still choose to attack your live, standing trees.

pine engraver-infested trees
A stand of pine engraver-infested trees with a slash pile of diseased wood circled in red.

Some other methods of mitigation include completely removing slash from the site. Just be sure not to move it somewhere that may cause damage to someone else’s property. You may also debark larger slash, but this method is hard work and very time consuming if you have a lot of slash. Debarking is best when only one or two trees have been felled. Another alternative is to chip slash, but remember, even though the beetles cannot live and breed in the chips, they will be attracted to the area by the volatiles (chemicals trees give off that can attract bark beetles) that are emitted from the chips and the beetles may start attacking standing trees in the area around the chips. This is also true of the “lop and scatter” method. You can lop all slash greater than three inches in diameter into small pieces and scatter these pieces in a sunny area to dry out the phloem rendering the slash unsuitable for pine engraver infestation, but again, the volatiles emitted may attract beetles to the area and they may then attack your standing trees since the lopped up slash is not viable.

Finally, limit pruning activities to October through December if possible, as pruning wounds also release volatiles that may attract pine engraver. And never stack freshly cut wood next to live, standing trees during high-risk months.

*DBH = Diameter at breast height or diameter of the tree’s bole at 4.5 feet.

Read more in the US Forest Service’s Management Guide for Pine Engraver