Insect and Disease Update
Insects and disease are opportunists: they take advantage of weakened and vulnerable trees.
Prolonged drought and rising temperatures associated with climate change have increased vulnerability of trees to bark beetles, in particular. Between 2008 and 2015 the number of standing dead trees in Colorado increased almost 30%4. Much of the increase is due to attacks by insects, infection by disease, or a combination of the two. Mountain pine beetle, spruce beetle, Douglas- fir beetle, sudden aspen decline, and subalpine fir decline are all pervasive on the landscape. Over the past 20 years, populations of these mostly native insects and fungi have exploded, aided significantly by heat and drought.
Over the course of millions of years, trees and wood-boring insects have evolved together. When insects develop new ways of attacking trees, the trees in turn develop new defenses. The primary defense trees have against bark beetle is their ability to produce resin or pitch. When bark beetles burrow into the bark of a tree, the tree responds with a stream of sticky resin. Resin forces beetles out and carries chemicals which are toxic to both the beetles and the fungi they carry. During moderate and severe drought, a tree’s ability to produce resin and chemical defenses is impaired, making them much more vulnerable to bark beetles and fungi.
Bark beetles are always present in the forests. At low or endemic levels, they attack and kill already weak trees but are often unable to impact healthy trees. During severe droughts, weak trees are more common. An increasing number of susceptible trees means bark beetles are more likely to be successful. Warmer temperatures reduce winter beetle mortality, and increase the rate of beetle reproduction5. Under these conditions, bark beetle populations can reach epidemic levels (aka “outbreaks”) and even healthy trees can be overwhelmed by mass attacks. Climate change has already made drought conditions more common, and predictions of future climate change suggest that warm and dry years like 2018 will be more frequent. These changes will lead to further bark beetle epidemics and widespread tree mortality in Colorado’s coniferous forests.
In 2017 the Roaring Fork watershed saw the fewer acres impacted by insects and diseases than any other year since aerial surveys began in 2006. Western balsam bark beetle continues to be the most impactful insect on the landscape but number of acres impacted has declined significantly from highs in 2014. Mountain pine beetle remains at very low levels. While Douglas-fir beetle has declined significantly from its peak in 2013 it is still present and expanding in several areas including Aspen Mountain. Spruce beetle remains at low levels despite the epidemic outbreaks taking place to our south in the Gunnison National Forest. Drought and increased activity by western spruce budworm (a defoliator) may weaken trees and increase the possibility of future outbreaks.
Between 2008 and 2015, the number of standing dead trees in Colorado increased almost 30%