Latitudinal differences in diapause related photoperiodic responses of European Colorado potato beetles (Leptinotarsa decemlineata)

Many organisms use photoperiodic cues to assess seasonal progression and pace their phenology. As photoperiod correlates with latitude, range expansions in seasonal environments may require re-synchronization of phenology and life-history traits with novel season length. Adaptive resynchronization takes time, and hence might be one factor explaining why range expansion to higher latitudes often is slow. Studies investigating latitudinal clines in photoperiodic traits often focus on species or populations which are well established. However, studying organisms which are in the process of expanding their range can provide valuable information on the evolutionary ecological mechanisms driving the adaptive synchronization to seasonal environments. The Colorado potato beetle, Leptinotarsa decemlineata, is a pest of potato, which rapidly has spread towards higher latitudes. We studied whether beetles from six European populations along a latitudinal axis are synchronized with their local photoperiodic environmental conditions. Variation in critical photoperiod (when 50 % of individuals make the decision to overwinter), diapause incidence, burrowing age for diapause and resurfacing behaviour were investigated by maintaining beetles under six photoperiods. The beetles showed a clear latitudinal pattern in diapause incidence and burrowing age for diapause but not in critical photoperiod. Resurfacing behaviour of burrowed beetles increased with the length of the photoperiod, which through unsynchronized overwintering behaviour could lead to high overwintering mortality. Thus, while synchronization of diapause preparation with local photoperiodic conditions can be one reason explaining the success of L. decemlineata in expanding to higher latitudes, further northward range expansion could be constrained by inherent difficulties to initiate overwintering under very long photoperiods.