Spruce Beetle

Windthrown trees can set up outbreaks

Spruce Beetle on the Rio Grande National Forest
Spruce Beetle on the Rio Grande National Forest

The spruce beetle, Dendroctonus rufipennis, is the most significant natural mortality agent of mature spruce. Outbreaks have occurred in spruce forests from Alaska to Arizona. Following a 1939 windthrow event, a very large spruce beetle outbreak that impacted thousands of acres and spanned more than a decade occurred on the White River and Grand Mesa National Forests and adjoining lands. Outbreaks cause extensive tree mortality and can alter stand structure and composition. Average tree diameter, tree height, and stand density are all reduced following large outbreaks. As a result of the 1940s outbreak, many stands once dominated by mature Engelmann spruce are now dominated by subalpine fir.
The earliest sign of infestation is the presence of fine, bark-colored boring dust in bark crevices and around the base of standing trees. Pitch tubes may or may not be evident. Spruce beetles prefer down spruce to standing trees. On down (windthrown and cut) trees, spruce beetles commonly colonize the lower, well-shaded surfaces and may colonize the entire length of the trunk, typically up to an 8-inch (20-cm) top. In standing trees, beetle activity is most common in the lower 30 ft (9 m) or so of the trunk. Strip attacks (attacks that impact only a portion of the tree’s circumference) may be common, especially when beetle populations are small. Trees may live despite a strip attack, but such trees are often colonized again and killed in subsequent years.  Tree crowns typically remain green for up to a year after attack. By the second year, needles have faded and soon fall from the tree. The aerial “signature” of spruce beetle-infested spruce is not as striking or long-lasting as that of pine beetles in pines. Therefore, aerial detection of spruce beetle is extremely difficult.
In the winter, infested trees are often easily identified by the abundance of bark flakes on the snow, which is evidence of feeding activity by woodpeckers. Forest stands most susceptible to attack are located along drainage bottoms, have an average DBH of 16 inches (40 cm) or more, have a basal area of over 150 square ft per acre (34 square m per hectare) and have a canopy comprised of more than 65% spruce.

Source: Rocky Mountain Region, Forest Health Protection. 2010. Field guide to diseases & insects of the Rocky Mountain Region. Gen. Tech. Rep. RMRS-GTR-241 Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 336 p.