The spruce beetle remains a serious threat in Southwest Colorado, according to results from the recently released 2010 forest health annual aerial survey.
Each year, observers with the Colorado State Forest Service and U.S. Forest Service conduct an aerial survey to map insect and disease activity in forested areas. Survey results provide a snapshot of landscape-level conditions that may be monitored and addressed more closely by on-the-ground foresters. Aerial survey results provide estimates of the number of acres affected by recent insect and disease activity in Colorado.
The insect of most concern in Southwest Colorado is the spruce beetle. This beetle’s populations typically build in older spruce forests, especially where wind-thrown trees provide habitat for outbreak populations to develop. During the past 14 years, a high number of wind events have resulted in thousands of acres of fallen spruce trees in Colorado.
Active spruce beetle infestations were detected on 208,000 acres in 2010, an increase of more than 80 percent from 2009. The largest outbreak is located in high-elevation, mature Engelmann spruce in the Weminuche Wilderness and San Juan and Rio Grande national forests. Spruce beetles impacted 136,000 new acres in 2010. Since 1996, spruce beetle populations have impacted 571,000 cumulative acres in Colorado.
Active spruce beetle infestations are difficult to detect during aerial surveys because of the less dramatic change in color spruce trees display when impacted by the beetle, compared to trees impacted by mountain pine beetle; estimates of acreage impacted by spruce beetles therefore may be low.
Douglas-fir beetle infestations increased from 23,000 observed acres in 2009 to 37,000 acres in 2010. Locally, the largest increases in Douglas-fir beetle activity were observed in Archuleta, La Plata and San Miguel counties.
Fewer acres of aspen overstory dieback and mortality (also referred to as sudden aspen decline) were mapped in 2010. All indications are that this condition peaked in 2008 and expanded very little over the past year. Our scientists believe that dieback and mortality of large groves of aspen trees were mostly triggered by a long-term drought starting around 2002.
The forest area defoliated by western spruce budworm dropped from 382,000 acres in 2009 to 213,000 acres in 2010, probably because of the cyclical nature of these outbreaks. This insect is a defoliator that feeds on the new needles of white fir and Douglas-fir trees.
Large expanses of forest within the same age class and same species provide little resistance against epidemic levels of forest insects or disease. The best defense against these native pest populations is to maintain healthy forest conditions through ecological processes and the use of forest management. Managing the forest to create landscapes consisting of diverse tree species, age classes and densities will provide resiliency to future insect and disease epidemics.
If you would like further information or have specific questions, please visit the Colorado State Forest Service website at www.coloradoforest.co or call the Durango District office at 247-5250.
Lindsay Gartner is assistant district forester with the Colorado State University Forest Service in Durango. As an educational resource for the community, the State Forest Service works closely with the Extension office in disseminating information to the public on topics concerning our natural lands and forests.