Barbara Bentz, Rocky Mountain Research Station, Kier Klepzig, Southern Research Station
An archived version of this topic paper is available
Bark beetles that infest and reproduce in live trees are capable of causing landscape-wide tree mortality. In the United States (US), species in the genera Dendroctonus and Ips are the primary culprits. Between 1997 and 2010 more than 5 million hectares were affected by bark beetles in the western US, most notably mountain pine beetle (D. ponderosae), spruce beetle (D. rufipennis), and pinÌƒon ips (I. confusus)(1), and the amount of carbon (C) in trees killed by these insects exceeds that of C in trees killed by fire (2). In the southeast and northeast US, southern pine beetle (D. frontalis) has affected more than 14,000 hectares since 2008, particularly in New Jersey and Mississippi. Prior to the activity during the late 2000s, expanding infestations of southern pine beetle had not been detected in the southeast or northeast since 2002 (3). It is clear that bark beetle outbreaks significantly influence forest ecosystem dynamics and carbon cycles, and research suggests warming summer and winter temperatures are major drivers of beetle population outbreaks across the US, and apparent range expansion in some species (4, 5, 6). Mountain pine beetle, spruce beetle and southern pine beetle are examples of bark beetles with the capacity for irruptive population growth. Populations exist at low levels for many years until triggered by factors such as drought (7, 8, 9), windfall (10), and pathogens that stress trees (11). Other species of bark beetles such as pinÌƒon ips can be triggered by similar conditions, although their population dynamics are more directly tied to the condition of the host tree (12). Once a trigger occurs, population growth depends on the scale of the trigger, continued favorable conditions including suitable host trees throughout the landscape (13), and temperatures that favor winter beetle survival (14, 15) and successful tree attacks in the summer (16).
Temperature drives bark beetle physiological processes such as larval development and cold hardening, thereby directly tying temperature to population growth. In general, warmer temperatures result in higher survival and faster development, although there are temperatures above which survival and development go down (17). Bark beetles kill their host trees through mass attacks, a process that requires synchronized adult emergence. Emergence synchrony occurs via temperature-driven thresholds in development (17) and diapause (18), a dormant state of reduced respiration. The strong role of temperature in population growth, and the role that reduced precipitation can play in host tree stress, suggest that climate change-associated shifts in temperature and precipitation will influence bark beetle populations in future forests. This could lead to increases in bark beetle populations, or in some cases decreases, depending on the species and geographic location (4, 16).
Mountain pine beetle female chewing through the phloem of a host tree to deposit eggs.
To be successful across expansive geographic distributions, adaptations to local environmental conditions have occurred within bark beetle species (19) and among species. Species that infest and reproduce in trees in warm habitats (e.g., some southern pine beetle populations and pinÌƒon ips) have evolved physiological mechanisms that allow for multiple generations in a single year. Species with host trees in colder climates (e.g., mountain pine beetle and spruce beetle) have evolved to survive during cold winters and emerge as adults to attack trees during warm summer months. The effect of warming temperatures will therefore differ depending on the species and the seasonality of warming. For example, the southern pine beetle can have up to 7 generations per year in the warmest part of its range (3). However, even with warming temperatures, the mountain pine beetle throughout its current distribution may not be able to produce multiple generations in one year since it is constrained by evolved adaptations16. Apart from differential effects of warming on developmental timing, warming during winter will have a positive effect on population growth of the majority of bark beetle species through a reduction in cold-induced mortality (5, 14, 15, 20).
Species that do not currently occupy the full extent of their host tree range are considered to be at least in part limited by climate. Other factors such as competition may play a role, but temperature can be particularly limiting. Recent temperature increases have had the greatest influence on bark beetle populations in marginally cool habitats, allowing population levels to increase. For example, outbreak populations of southern pine beetle are now persistent in New Jersey (21), and mountain pine beetle outbreaks in high elevation forests are continuing at paces not seen over the past century (22). Increasing minimum temperatures is one factor attributed to northward range expansion of the mountain pine beetle in Canada. Trees are being attacked further north than historical records from the past 100 or so years suggested was possible (23). Increased population success and range expansion due to release from climatic constraints has been recently observed and most studied in northern latitudes. There is also potential for species currently limited to southern latitudes within the US to expand their ranges northward with projected increases in temperature, although detailed information regarding thermal influences on fitness is lacking for the majority of bark beetle species. Moreover, there are numerous species of conifer-infesting bark beetles in Mexico that could expand their range into the southwestern US (4).
Changes in temperature and precipitation patterns will also indirectly affect bark beetles through effects on community associates (24, 25, 26) and host trees (27, 28). Although there is more uncertainty in predicting precipitation patterns than temperature, drought and high temperatures can both play roles in outbreak initiation (7,8, 9). Stressed host trees coupled with warm summers and winters can result in outbreak progression across a landscape of suitable host trees. Community associates that are both beneficial and harmful to bark beetles will also be influenced by changing temperature (29) and precipitation patterns, although less is known about these interactions.
Options for Management
Forest ecosystems have evolved with native bark beetle outbreaks. The economic and social costs of outbreaks, however, can be significant. Some practices can increase landscape resiliency to bark beetle outbreaks. Long-term strategies such as thinning and prescribed burning can optimize stand development trajectories, alter microclimates within stands, affect dispersal and host finding by beetles, and promote tree vigor and a diversity of species and age classes (13, 30). It is clear, however, that these types of management strategies are only efficacious prior to the onset of an outbreak. Short-term tactics can be used to reduce ongoing infestations by directly manipulating beetle populations, although due to cost these are limited to use in high value areas such as campgrounds and the wildland urban interface. Semiochemicals, communication compounds released by beetles and trees, can be used to attract and repel beetles of some species (31, 32, 33). Other tactics include a combination of insecticides sprayed directly on tree boles and sanitation harvests whereby infested trees are removed (13). These types of control can have a significant influence on non-target species such as other invertebrates, fish and birds however, which must be considered.
Bark beetles and fire are two important disturbance agents in forest ecosystems that can have reciprocal interactions. Bark beetle-caused tree mortality can influence subsequent fire behavior, although the spatial and temporal dynamics are complex (34). Removal of beetle-killed trees in the wildland urban interface and near recreation areas is a viable option that could reduce the risk of fire in these priority areas. Following wild and prescribed fire, bark beetles can preferentially attack fire-injured trees and contribute significantly to mortality of trees that would otherwise survive their injuries. Descriptive models have been developed to aid managers in choosing trees for removal following fire to reduce additional tree mortality (35). Large scale beetle population outbreaks have not been observed following wild or prescribed fires, however, most likely because fire-injured trees provide beetles with a single episode, or ‘pulse’ of stressed resources, that are localized and temporary and therefore not sufficient for landscape-scale population growth (36, 37).
With continued changes in climate, many tree species will be exposed to conditions that are potentially less suitable for optimal growth, thereby making them more susceptible to bark beetle attacks. To predict the role of bark beetles in future tree mortality, information that is based on a mechanistic understanding of processes is needed. For example, we know that bark beetles may respond to drought-stressed trees (8, 27), but specific precipitation thresholds and the role of moisture deficit in the bark beetle/tree relationship remains unclear. Similarly, we know that temperature is a strong driver of bark beetle populations, but for most species, the specific mechanisms that may be influenced in a changing climate are unknown.