Douglas-fir beetle (Dendroctonus pseudotsugae) is the most destructive bark beetle of mature Douglas-fir forests in western North America.
Douglas-fir beetle usually kills only small groups of trees, but during major outbreaks groups of 100 or more infested trees are not unusual. Infested trees may occur on a landscape-scale across multiple drainages during outbreaks.
- Adult males are stout, cylindrical beetles less than ¼-inch (4-6 mm) long, or smaller than a grain of rice. The head and midsection are black and the wing covers are typically reddish-brown.
- Adult females typically initiate an attack on new host trees, and mating then occurs near the entrance hole bored through the bark.
- Larvae are white, C-shaped, legless grubs with an amber colored head capsule.
Symptoms of Infestation
Outbreaks tend to be associated with mature Douglas-fir forests, especially following extended periods of below normal precipitation. The most apparent indicators of Douglas-fir beetle infestation are small groups of dead and dying Douglas-fir trees. When a tree is dying from Douglas-fir beetle, its needles fade from green to red-brown before starting to drop off. Fading foliage occurs about one year after trees have been attacked, typically after adult beetles have emerged. 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.
- Streaming resin along the main trunk (not always present), usually white and/or clear in appearance.
- Vertically oriented galleries under the bark, with alternating larval side galleries.
- 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.
- Exit holes on the bark surface, after the adult beetles emerge from infested trees.
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 the competition between trees, is the most successful forest management strategy to promote tree vigor. Options for management include:
- Remove infested trees and slash, wind-thrown trees susceptible to Douglas-fir beetle infestation, Trees already infested by other insects and diseases, excess numbers of older trees, as the beetles most frequently attack the largest trees first .
- Utilize packets containing the pheromone Methylcyclohexanone (MCH) disrupt the attraction of incoming beetles and can be used to reduce attacks on Douglas-fir trees.
- The use of insecticides has proven effective in preventing Douglas-fir beetle infestation. Certain formulations of carbaryl and pyrethroids that are registered and have been tested for effectiveness are the primary insecticide sprays used to help reduce the likelihood of attacks on individual trees.
- Solar treatments can be used to reduce Douglas-fir beetle populations in 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, forcing them to either relocate or die.
- 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, while all of the Douglas-fir beetles are still inside.
Learn More About Douglas-fir Beetle
Douglas-fir Beetle (561 KB PDF) – New 2016