Andrew Liebhold, Northern Research Station, US Forest Service, Morgantown, WV and Barbara Bentz, Rocky Mountain Research Station, US Forest Service, Logan, UT.
Since forest insect populations are influenced by environmental conditions, future changes in climate can be expected to significantly alter the outbreak dynamics of certain forest insect species. In some cases, larger and more frequent insect outbreaks may occur, but in other cases recurring outbreaks may be disrupted or diminished. Shifts in temperatures that directly influence insects, as well as reduced host tree resistance caused by changes in precipitation can contribute to forest insect population growth. Alternatively, disruption of local adaptation to climate could result in localized population extirpation. Much is known about the influence of biotic and abiotic factors on some forest insect population eruptions. From this research it is clear that the effects of climate change on outbreaks will vary regionally as well as among different insect/host associations. Due to the complexity of the food webs and host tree dynamics that most forest insects are part of, in addition to the uncertainty of climate forecasts, predicting the effects of future climate change on insect-caused forest impacts will be challenging. Current research is aimed at increasing our understanding of the complexity of forest insect dynamics and enhancement of models for predicting forest impacts associated with future changes in climate. Options for applied forest management to mitigate the associated impacts can then be addressed.
Insects are ubiquitous disturbance agents that play important roles in the long-term dynamics of forest ecosystems. While most forest insect species remain at low densities and are rarely noticed, a few species exhibit eruptive population dynamics and episodically reach outbreak levels, causing massive defoliation, dieback or mortality in host trees.
The processes responsible for triggering insect outbreaks remain poorly understood. Most forest insect species are embedded in complex food webs. Interactions with higher (i.e., predators, pathogens and parasitoids) and lower (i.e. trees) trophic levels play key roles in shaping the dynamics of their populations. In addition, weather can directly or indirectly affect forest insect population dynamics. Temperature directly affects insect population dynamics through modification of developmental rates, reproduction and mortality. Weather can also affect insect populations indirectly via alteration of the abundance, distribution and physiology of host trees. Indirect effects also occur at higher levels of the food chain, such as effects of temperature on the abundance and developmental timing of predators, pathogens and associated microorganisms.
Issues critical to more fully understanding and predicting effects of climate change on forest insect impacts include:
- What are the species-specific, direct effects of climatic variability on forest insect population dynamics?
- How does weather indirectly affect forest insects via their host plants and what are the consequences?
- What are the indirect effects of weather on insect population dynamics acting via predators, parasitoids, pathogens and associated microorganisms?
- What is the adaptive capacity of forest insect species to rapidly changing environmental conditions?
Due to differences in critical feedbacks driving insect population dynamics, effects of climate change on outbreak dynamics will vary among regions and among insect-host species associations. For example, decreased precipitation and consequential reduction of host tree resistance is believed to have played a primary role in triggering outbreaks of piñon ips (Ips confusus) and Arizona fivespined ips (Ips lecontei) across the American southwest, which resulted in widespread mortality of piñon (Pinus edulis) and ponderosa pine (P. ponderosa) (1). The outbreaks ended as the supply of drought-stressed trees was exhausted. In contrast, although drought stress facilitated progression from an incipient to epidemic mountain pine beetle population (Dendroctonus ponderosa) in British Columbia, a significant correlation with precipitation was no longer found after the beetle population became self-amplifying (2). These examples illustrate how species responses to weather differ dramatically depending on the feedback mechanisms that have evolved within the insect-host species complex. Because insect species typically have adapted to local climates and host trees that differ across their range, we can anticipate significant within-species regional variability in temperature response and expect this to result in regional variability in response to a changing climate (3).
As climate continues to change, we can expect more situations, particularly at the margins of tree ranges, where sub-optimal conditions for tree growth and reduced tree vigor can lead to outbreaks of forest insects. These conditions must coincide, however, with appropriate conditions for insect populations. This is potentially the case with recent mountain pine beetle outbreaks in high elevation, five-needle white pine forests. Evidence suggests that elevated temperatures at high elevations across western North America have allowed mountain pine beetle populations to develop in a single year in areas where two or more years were previously required (4). This shift triggered greater population growth rates and resulted in increased high-elevation pine mortality in areas where outbreak populations were previously recorded only infrequently. Although we have limited data on the physiological response of high elevation, five-needle white pines to current precipitation and temperature regimes, reduced host tree vigor could also be playing a role in mountain pine beetle success in these forests.
Higher temperatures can also play a role in insect population success and potential range expansion. For example, increased winter minimum temperatures are expected to promote hemlock wooly adelgid (Adelges tsugae) expansion northward into the hemlock forests of Canada (5), and the expansion of mountain pine beetle northward in British Columbia and into eastern Alberta (6).
There are other examples of such temperature-induced shifts in insect ranges. Because insects have evolved life-history strategies allowing for adaptation to local climate, the likelihood of continuing range shifts will depend on the capacity of insect species to adapt to the rapidly changing environmental conditions, and/or a thermal regime that maintains an appropriate seasonality. In some cases, seasonality may be disrupted, potentially resulting in population extirpation. Additionally, the resistance potential of both alien and native host tree species in a changing climate will be critical knowledge for predicting insect population success.
Because the population dynamics of forest insects are affected by complex interactions with predators, parasitoids, microorganisms (including symbiotic relationships), host trees and pathogens, there are numerous opportunities for indirect effects of climate variability. Increases in forest insect outbreak area and frequency, therefore, are not the only consequence of climate change. Elevated temperatures in the European Alps, for example, are believed to have caused the cessation of recurrent outbreaks of the larch budmoth (Zieraphera dineana) in forests that have been periodically defoliated over the last 1200 years (7). When interactions between forest herbivores and their natural enemies are altered under changing climates, the density-dependent processes that govern population cycles of forest herbivore species can be disrupted and thereby lead to reduction or total cessation of outbreak episodes. Unfortunately, these interactions are complex and difficult to predict.
Options for Management
Stand susceptibility to forest insect outbreaks can be strongly influenced by the landscape pattern of host tree composition, stand structure and density. These can be manipulated through management actions including silviculture and prescribed fire (8). For a few forest insect species, temperature-driven models are available that can be used to forecast population success in a changing climate (9). By combining knowledge of stand and host conditions favorable to insect outbreaks with temperature-driven insect population models, it may be possible to forecast catastrophic outbreaks resulting from direct and indirect effects of climatic variability on insect population dynamics. Recognizing the uncertainty in forecasting future temperature and precipitation patterns, appropriate management practices for reducing future landscape susceptibility may then be applied. For example, in locations where climatic conditions favorable to bark beetle outbreak development are anticipated, silvicultural options for increasing landscape heterogeneity (i.e., altering tree species and age diversity) and reducing tree density can be used to alter stand susceptibility (8).
Not all forest insect species are strongly affected by tree vigor, and improving tree growing conditions cannot be expected to always increase stand resistance to insects. However, even for insects with dynamics that are not strongly tied to tree conditions (e.g., foliage-feeding insects), stand management might be applied to increase tree tolerance to insect feeding (e.g., defoliation). There is also some evidence that tree diversity increases landscape resistance to forest insects, many of which are monophagous on a particular genus or species, and avoiding reliance on a single tree species may be beneficial. Insecticides are a proven option for protecting individual trees from forest insect-caused mortality, although this tactic is not efficacious over large forested areas. Considerable uncertainty exists in predicting not only changes in climate (i.e., temperature and precipitation), but also the response of forest insects and their host trees and community associates to these changes. Current research is aimed at increasing our understanding of these processes which ultimately can be of use in planning and managing future forests.