Insect Profiles: Douglas-fir Beetle (Dendroctonus pseudotsugae)
Douglas-fir beetle (Dendroctotonus pseudotsugae) is a bark beetle that infests Douglas-fir. It is one of the most destructive pests of Douglas-fir. Outbreaks of Douglas-fir beetle can persist for several years. When populations are dense, Douglas-fir beetle can decimate forests and stands, killing hundreds to thousands of Douglas-firs.
Distribution & Habitat
Douglas-fir beetle occurs globally, wherever susceptible trees are present.
Douglas-fir beetle primarily infests Douglas-fir. Western lerch is occasionally infested, albeit with less frequency. The beetle is generally attracted to trees that have been weakened by environmental stressors, disease, or other infestations. Trees that have suffered fire scorch or have been felled are more prone to infestation.
The eggs laid by Douglas-fir beetle are white in coloration. They are small, measuring up to 1 mm in length. The larvae are white grubs bereft of legs, with light brown heads. When mature, the larvae can measure up to 6 mm long. The pupae are white to cream colored. The adults are initially light brown. As the adults mature, they deepen in color, turning dark brown to black. Mature beetles may be completely black. The wing covers have a reddish tinge. The adults are cylindrically shaped, and can grow up to ¼ of an inch in diameter.
Douglas-fir beetle produces one generation per year. The adults, as well as some larvae that have not yet completed their development, overwinter beneath the bark of infested trees. The following year, depending on the geographic location, the adults and larvae emerge from spring to early summer. The larvae conclude their development by early to mid summer, whereupon they morph into adults. The adults mate. The males expire shortly thereafter. The females navigate to host trees, and penetrate into the phloem, where they construct vertical egg galleries. The eggs are laid in groups, alternately along each side of the galleries. The eggs hatch within 1 to 3 weeks, revealing masses of tiny larvae.
The newly hatched larvae vacate the egg galleries, mining away from them at right angles. The larvae proceed to feed under the bark in the phloem layer. This creates perforations in the wood, which can cause the host to be invaded by disease pathogens, and other insects. Once the larvae reach maturity, they craft pupal cells on either side of the feeding galleries. The mature larvae pupate within the cells during the winter months. They appear as adults the following spring. Any larvae that have not yet matured overwinter beneath the bark, and resume activity in spring. When beetle populations are low, small batches of trees will be targeted, and infestations will remain relatively concentrated. As populations increase, outbreaks can occur, with Douglas-fir beetles infesting hundreds to thousands of vulnerable trees. Outbreaks can persist for 2 to 4 years.
Symptoms of Infestation
As the adults and larvae bore through the phloem, they produce a reddish-brown dust, which can be found in bark crevices and exit holes on the lower portion of the tree, or on the ground at the base of the tree. Clear resin may be exuded from the upper portion of infested trees. The resin is conspicuous, and may be visible from afar. Removing sections of the bark will reveal the egg galleries, eggs, and larvae. Infested trees will exhibit needle discoloration. The needles will transition from green to yellow, before deepening to orange, and then finally, a reddish-brown. Needle discoloration will not become apparent until several months to a year following the initial infestation. During years when conditions are dry, needle discoloration may be hastened. Woodpeckers will often create holes in infested trees while attempting to extract the larvae.
- When planting, select trees that are immune to infestation from Douglas-fir beetle.
- Maintain trees through sound cultural practices. Ensure that trees are sufficiently watered, especially during extended periods of drought. Each year, apply a layer of organic mulch around the base of susceptible trees. This will help to improve the soil quality, moderate the soil temperature, and retain soil moisture.
- Cull severely weakened trees to provide some natural control to infested areas.
- Anti-aggregation pheremone packets containing Methylcyclohexanone can be hung on vulnerable trees. The packets will disrupt beetle aggregation, and limit population growth.
- Registered insecticides can be applied to trees to combat infestations. Applications should be performed in spring, as the larvae and adults emerge.
- Cold, dry winters can kill overwintering broods.
- Several parasites and predators feed on Douglas-fir beetle. A Braconid wasp parasitizes the larvae. Many Dolicopdid flies prey on the larvae, while Clerid beetles consume the larvae and adults. Woodpeckers feast on the larvae, and will often convene around infested trees.
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Photo courtesy of USDA Forest Service