• Rocky Mountain Research Station
• 240 West Prospect
• Fort Collins, CO 80526
• (970) 498-1100

Hot Topic: Bark Beetle

A number of native bark beetles can cause tree mortality in Colorado’s forests and urban environments. These insects have co-evolved over thousands of years with their host trees an are an integral part of forest ecosystems. Bark beetles help shape forest structure and composition and influence forest productivity and biogeochemical cycling—the movement of elements through the ecosystem. They also play important roles in providing habitat for many wildlife species, arthropods (such as insects and millipedes for example) and a great diversity of fungi. Under certain circumstances bark beetles can come in conflict with land managing objectives, whether for a forester or a homeowner and tree mortality can become problematic.

Researchers are working through a long list of questions about how bark beetles affect ecosystems—aside from changing the scenery. Bark beetle infestations create patches of forest that have trees of various ages, densities, species, and successional stages. This variation helps keep the forests healthy. Researchers are looking at ways to connect bark beetle activity with landscape patterns so they can better measure—and better understand—the beetle’s ecological role.

Research suggests that practices such as forest thinning can help mitigate extensive tree mortality caused by bark beetles. However, most studies addressing vegetation management have been conducted on small plots, areas where even-aged management is practiced and in limited forest types. It is also not well understood what levels of insect populations can simply overwhelm managed stands.

Another research priority is sorting out the complicated interactions between bark beetles and forest fires. Interactions between bark beetles and fire take two forms. First, fire can injure trees and change the volatile emissions of conifers, a process that often increases the trees’ susceptibility to some bark beetles. Second, bark beetles can change the forest environment by influencing forest structure and transforming fuels. Overall, wildfire risk following beetle outbreaks, although not well understood, might depend on interactions with several factors. Some of these include weather and climatic patterns, forest type and tree cover type, site characteristics, changes in forest structure, time since mortality, and past management history—among others.

Scientists also are looking at the effects of climate change on beetle biology and outbreaks, which recently have coincided with increased winter temperatures and changing precipitation patterns. Climate change affects bark beetles by altering their development and temperature-induced mortality. Climate change may also affect trees’ defense mechanisms against bark beetle attacks.

Although researchers are considering the potentially good effects of outbreaks, they continue to look for ways to mitigate tree mortality. One area that shows promise is the use of semiochemical, substances that insects produce and use to communicate and regulate the attack process in a tree. For example, many bark beetles produce aggregation chemicals, or pheromones, that concentrate insect populations on a tree in an effort to overcome tree defenses. When a tree is fully colonized by the beetles, an anti-aggregation pheromone is released to prevent additional attacks on the tree. This is a way for the insects to reduce competition for resources. Scientists have identified many of these chemicals and studies are being conducted to examine their effectiveness in managing insect populations. Scientists also continue to test insecticides that can be used to prevent trees from being attacked by the bark beetles.

Bark beetles are causing a significant change to western forest scenic landscapes. Scientists are pursuing several lines of research that address the negative effects of beetles and help us understand the beetle’s positive influences.

Important Bark Beetles in Colorado and their Host Trees