Western U.S. Bark Beetles (2008)
Barbara Bentz, FWE, Rocky Mountain Research Station, Western Bark Beetle Research Group (WBBRG)
The current version of this paper (2014) is available at www.fs.usda.gov/ccrc/topics/insect-disturbance/bark-beetles
Since 1990, native bark beetles have killed millions of trees across millions of hectares of forest from Alaska to southern California. Although bark beetle infestations are a regular force of natural change in forested ecosystems, several of the current outbreaks, occurring simultaneously across western North America, are the largest and most severe in recorded history.
Bark beetle outbreak dynamics are complex, and a variety of circumstances must coincide and thresholds must be surpassed for an outbreak to occur on a large scale. Moreover, large areas of suitable hosts are an essential requirement for a widescale outbreak. Although outbreak dynamics differ from species to species and from forest to forest, climate change is one factor that appears to be driving at least some of the current bark beetle outbreaks. Temperature influences everything in a bark beetle's life, from the number of eggs laid by a single female beetle, to the beetles' ability to disperse to new host trees, to individuals' over-winter survival and developmental timing. Elevated temperatures associated with climate change, particularly when there are consecutive warm years, can speed up reproductive cycles and reduce cold-induced mortality. Shifts in precipitation patterns and associated drought can also influence bark beetle outbreak dynamics by weakening trees and making them more susceptible to bark beetle attacks.
The recent large-scale dieback of piñon (Pinus edulis Engelm.) and ponderosa pine (P. ponderosa Dougl. ex Laws.) and associated bark beetle outbreaks in the Southwestern United States has been linked to the "climate change type drought" (e.g., dry and warm) that occurred in this region in the early 2000s. Several bark beetle species, including piñon ips (Ips confusus Leconte), Arizona fivespined ips (Ips lecontei Swaine) and the western pine beetle (Dendroctonus brevicomis LeConte), responded to the vast landscapes of drought-stressed trees, contributing significantly to the widespread tree mortality. Because elevated temperatures potentially influence the number of generations of these species reproducing in a single year, similar outbreaks could occur again as precipitation and temperature patterns continue to shift.
We have database models to describe and project the effect of temperature, but not other climate variables, on life-cycle timing of the mountain pine beetle and spruce beetle (D. rufipennis Kirby). For both species, the influence of elevated temperatures on outbreak dynamics is most notable at higher elevations and latitudes where some beetles have shifted to completing their development in a single year (univoltine) rather than 2 or even 3 years. Assuming other inputs to the system remain constant, this decrease in generation time translates to a doubling in the rate of population growth.
Model predictions suggest that the greatest risk to spruce forests in the next 30 years will be in Alaska, where elevated temperatures caused outbreaks of spruce beetles in the mid 1990s, and at the highest elevations of the Western States where spruce (Picea spp.) grows. Proportionately, mountain pine beetles in high-elevation five-needle pine forests will also continue to increase. At low elevations, however, under a conservative climate change scenario, the amount of area in which we predict that mountain pine beetle populations will do well in the next 30 years could actually decrease as temperatures warm excessively and disrupt the insects’ seasonality. These predictions, however, are based on model simulations that assume a population must be 100 percent univoltine to be successful. Recent field data suggest mountain pine beetle outbreaks occur in forests with a mix of univoltine and semivoltine (2 years required for a single generation) beetles, and 100 percent univoltinism is not necessarily a requirement. We are revising our phenology model to address these issues.
A separate model that describes the probability of temperature-dependent survival suggests that elevated minimum temperatures (which are rising faster than maximum temperatures) have increased the survival probability of mountain pine beetle in many areas. This trend is predicted to increase in the next 30 years, particularly at high elevations throughout the Rocky Mountains and Great Basin. However, it should be noted that increased winter survival does not always coincide with increased population success based on developmental timing. Each process is affected by temperature patterns occurring at different times of the year. It is therefore necessary to consider both temperature-dependent processes in predicting future mountain pine beetle outbreak dynamics.
Our models currently do not take into account any genetic variability in developmental timing across the range of the species, or inherent capacity to rapidly adapt to the changing conditions. The influence of climate-change-induced shifts in the distribution of host trees will also need to be considered. In particular, the distribution of ponderosa pine is expected to expand with climate change, and associated bark beetle species undoubtedly will also be affected. Also, many other bark beetle species, in particular in the Southwestern United States, have the potential for dramatic shifts in outbreak dynamics and geographic range as temperatures continue to rise. However, there are virtually no data describing the role of temperature in the population dynamics of these bark beetle species.
Options for Management
In addition to climate change, forest history and management have also influenced recent bark beetle outbreaks. In some areas, over the past century natural disturbances and human activities have produced large areas of host trees that are very similar in size and age, a virtual bark beetle "smorgasbord." Also, when trees must compete for resources in overcrowded conditions, bark beetles can more easily overcome these stressed trees’ defenses and initiate a severe outbreak.
The recent infestations may result in dramatic changes to the long-term ecological pathways of some ecosystems, radically shifting vegetation patterns in some hard-hit forests. Although there are no known management options to prevent the spread of a large-scale bark beetle outbreak, land-use activities that enhance forest heterogeneity at the landscape scale-such as creating patches that contain diverse species and ages of trees-can reduce susceptibility to bark beetle outbreaks. However, because resource objectives will differ and the factors influencing a bark beetle outbreak differ depending on the species of bark beetle, host tree species, local ecosystem, and geographical region, there is no single management action that is appropriate across all affected forests.
Bentz, Barbara. 2008. Western U.S. Bark Beetles and Climate Change. (May 20, 2008). U.S. Department of Agriculture, Forest Service, Climate Change Resource Center. http://www.fs.fed.us/ccrc/topics/bark-beetles.shtml