Bark Beetles
FIELD GUIDE TO INSECTS AND DISEASES OF ARIZONA AND NEW MEXICO FORESTS
Douglas-fir Beetle
Dendroctonus pseudotsugae (Hopkins)

Host:  Douglas-fir

Figure 122. Figure 122. Egg and larval galleries of Douglas-fir beetle. Note larval galleries occur in alternating groups.

Symptoms/Signs:  Reddish-orange frass is the first sign that a tree has been attacked. At times, the most evident sign of attack is the clear resin exuding from entrance holes on the upper portions of the stem. Pitch flow, however, is not always present on successfully attacked trees. Needles of successfully attacked trees change from green to yellow to sorrel to reddish brown in the year following attack. Egg galleries run parallel to the wood grain and are 20 to 25 cm long. Adult beetles are stout, cylindrically-shaped, and 4 to 6 mm long. The head and thorax are black; the wing covers reddish brown, becoming darker with age.

Biology:  The Douglas-fir beetle has one generation per year. Although adult flight times vary by year, most new attacks occur in late spring to early summer. Broods develop under the bark throughout the summer and early fall. Adult, pupal, or larval life stages can overwinter. Brood that develops to pupae or adults by winter usually emerges between April and June. Overwintering larvae emerge as adults in summer. A small percentage of adults that overwinter will re-emerge from the spring-attacked trees and attack additional trees in the middle of summer. Typically, late-season attacks account for less than 20 percent of all attacks in one season.

Figure 123. Figure 123. Douglas-fir beetle caused mortality of dwarf mistletoe-infected Douglas-fir on the San Francisco Peaks, Arizona.

Effects:  The direct effect of successful attack is tree mortality. At low or endemic levels, the beetle infests scattered trees that are infected with dwarf mistletoe and/or armillaria root disease, injured by fire, or stressed by defoliating agents. During outbreaks, groups of 100 or more attacked trees are common. Douglas-fir beetle epidemics can have both immediate and long-term effects on mixed conifer forests. Immediate effects include changes in stand structure and composition. Successional (long-term) changes result from alterations in microclimate.

Similar Insects and Diseases:  Other bark beetles, Scolytus monticolae and Douglas-fir pole beetle (Pseudohylesinus nebulosus), often occur in large branches, boles of pole-sized trees and the top of trees killed by Douglas-fir beetle. They may be separated by gallery patterns and characteristics of adult beetles.

References:  24, 90

Figure 124a. Figure 124a. Successful attack by Douglas-fir beetle is indicated by frass (boring dust) in bark crevices, spider webs, or at the base of Douglas-fir.
Figure 124b. Figure 124b. Successful attack by Douglas-fir beetle is indicated by pitch streaming
Figure 125. Figure 125. Galleries of Douglas-fir pole beetle on small diameter Douglas-fir. Galleries are short, longitudinal and often with two branches.