Occurrence of bark beetles following wildfires

The wildfires of 2017 took a direct toll on millions of trees across the Montana landscape. In some cases, trees on the periphery of these fires and in areas of lesser severity were merely scorched. Although these scorched trees may not die outright, they could be poised for attack by a variety of beetles. Partially burned Douglas-fir are particularly susceptible to Douglas-fir beetle.

Douglas-fir beetle is not an especially aggressive beetle and generally attacks Douglas-fir trees that are stressed by root disease, windthrow or fire scorch. As the beetles bore into the tree, they kick out red-brown dust that accumulates in bark crevices. This boring dust is a key indicator that the tree is infested with beetles. These beetles will lay eggs that will develop into adults over the course of a year and then emerge in mid-May to attack additional trees.

To overcome the tree's natural defenses, the beetles work in concert with each other and collectively attack a single tree. Aggregation pheromones are chemical messages the beetles use to rally additional beetles, but the tree provides only a limited amount of nutrients so the beetles eventually switch to an anti-aggregation pheromone that essentially conveys a "no vacancy" message. These pheromones have been isolated by chemical ecologists and synthesized for commercial production. Methyl-cyclo-hexanone, commonly referred to as MCH, is commercially available and fairly easy to apply on single trees or small acreages. MCH can be found online through a number of different retailers. MCH only protects the tree against Douglas-fir beetle and is not effective at controlling beetles already inside the tree. MCH should be in place by mid-April to deter attack from the adult beetles that typically fly in mid-May. MCH only lasts for one year.

If a tree or stand is already infested, it is best to remove the tree prior to May when the beetles emerge. Beetles will continue to develop in a tree even after the tree is cut, so be sure to remove all bark from the tree or remove it entirely from the property.

Scorched trees may not be attacked by Douglas-fir beetle until years after the injury, or may not even be attacked at all. The likelihood of Douglas-fir beetle activity following wildfires is variable across the region thus, landowners are encouraged to assess impacts on their individual property and consider their unique forest management goals. Your local DNRC service forester can help assess the status of scorched trees and offer advice that aligns with desired forest conditions.

Landowners who wish to speak to a DNRC Service Forester about possible insect activity and landowner resources are encouraged to call the Lincoln Field Office at 406-362-4999.