Read about the life stages of bark beetles and their impact on native conifers of Western U.S. forests.
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Throughout the West Coast through the Rocky Mountains, bark beetles have affected tens of millions of acres of forest. While bark beetles are native to U.S. forests and play important ecological roles, they can cause extensive tree mortality and negative economic and social impacts. Climate change has led to an increase in these damaging effects, and the Forest Service is working to better understand bark beetle ecology and to improve forest management.
Bark beetles are tiny insects with hard, cylindrical bodies that reproduce under the bark of trees. There are 600 different species of bark beetles in the United States. Several species, such as the mountain pine beetle (Dendroctonus ponderosae), attack and kill live trees. Most species of bark beetles live in dead, weakened, or dying hosts.
Bark beetles are important disturbance agents in western coniferous forests. Population levels of a number of species oscillate periodically, often reaching high densities and causing extensive tree mortality when favorable forest and climatic conditions coincide. These events are part of the ecology of western forests and positively influence many ecological processes, but their adverse economic and social implications can also be significant.
During the past decade in the west, tree mortality caused by bark beetles has increased in spruce, lodgepole, pinyon-juniper, and ponderosa forests. This increase is correlated with shifts in temperature and increased water stress, which create conditions within trees that are favorable to beetle survival and growth.
LIFE HISTORY: Bark beetles derive their name from their habit of living and mining between the bark and wood of trees and shrubs. Adults excavate egg galleries in bark phloem. All bark beetle life stages are spent in the phloem, inner bark and bark, except when adults leave the tree in which they developed to fly to new host material. Bark beetles feed on the phloem during adult and larval stages.
Most bark beetles are considered secondary mortality agents because they prefer weakened host material. However, during environmental conditions favorable for beetle development, populations may build up rapidly and successfully attack healthy trees. Most bark beetles have a symbiotic relationship with blue-stain fungi. The blue stain fungi can completely penetrate the sapwood within a year. The fungi occlude the outer conducting tissues in the xylem that halts upward water translocation. This action, plus that of the bark beetle feeding, causes the death of a host tree.
Bark beetles produce chemical compounds called pheromones that are used to communicate with other beetles. Aggregation pheromones cause beetles to congregate in certain areas and mass-attack trees. Anti-aggregation pheromones cause beetles to disperse to neighboring trees or other areas. Pheromones of many bark beetles have been identified and synthetically produced. Both aggregation and anti-aggregation pheromones have been effective to mitigate impacts caused by some bark beetles in the western United States.
Crowns of successfully attacked trees turn from green to yellow to reddish brown. This color change, an indication of a dying tree, may occur from a month to more than 2 years after successful attack depending on the temperature, moisture conditions, and density of beetles in the tree. Close inspection of infested tree trunks will show either small globules of resin, small holes through the bark, or reddish boring dust in bark crevices and around the tree base. The removal of bark from infested trees will reveal two types of galleries, egg and larval. Egg galleries constructed by adult beetles are rather uniform in width. Larval galleries depart at right angles from egg galleries and increase in size as the young grow