Aerial forest surveys track continued spread of spruce beetles across Colorado forests

State, federal scientists track forest health

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Aerial survey results show how spruce beetles are taking a toll across Colorado’s forests, with new areas of infestation in the Sange de Cristo, the West Elks and even the northern mountains.
Spruce beetle populations are surging in the southern Rocky Mountains. bberwyn photo.
Spruce beetles are still spreading in the southern Rocky Mountains. @bberwyn photo.

By Bob Berwyn

The latest results from aerial surveys of Colorado forests shows that spruce beetles are doing the most damage, with infestations detected on 409,000 acres across the state, expanding onto 182,000 acres of previously unaffected forests. Since 1996, spruce beetle outbreaks have caused varying degrees of tree mortality on more than 1.5 million acres in Colorado.

The mapping shows spruce beetles spreading outward from the San Juans to the West Elk Mountains, the Sangre de Cristo Mountains and into the northern part of the state around Rocky Mountain National Park. See the full report here: http://www.fs.usda.gov/goto/USFSR2ForestHealth.

State forest experts said it was the fourth year in a row that spruce beetle outbreaks caused widespread tree mortality. As populations of spruce beetles expand, they are starting to affect higher-elevation stands of Engelmann spruce. The report says blowdown events, combined with long-term drought stress, warmer temperatures and extensive amounts of older, dense spruce, have all contributed to the ongoing spruce beetle outbreak.

By contrast, mountain pine beetle activity remained low, with 5,000 acres affected statewide. The epidemic has ended in many areas of Colorado as mature pine trees have been depleted in the core outbreak areas, but the report also notes that mountain pine beetles have stopped spreading in areas where suitable mature host trees remain.

Mountain pine beetles did spread upward, with mortality across 2,000 acres of limber pines. There was also mortality across about 1,800 acres of lodgepole pines along the Continental Divide in southern Colorado, which may have been caused by other types of western bark beetles.

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Pine beetle activity in Colorado is at its lowest level since the mid-1990s.

Other highlights from the report:

  • Outbreaks of two defoliators of conifer trees – western spruce budworm and Douglas-fir tussock moth – also expanded in 2015.
  • The area impacted by western spruce budworm, Colorado’s most widespread forest defoliator, increased from 178,000 acres in 2014 to approximately 312,000 acres in 2015. This insect typically feeds on developing buds and new needles of fir, Douglas-fir and spruce in southern Colorado.
  • Douglas-fir tussock moth caterpillars consumed needles on Douglas-fir, white fir and spruce on approximately 26,000 acres along Colorado’s Front Range. Impacts were observed primarily in the South Platte River Basin and areas just west of Colorado Springs. Ground surveys have documented the presence of a naturally occurring virus among Douglas-fir tussock moth caterpillars, which has historically been a key indicator of imminent population collapse.
  • Trees weakened by these defoliators may become susceptible to Douglas-fir beetle, which can kill impacted trees.
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