Secondary Ambrosia Beetles in Apparently Healthy Trees: Adaptations, Potential Causes and Suggested Research

Some ambrosia beetles are primary attackers of healthy, living trees, but in recent years normally secondary species have been increasingly observed attacking living trees, either as exotics or in their native geographic ranges. We identified five factors that could underlie an increasing prevalence of attack by secondary ambrosia beetles on living trees: (1) early flight before the host tree has recovered the ability to resist attack in the spring, possibly associated with climate change; (2) nutritional independence from the host that may enable ambrosia beetles to feed on ambrosia fungi that live on dead tissue in a living tree; (3) potentially pathogenic fungi that could become more pathogenic with climate change or through hybridization with exotic strains; (4) cryptic behavior that facilitates international transport and the establishment of exotic species and genotypes; and (5) a complex chemical ecology that enables secondary ambrosia beetles to locate stressed living trees that may temporarily appear to be suitable hosts for secondary beetles. We propose four avenues of research that will lead to an increased understanding of attack of living trees by ambrosia beetles, and may facilitate the implementation of effective pest management strategies and tactics: (1) intensive surveys, particularly for exotic beetle species and associated fungal strains; (2) molecular genetics studies that would facilitate the identification of known and new strains and genotypes, particularly of ambrosia fungi; (3) studies of the pathogenicity of ambrosia fungi as well as other fungi that could predispose trees to attack; and (4) investigations of the chemical ecology of tree-attacking species that could lead to new pest management tools and tactics.