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.
Highlights
Bark beetles are common insects in forest ecosystems (Hulcr et al 2015; Raffa et al 2015)
Under SD conditions mean egg numbers were in the same range and not significantly different among each other
Data of post-diapause beetles provided information on reproductively active (= non-diapausing) individuals, which were subsequently used as control groups for comparisons with females developing at various photoperiodic treatments
Summary
Bark beetles are common insects in forest ecosystems (Hulcr et al 2015; Raffa et al 2015). Journal of Pest Science spruce-dominated forests (Kausrud et al 2012; Schelhaas et al 2003; Wermelinger 2004) It shows extensive population outbreaks, which are usually triggered by abiotic disturbance events, e.g. storm, snowfall or drought, providing high amounts of suitable breeding material. This usually results in a rapid increase of beetle population densities, followed by severe Norway spruce mortality (Marini et al 2017; Seidl et al 2016; Stadelmann et al 2014; Wermelinger 2004)
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