Douglas-fir beetle (Dendroctonus pseudotsugae) is the most destructive bark beetle of mature Douglas-fir forests in western North America.
Douglas-fir beetle (Dendroctonus pseudotsugae), another close relative of the spruce beetle and mountain pine beetle, is an important native bark beetle of mature Douglas-fir (Pseudotsuga menziesii) forests across most of the West. Outbreaks tend to be associated with mature Douglas-fir forests (average stand diameters greater than 14 inches at 4.5 feet from the forest floor) coupled with periods of below-normal precipitation. Adults typically seek new trees to attack from late spring through early fall. Unlike other bark beetles in Colorado, Douglas fir beetles can use trees that have fallen or been cut and left for removal at a later date.
Signs & Symptoms
The most apparent indicators of Douglas-fir beetle infestation are small groups of dead and dying Douglas-fir trees. Infested trees have their needles fade from green to red-brown (rust colored) before starting to drop off. Fading foliage occurs about one year after trees have been attacked, typically after adult beetles have emerged and left for another green tree.
Other symptoms of Douglas-fir beetle infestation may include the presence of reddish-brown boring dust around the base of trees and within the cracks and crevices of the bark. Woodpecker damage, where the birds have stripped portions of the bark from infested trees in search of larvae (leaving accumulations of bark at the base of trees) is often an indicator of bark beetle presence. Exit holes on the bark surface may be seen after the adult beetles emerge from infested trees.
CSFS Forest Entomologist Dr. Dan West produced an Esri Story Map for the 2019 Forest Health Report, which offers users an interactive way to find out about forest conditions and insect activity in their area.
Ongoing management of Douglas-fir forests is the most effective long-term strategy for reducing tree losses from Douglas-fir beetle. Thinning overly dense stands of trees, to reduce competition and promote tree age and species diversity, is the most successful forest management strategy to promote tree vigor.
Options for management include:
- Remove infested trees and slash from thinning or pruning, wind-thrown trees susceptible to Douglas-fir beetle infestation, trees already infested by other insects and diseases, and excess numbers of older trees, as the beetles most frequently attack the largest trees first.
- Solar treatments can be used to reduce Douglas-fir beetle populations in small infested stands. These treatments involve felling infested trees and stacking cut logs in an area with full sun before covering them with clear plastic. The solar treatment of infested trees creates conditions unsuitable for survival of Douglas-fir beetles.
- Trap trees serve as traps for recently emerged, adult beetles. After the trap trees become infested with beetles, trees are removed and destroyed, usually during the fall or winter when all of the Douglas-fir beetles are still inside.
- Hanging pheromone packets containing Methylcyclohexanone (MCH) disrupt the attraction of incoming beetles and can be used to reduce attacks on Douglas-fir trees.
- The use of preventive insecticide sprays has proven effective in preventing Douglas-fir beetle infestation. Certain formulations of pyrethroids that are registered and tested for effectiveness are the primary preventive insecticidal sprays used to help reduce the likelihood of attacks on individual trees. The Colorado State Forest Service recommends spraying only high-value trees, such as those near homes, businesses or recreation sites. Follow the label of any chosen product.
Douglas-fir Beetle (561 KB PDF)