Bark Beetles: Biology and Ecology of Native and Invasive Species

Bark beetles exhibit incredible diversity and complexity in their behavior, ecology, and population dynamics. The vast majority of the 6,000-plus extant species colonize dead or dying host plants, and are important nutrient recyclers. Only about 1% of species attack and kill healthy hosts, but several of these are among the most destructive tree pests on a global and regional scale. Considering the diversity and ecological success of bark beetles, it appears that their transition, some 90–120 million years ago, from “life in the green” to “life under bark” was a beneficial lifestyle change. Other key innovations in this group include symbioses with microbes, social behavior, and unusual reproductive strategies. In Bark Beetles: Biology and Ecology of Native and Invasive Species, 33 authors contributed their expertise, resulting in the first detailed and thorough synthesis of the biology, ecology, phylogeny, and management of bark beetles from a global perspective. Despite the multi-author format, continuity among writing styles makes the book seamless to read and the 15 chapters are arranged in a logical order from fundamental concepts to management strategies and impacts. The book is particularly well timed due to continued outbreaks of tree-killing bark beetles, range expansions associated with climate change, and proliferation of exotic species in non-native habitats. Bark beetle taxonomy originated with Carl Linnaeus, who provided the first four species descriptions in 1758—although he grouped them with the dermestids. Bark beetles are currently recognized as specialized members of the “true weevils” in the subfamily Scolytinae. Many scolytine species are ambrosia beetles that feed on symbiotic fungi rather than host-plant tissues; Bark Beetles appropriately handles the ambrosia beetles as an ecological guild and not a taxonomic designation, since their fungus-farming lifestyle arose multiple times (about 40–60 million years before humans began domesticating plants for agriculture). Yes, fungus farming also originated once in ants and once in termites, but ambrosia beetles hold the distinction of multiple independent origins. Many of the remaining bark beetles that are not fungus farmers still have obligate associations with microbes. Bark Beetles covers some key mechanisms, and also emphasizes outstanding questions about the impact of these microbial symbionts on bark beetle biology and population dynamics. The authors also address contemporary alternatives to classic paradigms on symbiont pathogenicity and host defenses. For instance, fungal symbionts may not be required to overwhelm the defenses of healthy trees, as once proposed; instead, they may be more important as a nutritional supplement for the beetles. Perhaps the most impressive theme evident throughout Bark Beetles is that the diversity and complexity of bark beetle biology and ecology arguably rivals that of most other insects. While not particularly well known, higher forms of sociality have evolved repeatedly in the Scolytinae, including gregariousness; parental care by the mother, father, or both; and delayed dispersal of adult offspring that aid in the care of younger siblings. Most male insects leave immediately or soon after copulation, but males of some scolytine species exhibit prolonged residency and often remain around the gallery until oviposition ceases. (Would Sigmund Freud have been intrigued by the millions of years of incestuous breeding between brothers and sisters and, occasionally, mothers and sons?) As evident in Bark Beetles, these and other biological complexities have resulted in several species being used as model organisms for diverse disciplines, including biological invasions, population dynamics, disturbance ecology, chemical ecology, microbial symbioses, social evolution, and coevolution. Outbreaks of tree-killing bark beetles in North America and Europe are among the most destructive forest disturbances, in some cases outranking fires, hurricanes, and tornadoes in their devastation. Some species are even recognized as “landscape engineers” due to their detrimental impact on stand composition during outbreaks. Warming climates have resulted in several species expanding their ranges and exhibiting outbreaks in regions that previously were too cold. The variable impact of climatic extremes on host tree susceptibility is also considered in Bark Beetles. While few bark beetles kill healthy trees, those that do are some of the most destructive pests of trees, including members of the genera Dendroctonus, Ips, Tomicus, and Scolytus that are specifically highlighted in several chapters. Destructive pests of horticultural trees, such as the coffee berry borer, Hypothenemus hampei (Ferrari), are also individually addressed. Bark Beetles also covers the diverse interactions that drive bark beetle abundance—from symbiotic microbes to predators, parasitoids, and varied host resistance mechanisms. The book does a fine job synthesizing decades of research on the mechanisms of bark beetle outbreaks, but recognizes that their population dynamics are notoriously complex and outbreaks are difficult to predict. Challenges still lie ahead, particularly as a result of climate change and range expansion. Undoubtedly, Bark Beetles: Biology and Ecology of Native and Invasive Species will be required reading for students, researchers, pest management professionals, tree care specialists, and policy makers in need of an expansive and contemporary synthesis of the bark beetle literature. Many of the chapters also contain exceptional images that are helpful in visualizing important concepts and understanding morphological characteristics for identification purposes.