Conifer defense and resistance to bark beetles

Conifers are the dominant trees over huge areas worldwide and include many species of great economic and ecological importance. Some conifer species, particularly members of the pine family, are susceptible to attack by tree-killing bark beetles and their symbiotic microorganisms. The beetle–symbiont complex can overwhelm the defenses of healthy trees through pheromone-mediated mass attacks. However, most of the time, the trees’ effective preformed and inducible defenses protect them from attack and regulate bark beetle populations at low, endemic levels. Conifers integrate multiple defense mechanisms, including preformed and inducible specialized metabolites (particularly terpenoids and phenolics), though mechanical barriers (periderms, lignified cells, crystals), and other chemical, molecular, and mechanical defenses. Anthropogenic disturbances, such as climate change and range expansion of invasive species, reduce tree vigor and create new pest–conifer associations at an increasing rate. These threats accentuate the need for a more detailed understanding of the interactions between conifer defenses and the bark beetle–symbiont complex.