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Climate change is associated with increases in temperature and the frequency and intensity of abiotic disturbances such as drought that can affect forest resilience at multiple scales. Higher temperatures leading to extended growing seasons alter the phenology, distribution, and population dynamics of herbivorous insects. Bark beetles (Coleoptera: Curculionidae: Scolytinae) are among the most ecologically and economically important herbivorous insect taxa and can undergo population eruptions and cause widespread coniferous tree mortality. Specifically, Ips bark beetles preferentially attack conifers that are stressed, dying, or dead—such tree conditions are predicted to become more common due to climate change. Our chapter objectives were to synthesize knowledge on the effects of drought on (1) the incidence of Ips beetle outbreaks and (2) changes in conifer host physiology that increase their susceptibility to bark beetles. We documented 132 Ips beetle outbreaks globally between 1919 and 2018 that were reported in association with drought or water stress. However, very few studies quantitatively demonstrated this relationship and/or defined the mechanisms using experiments. Alterations in host physiology that lead to tree stress will increase colonization by Ips beetles, but their responses may be dependent on attributes at the tree, stand to landscape level. Given the shifts in precipitation and temperature regimes worldwide, we argue for more in-depth studies that quantify the damage and drivers of Ips beetle outbreaks, assessing simultaneous changes in host trees and Ips beetles, ways to mitigate damage for beetle outbreaks, and urgently needed proactive adaptive management strategies in drought-prone forests worldwide.

This chapter discusses the hypothesis that outbreaks in the southern pine beetle (SPB), Dendroctonus frontalis Zimmermann (Coleoptera: Scolytidae) are generated by environmental factors that affect host-tree suitability, especially water availability. Many species of bark beetles (Coleoptera: Scolytidae) undergo dramatic population fluctuations that lead to extensive damage of coniferous forests. Several general models have been proposed to explain population variation in herbivorous insects based on the physiological condition of host plants. An alternative physiological model has been tested for the effects of water availability on the suitability of southern pines for bark beetles. In the first attempt of testing these predictions, Dunn and Lorio manipulated the water balance of 11-year-old loblolly pine using a combination of rain-exclusion shelters and irrigation. Once the attack of the host tree by SPB is under way, a large assemblage of natural enemies prey upon or parasitize various stages of SPB, both inside and outside the tree. To estimate the mortality inflicted by adult T. dubius on SPB, laboratory experiments were conducted in which SPB were exposed to different predator densities. Laboratory studies of T. dubius have indicated a development time from egg to adult of about 110–240 days, depending on temperature. SPB population fluctuations show pattern in both time and space. Even infrequent climatic events may be adequate to synchronize population cycles across broad geographic areas. Thanasimus dubius has a long development time, which makes it a good candidate for producing outbreak cycles, whereas climatic effects on host-tree suitability may influence the amplitude of outbreaks and synchronize population cycles on a regional scale.