Infection by the spruce beetle (Dendroctonum rufipennis), also called the spruce bark beetle, results in the loss of 300 to 500 million board feet of spruce timber each year, from Arizona to Alaska. Outbreaks of spruce beetle reduce the growth of trees, resulting in less dense stands dominated by small and medium-sized trees.
Adult spruce beetles usually emerge in May to October. They attack trees in early summer shortly after they emerge. Females bore through the outer bark of trees to deposit their eggs, which usually hatch by August. The larvae bore outward and dominate the tree during the winter. Most larvae become pupae about one year after the adults attacked the tree the previous summer. Some beetles stay put, but 95 percent of the adults emerge and move to the base of the tree, boring into the bark, where they spend the next winter. Two years after the first attack, the adults emerge again to attack the tree.
Signs of Infestation
Spruce beetles leave a reddish-brown dust at the entrance of their holes, in the crevices of bark and on the ground. The tree may produce masses of pitch around the holes. These signs will be most visible during the first summer of attack. The beetle may attack only one side of the trunk. The infested strip may die, but the tree will still live. The infested strips may host two or more generations of spruce beetles.
Woodpeckers may strip infected areas of bark. The needles usually do not fade or discolor in the first year of infection. Beginning with the second summer, needles may turn yellowish-green or orange-red. Needles on separate branches may discolor at different times. After the second summer, wind and thunderstorms may strip infected needles, turning the exposed twigs on the upper crown a yellowish-orange to reddish color.
Adult Spruce Beetles are round, dark brown to black and about ¼ inch long; they have reddish brown to black wings. They lay oblong white eggs about 1/16 inch long. The cream-colored larvae are round, stout and have no legs. They are about ¼ inch long at maturity. The inactive, opaque white pupae look similar to adults. Most of the eggs hatch by August.
Where Infections Occur
More spruce beetles live in trees that have been blown down. When populations are high in downed areas, they move to large diameter standing trees. They attack larger trees in stands where spruce trees dominate.
Prune the lower third of the live crown of small diameter trees. Create a trap by felling green trees with a diameter of less than 12 inches. Place them in the shade and remove and destroy them after they die.
Do not leave spruce logs in piles. Cut, split and cross-stack infected trees and place them in the sun. Remove bark from infected trees at the base or cut them as low as the ground as possible. Remove older trees. The younger ones will grow faster and are able to resist attack.
Thin heavy stands, to allow sunlight and moisture to reach the remaining trees. Prune spruce trees in the fall. If you prune in the spring or summer, sap from the pruning cuts may attract spruce beetles.
Synthetic pheromonescan be used to draw beetles to trap trees. Preventative applications of insecticides containing carbaryl and pyrethroids may be applied the trunks of heavy trees. If you suspect an infection, contact the U.S. Forest Service for help.