While many bark beetles attack only dead or weakened trees, species in the genus Dendroctonus have the ability to kill healthy conifers through mass attack of the host tree, and may exhibit devastating outbreaks that significantly alter the landscape. Other species in this group are able to successfully colonize trees in small numbers without killing the host. We reconstruct the evolution of these ecological and life history traits, first classifying the extant Dendroctonus species by attack type (mass or few), outbreaks (yes or no), host genus (Pinus and others), location of attacks on the tree (bole, base, etc.), whether the host is killed, and if the larvae have individual galleries or are gregarious. We then estimated a molecular phylogeny for a data set of cytochrome oxidase I sequences sampled from nearly all Dendroctonus species using Bayesian methods. We accommodated phylogenetic uncertainty by performing ancestral state reconstructions on a representative sample of 100 trees from the posterior distribution produced using MrBayes 3.1.2.
Our analyses suggest that extant Dendroctonus species likely evolved from an ancestor that killed host pines through mass attack of the bole, had individual larvae, and exhibited outbreaks. The ability to colonize a host tree in small numbers (as well as gregarious larvae and attacks at the tree base) apparently evolved later, possibly as two separate events in different clades. It is likely that tree mortality and outbreaks have been continuing features of the interaction between conifers and Dendroctonus bark beetles.