Severe droughts have occurred recently in the United States, and given future climate change, the extend, frequency, and severity of future droughts are expected to increase associated with warming and changes in precipitation. Widespread tree mortality from insects and pathogens has occurred in the United States during the last few decades, suggesting a role associated with climate change. Here we review and synthesize the scientific literature on the role of drought in influencing outbreaks of insects and pathogens.
For many forest insects, especially bark beetles, moderate drought results in reduced beetle performance, whereas severe drought results in increased susceptibility of host trees to attack. Sapfeeders appear to have the greatest effects during moderate drought. For some pathogens (primary pathogens and those whose lifecycle depends on moisture), drought reduces performance. For other insects and pathogens, drought either has little effect or can reduce outbreak probability. Additional research needed to increase understanding of the role of droughts is summarized.