Why Are Beetles Destroying Western Forests?
Recently Susan visited Colorado for a vacation where she has been many times before to visit family. She always enjoys getting out of the city to experience the beauty of the Rocky Mountains. But this time, she was truly alarmed by the sight of dead trees everywhere. She asked me to look into this and write a post about it.
The culprit is the native mountain pine beetle, an insect about the size of a grain of rice, which has been destroying trees in British Columbia, Canada, and across Colorado and Wyoming.
The mountain pine beetle is a predator of many western pine trees, particularly mature lodgepole pines. It must kill the trees in order to successfully reproduce. The beetle typically kill trees already weakened by disease or old age. But even a healthy tree isn’t able to fight off the beetles when they are at epidemic levels.
The beetles attack pines in late summer, dispersing a chemical signal that attracts other beetles to mass-attack the tree. When the beetles bore through the bark of the tree, they introduce blue-stain fungus, which can work quickly to kill the tree. The beetles form tunnels and lay eggs underneath the bark, which hatch into larvae. The larvae spend the winter underneath the bark and emerge as adults in the summer, beginning the cycle again.
The U.S. Forest Service estimates that by 2012, the majority of lodgepole pines in northern Colorado and southern Wyoming will be killed by the beetle. Extensive beetle kill has resulted in ecosystem-wide impacts such as increased potential for wildfires and some loss of other tree species such as Douglas fir.
What has caused this infestation? It appears that global warming is a major factor in the infestation. The beetle eggs, pupae, and young larvae are the most susceptible to freezing temperatures. According to a British Columbian government site, in the winter, temperatures must consistently be below -35 Celsius to -40 Celsius (-31 to -40 degrees Fahrenheit) for several straight days to kill off large portions of beetle populations. In the early fall or late spring, sustained temperatures of -25 Celsius (-13 degrees Fahrenheit) can freeze mountain pine beetle populations to death.
Amy Nicholas, a U.S. Fish and Wildlife Service biologist in Rock Springs, Wyoming, says: “Because of warming temperatures, scientists are now seeing significant beetle impacts even in the high elevation sites occupied by whitebark pine. These sites are usually quite cold and unfavorable for epidemic levels of mountain pine beetle. That no longer seems to be the case.”
In addition to global warming, other factors, such as extended droughts and dense forests, have created a “perfect storm” for the infestation.
Lodgepole pine forests dominate the forested ecosystems of western North America and can provide breeding and foraging habitat for many coniferous wildlife species, including song birds, woodpeckers, and red squirrels.
Research by biologists with the U.S. Fish & Wildlife Service aims to determine which alternative stand types may best support wildlife species until lodgepole stands can regenerate after the ongoing infestation.
Unfortunately the spread of the beetle and its deadly fungus reminds us at Frogs Are Green of another disease linked to climate change, the chytrid fungus, that is wiping out populations of frog species worldwide.
Most of the information for this post came from a press release issued by U.S. Department of Fish and Wildlife, Wyoming: Perfect Storm Fuels Mountain Pine Needle Epidemic