Insects and diseases cause changes in forest structure and function, species succession, and biodiversity, which may be considered negative or positive depending on management objectives (Edmonds and others 2011). An important task for forest managers, pathologists, and entomologists is recognizing and distinguishing between natural and excessive mortality, a task that relates to ecologically based or commoditybased management objectives (Teale and Castello 2011). The impacts of insects and diseases on forests vary from natural thinning to extraordinary levels of tree mortality, but insects and diseases are not necessarily enemies of the forest because they kill trees (Teale and Castello 2011). If disturbances, including insects and diseases, are viewed in their full ecological context, then some amount can be considered “healthy” to sustain the structure of the forest (Manion 2003, Zhang and others 2011) by causing tree mortality that culls weak competitors and releases resources that are needed to support the growth of surviving trees (Teale and Castello 2011).
Potter, Kevin M.; Paschke, Jeanine L.; Koch, Frank H.; Zweifler, Mark O. 2018. Chapter 2 - Large scale patterns of insect and disease activity in the conterminous United States, Alaska, and Hawaii from the national Insect and Disease Survey, 2017. In: General Technical Report SRS-239. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southern Research Station. 21-47 p.