Abstract
Bark beetle species within the scolytid genera Dendroctonus, Ips, Pityogenes and Tomicus are known to cause extensive ecological and economical damage in spruce and pine forests during epidemic outbreaks all around the world. Dendroctonus ponderosae poses the most recent example having destroyed almost 100,000 km2 of conifer forests in North America. The success and effectiveness of scolytid species lies mostly in strategies developed over the course of time. Among these, a complex system of semiochemicals promotes the communication and aggregation on the spot of infestation facilitating an en masse attack against a host tree’s defenses; or an association with fungi that evolved either in the form of nutrition (ambrosia fungi) or even by reducing the resistance of host trees (blue-stain fungi). Although often specific to a tree genus or species, some bark beetles are polyphagous and have the ability to switch on to new hosts and extend their host range (i.e., between conifer genera such as Pityogenes chalcographus or even from conifer to deciduous trees as Polygraphus grandiclava). A combination of these capabilities in concert with life history or ecological traits explains why bark beetles are considered interesting subjects in evolutionary studies. Several bark beetle species appear in phylogeographic investigations, in an effort to improve our understanding of their ecology, epidemiology and evolution. In this paper investigations that unveil the phylogeographic history of bark beetles are reviewed. A close association between refugial areas and postglacial migration routes that insects and host trees have followed in the last 15,000 BP has been suggested in many studies. Finally, a future perspective of how next generation sequencing will influence the resolution of phylogeographic patterns in the coming years is presented. Utilization of such novel techniques will provide a more detailed insight into the genome of scolytids facilitating at the same time the application of neutral and non-neutral markers. The latter markers in particular promise to enhance the study of eco-physiological reaction types like the so-called pioneer beetles or obligate diapausing individuals.
Highlights
Among the many subfamilies that belong to the class of Insecta, only few have gained as much popularity, attracting the focus of the public, as have bark beetles [1]
The analyses revealed first a clear genetic differentiation between the two Ips species, and a genetic divergence between the European and Asian I. duplicatus populations which would be associated to behavioural differences in the pheromone bouquet [85,89,90,91]
In each of these refugia, the bark beetle evolved on separate pine species, P. pinaster in Portugal and P. halepensis-P. pinea in France and Spain, confirming the strong association of T. destruens with its host trees
Summary
Among the many subfamilies that belong to the class of Insecta, only few have gained as much popularity, attracting the focus of the public, as have bark beetles [1]. Chemical ecology plays a vital and essential role in the life cycle of bark beetles, since beside finding a mating partner, bark beetles use volatiles to locate the most suitable host [30,31,32,33,34,35] and to avoid non-hosts [36,37,38,39,40], to aggregate more individuals on the most suitable host in order to easier overcome a host’s resistance [14,29,41,42,43] and to avoid inter- or intraspecific competition by discouraging the further aggregation of other individuals [16,32,44,45,46,47] These pheromone responses could vary geographically and are often associated with the genetic structure of a population [48]. In order to be able to draw comparisons between the different studies, only investigations based on direct sequencing will be included in the current review
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