Pleistocene events and ongoing population outbreaks affect the genetic structure of the Eurasian spruce bark beetle Ips typographus in the Southern Alps and the Apennines

The bark beetle Ips typographus (L.) is the most important pest in European Norway spruce forests. Especially after abiotic disturbance events, like storms or snowfall, this beetle can utilise vast amounts of trees with impaired defences, leading to mass outbreaks and further mortality of spruce. In 2018, the 'Vaia' storm damaged huge areas of Norway spruce -dominated forests in the Southern Alps. In the following years, the same regions were additionally affected by heavy snowfalls providing high amounts of suitable material for I. typographus brood establishment and development. Since then, these forests have faced unprecedented population outbreaks of I. typographus with severe effects for forest ecosystems, from both an ecological and an economic perspective. A comprehensive understanding of this beetle's genetic structure, including effects of the last ice ages as well as consequences of current population outbreaks, can give insights into the population dynamics of an eruptive species. Here, we studied the genetic structure of five epidemic populations from the Southern Alps and one from a potential glacial refugium in the Apennines. Our results revealed a low genetic diversity and a weak population structure among all studied locations. In the six analysed populations, four already described haplotypes, belonging to three slightly divergent haplogroups, were found. These haplotypes were evenly distributed among populations, including the potential glacial refugium in the Apennines. High gene flow among populations is probably maintained by the beetle's high dispersal capacity and high population densities, resulting in a panmictic population structure.