Facultative and obligate diapause phenotypes in populations of the European spruce bark beetle Ips typographus

Abstract The bark beetle Ips typographus is the most destructive insect pest in Norway spruce-dominated forests. Its potential to establish multiple generations per year (multivoltinism) is one major trait that makes this beetle a severe pest. Ips typographus enters diapause to adjust its life cycle to seasonally changing environments. Diapause is characterized by developmental and reproductive arrest; it prolongs generation time and thus affects voltinism. In I. typographus a facultative, photoperiod-regulated diapause in the adult stage has been described. In addition, the presence of an obligate, photoperiod-independent, diapause has been hypothesized. The diapause phenotype has important implications for I. typographus voltinism, as populations with obligate diapausing individuals would be univoltine. To test for the presence of different I. typographus diapause phenotypes, we exposed Central and Northern European individuals to a set of photoperiodic treatments. We used two ovarian traits (egg number and vitellarium size) that are associated with gonad development, to infer reproductive arrest and thus diapause. We found a distinct effect of photoperiod on ovarian development, with variable responses in Central and Northern European beetles. We observed obligate diapausing (independent of photoperiod) individuals in Northern Europe, and both facultative (photoperiod-regulated) as well as obligate diapausing individuals in Central Europe. Our results show within-species variation for diapause induction, an adaptation to match life cycles with seasonally fluctuating environmental conditions. As the diapause phenotype affects the potential number of generations per season, our data are the basis for assessing the risk of outbreaks of this destructive bark beetle.