Climate Fact: Spruce Beetle Surge
While forests fires may be the most visible and dramatic events that reshape North America’s forests, outbreaks of insect “pests” actually affect an area 45 times larger than that affected by fire. Generally, insects attack trees weakened by things like drought, wind storms, and fire, as healthy trees are usually able to fend-off attacks from pests. If the pests attack the tree in large enough numbers, however, the tree’s natural defenses can become overwhelmed. Spruce beetle populations become “outbreak” when there is a large enough population of already weakened host trees (more than two clumps of five trees in a five acre area) and the right climatic conditions. In America’s Intermountain region, the right climate conditions for an outbreak include winter temperatures that do not drop below negative 29 degrees Fahrenheit (the temperature at which beetle larvae freeze to death), warmer fall temperatures (which allow the beetles to have more life cycles in a shorter period of time), and several years of drought (which weaken trees). Since 1976, there has been a 5.4 degree Fahrenheit increase in average wintertime temperatures and a 3.4 degree Fahrenheit increase in average autumn temperatures in the Intermountain region. Also, the percentage of the western United States that is in drought condition has doubled over the past century.
Seasons: Spring, Summer, Fall
Sources: Logan, JA et al. “Assessing the impacts of global warming on forest pest dynamics.” Frontiers in Ecology and the Environment 1 (2003) 130-137 and Hebertson, EG and Jenkins, MJ. “Climate Factors Associated with Historic Spruce Beetle (Coleoptera: Curculionidae) Outbreaks in Utah and Colorado.” Environmental Entomology 37 (2008) 281-292 and “U.S. Temperature and Precipitation Trends.” U.S. National Oceanic and Atmospheric Administration (NOAA): Climate Prediction Center. 5 January 2005. 26 June 2008 http://www.cpc.ncep.noaa.gov/charts.shtml> and United States. Climate Change Science Program. Weather and Climate Extremes in a Changing Climate. Synthesis Assessment Product 3.3: GPO. 2008.
