Forest Health

dead spruce

Many folks visiting the Rio Grande National Forest are noticing a big change in the high elevation forests as literally millions of trees are succumbing to the spruce bark beetle. A total of 617,000 acres of spruce-fir forest have been infested by spruce beetles since 1996 and the beetles are continuing to spread.

The native spruce beetle primarily attacks mature Engelmann spruce, although it sometimes infests blue spruce too. The tiny beetle is killing trees down to 5 inches in diameter Luckily, smaller spruce and all sizes of fir will continue to survive, and they will provide the base for creating the next forest.

For more current information on spruce beetle infestations and forest health visit the following links from the Colorado State Forest Service and the Rocky Mountain Region of the US Forest Service.

Spruce Beetle 

spruce trees

The spruce beetle infestation is still spreading through the Rio Grande National Forest (RGNF). Each year aerial surveys detect the spread of spruce beetle infestations. Over 617,000 acres have been infested since 1996. The rate of annual spread had declined from its peak on the RGNF because most of the spruce-fir forests and the Engelmann spruce components of mixed conifer forests have been infested.

Aspen Defoliators

Photo of a nest of the western tent caterpillar Photo: Colorado State Forest ServiceThe two most common defoliators of aspen on the Rio Grande National Forest are tent caterpillar and aspen tortrix. Defoliators are insects that eat the leaves of trees and shrubs. Tent caterpillars eat the leaves and build silken tent-like structures in trees and shrubs. The large aspen tortrix eats the leaves of aspen trees as larvae.

Aspen defoliators feed on the leaves of aspen trees and other broad-leafed plants. When populations are large enough, entire stands of aspen trees may be defoliated. As long as the aspen have sugar reserves in their root systems, the trees will sprout new leaves following defoliation. The new leaves are generally smaller than those that grew in the spring, but they can still make for magnificent fall colors.

Individuals and clones of aspen trees may begin to die after several years of repeated defoliation as the sugar reserves are used up or from other insects and diseases attacking the trees. Typically, populations of the defoliators crash before there is significant aspen mortality.

Photo: Western tent caterpillar, Colorado State Forest Service

Western Spruce Budworm

Photo of dying spruce needles from the western spruce budworm. While spruce budworm activity can vary greatly from year to year, infestations tend to be chronic on the Rio Grande National Forest, that is, there are always areas on the Forest with budworm activity.

The larvae of spruce budworms are defoliators that feed on the new needles of white fir, Douglas-fir, spruce, and subalpine fir. Brown, needleless branch tips on green trees are a telltale characteristic of a spruce budworm infestation. Individual coniferous trees may die within stands infested by spruce budworm for several years.

Photo: Western Spruce Budworm, Dan West, Colorado State Forest Service