Colorado Scientists Develop Consensus in Providing Key Information on Forest Insect Outbreaks and Fire Risk

As the natural rhythm of forests changes over time by insect outbreaks, fire or other natural disturbances, forest management teams are in need of ecological information to make crucial decisions in handling these natural occurrences.

Extensive outbreaks of tree-killing insects are occurring in several western areas of the United States, including Colorado. Along with recent high-intensity forest fires, these insect outbreaks are raising concerns about the health of forests across the nation. An independent group of scientists from the Colorado Forest Restoration Institute at Colorado State University, the University of Colorado and the University of Idaho have reached a consensus about some of the critical ecological information needed for forest management in Colorado.

The group developed a report that provides forest managers and policymakers with information about different kinds of insect outbreaks occurring in Colorado forests as a basis for making decisions about how to manage forests that are going through substantial changes. This information is essential, as Colorado recently experienced severe and extensive insect outbreaks.

The primary insects attacking and killing trees in Colorado forests are bark beetles and defoliators. These insects are Colorado natives and have co-existed with their host tree species for thousands of years. The insects typically are present in a forest in low numbers, killing the occasional weak tree.

Periodically, insect populations grow rapidly and kill large numbers of trees over large areas. Insect outbreaks are a natural occurrence and happen in nearly all forests. Major outbreaks do not happen often, and typically decades pass before another insect outbreak affects a forest.

Researchers believe that recent insect outbreaks in Colorado are particularly widespread and severe due to several factors. The past decade has brought severe drought to many parts of the state accompanied by relatively warm temperatures in both summer and winter. The combination of drought and hot summers probably stressed the trees and made them more susceptible to bark beetles. Additionally, warm summers may have accelerated the growth and reproduction of some bark beetle species, and mild winters allowed the survival of beetle larvae.  Landscapes dominated by large, old trees may also be particularly susceptible to beetle attacks.

It is widely believed that insect outbreaks set the stage for severe forest fires, but based on current knowledge, the assumed link between insect outbreaks and forest fires is not well supported or is so small as to be considered inconsequential for many forests in Colorado.

The researchers evaluated the likely effects of several possible measures that forest managers can use to help reduce the impacts of insects and fires in Colorado forests, including insecticide spraying, harvesting dead trees and better informed forest management teams.

For more information the full report is available at