Scolytus

Abstract: P lant disease epidemics resulting from introduction of exotic fungal pathogens are a well-known phenom- enon. Limited resistance in the host and excessive aggres- siveness in the pathogen (reflecting their lack of prior co- evolution) can result in an explosive outbreak of disease. Introduction events also present a window of evolutionary op- portunity for the pathogen. In its endemic location, a plant pathogen tends to be subject to routine selection constraints, favoring maintenance of a relatively stable, if fluctuating, population structure over time. When introduced into a new environment, it will often be subject to novel or episodic se- lection, reflecting sudden exposure to new biotic and abiotic influences, such as a new host population, new vectors, new competitors, or a different climate. These influences provide the potential for rapid evolution (Brasier 1995). Falling within the category of episodic selection is the sud- den contact that can occur between closely related but pre- viously geographically isolated pathogens as a result of in- troductions. Theoretically, this process presents an opportunity for rapid emergence of new or modified pathogens via interspecific gene flow (Brasier 1995). Until recently, this phenomenon has been little studied. Now Dutch elm disease, a major ecological accident of the 20th century (Heybroek 1993), is providing remarkable insights into rapid evolution of a plant pathogen outside its en- demic environment. This article describes the migratory events and unusual genetic events that have occurred in this fungus and other new examples of rapid pathogen evo- lution via interspecific gene flow, and discusses some of the wider environmental, evolutionary, and quarantine impli- cations. The Dutch elm disease pathogens Elm trees (Ulmus) are confined mainly to the temperate re- gions of the Northern Hemisphere. China and Japan have a total of about 25 elm species, while Eurasia, North Amer- ica, and the Himalayas each have about five or six species. Dutch elm disease (so called because the early seminal re- search was in The Netherlands (Holmes and Heybroek 1990)) is the elm's main enemy. It is a wilt disease, caused by ascomycete fungi of the genus Ophiostoma, that spreads within the tree's vascular system. The pathogens are trans- mitted from diseased to healthy elms by elm bark beetles of the genus Scolytus (Fransen 1935, Webber and Brasier 1984).

Abstract: Bark Beetles: Biology and Ecology of Native and Invasive Species provides a thorough discussion of these economically important pests of coniferous and broadleaf trees and their importance in agriculture. It is the first book in the market solely dedicated to this important group of insects, and contains 15 chapters on natural history and ecology, morphology, taxonomy and phylogenetics, evolution and diversity, population dynamics, resistance, symbiotic associations, natural enemies, climate change, management strategies, economics, and politics, with some chapters exclusively devoted to some of the most economically important bark beetle genera, including Dendroctonus, Ips, Tomicus, Hypothenemus, and Scolytus. This text is ideal for entomology and forestry courses, and is aimed at scientists, faculty members, forest managers, practitioners of biological control of insect pests, mycologists interested in bark beetle-fungal associations, and students in the disciplines of entomology, ecology, and forestry. * Provides the only synthesis of the literature on bark beetles* Features chapters exclusively devoted to some of the most economically important bark beetle genera, such as Dendroctonus, Ips, Tomicus, Hypothenemus, and Scolytus* Includes copious color illustrations and photographs that further enhance the content

Abstract: A vast community of organisms occurs on and within bark beetles and bark beetle-infested trees. The large diversity of symbiotic species covers a breadth of functional roles that are often redundant or substitutable. Symbiotic associations can be facultative or obligatory, and vary from antagonistic to mutualistic depending on the context at which the interactions occurs. Most symbiotic organisms are phoretic, and thus are transferred from tree to tree by bark beetles or other arthropods. Symbionts influence bark beetle communication, reproduction, nutrition and population dynamics, as well as tri-trophic interactions, competition among species, and host-tree utilization by bark beetles. Taxa considered symbiotic with bark beetles include fungi, bacteria, viruses, algae, mites, protozoa, and nematodes, among others. Interactions among symbionts are often mediated by host plant chemistry, abiotic factors such as temperature, or other phoretic organisms. Some of the symbionts, such as fungi, amplify our view of bark beetles as pests, as they may be tree pathogens or influence the coloration and texture of wood or plant products. However, some symbionts may also provide a solution to bark beetle population outbreaks and range expansion, as some symbiotic species are harmful to bark beetles and could be used as biological control agents.