Predicting forest beetle outbreaks

Researchers have developed a model that can help forest managers in the U.S. Southwest predict where future bark beetle outbreaks will occur. Scientists consider bark beetles to be one of the most destructive forces on conifer forests worldwide. They can transform forest ecological communities on a wide scale and cause large economic losses - though they may have positive ecological impacts in some areas, for example by allowing for the natural restoration of grasslands.

Historic management practices and human disturbance have likely made forests in North America more susceptible to bark beetle outbreaks. While scientists know bark beetle outbreaks are more likely to occur during drought, they have little understanding of why they affect some areas and not others.

Maria Santos and Thomas Whitham looked at the trees affected by a widespread bark beetle (Ips confusus) outbreak that occurred in Central Arizona during a period of severe drought in 2002. They found that trees with greater height, perimeter of trunk, and food availability for the beetle were more likely to be infested (the beetles feed on the tree ploem or inner bark).

Infestation of the adjacent tree increased the probability that a given tree would be infected. Other factors such as slope, elevation, dominant wind, and resistance to moths also explained infestation patterns. Based on these results, the researchers were able to develop a model that accurately predicts 95% of the variability in the infestation patterns.

Forest managers in the region can use this model to predict where future bark beetle outbreaks are likely to occur, monitor the situation, and take preventive measures, if necessary, such as conducting prescribed burns. This will likely become especially important as global warming increases drought occurrence potentially making forests more susceptible to bark beetle outbreaks. 

--Reviewed by Rob Goldstein

Santos, M., & Whitham, T. (2009). Predictors of Ips confusus Outbreaks During a Record Drought in Southwestern USA: Implications for Monitoring and Management Environmental Management DOI: 10.1007/s00267-009-9413-6