Bark Beetles Benefit From Bad Storms

After a severe wind or ice storm breaks and topples trees, the fallen debris can end up incubating an epidemic of tree-boring insects.

A synthesis of published research finds that bark beetles and long-horned beetles are the insects most capable of producing a population explosion after a destructive storm hits a forest.

In western North America, spruce beetles and Douglas-fir beetles can gain enough momentum from broods produced in a large mass of windthrown trees to start infesting live trees.

Some of the largest bark beetle outbreaks in the United States have started this way. A June 1939 windstorm over Colorado's White River Plateau led to a spruce beetle epidemic that killed Engelmann spruce trees, and even some lodgepole pine, in seven national forests, totalling 10 million m³ of wood. High winds in western Oregon and Washington during the early 1950s downed 27 million m³ of forest. Another 8.3 million m³ of trees were subsequently killed by the Douglas-fir beetles that flourished after the storms.

It takes 2 to 5 years after strong winds thin a forest for spruce beetles in the US Rocky Mountains to move into live trees. The massive Colorado infestation lasted over a decade.

Douglas-fir beetles respond more rapidly, with maximum tree mortality occurring one or two years after a wind storm. But these bark beetle epidemics can't be sustained in live trees alone. If more devastating weather has not generated new brood habitat, then an insect outbreak subsides in four years. This cycle of storms and Douglas-fir beetle epidemics has played out repeatedly in the mountains of Oregon, Washington, Idaho and Montana.

To a lesser extent, other bark beetle species in the Dendroctonus genus, including mountain, southern, western and Jeffery pine beetles, also respond to the tree damage caused by wind and ice storms in western and southern North America. The population dynamics of these insects are similar to those of Douglas-fir beetles.

Some live trees are more susceptible than others to beetle attack. Spruce beetles do best in stands that are thick with large spruce trees growing on well-drained soils. The beetles need tree bark that retains moisture. So they select shady sites and the undersides of logs that won't dry quickly from sun.