Physiological and biochemical analysis of overwintering and cold tolerance in two Central European populations of the spruce bark beetle, Ips typographus

Overwintering success is one of the key aspects affecting the development and outbreaks of the spruce bark beetle, Ips typographus (L.) populations. This paper brings detailed analysis of cold tolerance, and its influence on overwintering success, in two Central European populations of I. typographus during two cold seasons. Evidence for a supercooling strategy in overwintering adults is provided. The lower lethal temperature corresponds well to the supercooling point that ranges between −20 and −22 °C during winter months. The supercooled state is stabilized by the absence of internal ice nucleators and by seasonal accumulation of a mixture of sugars and polyols up to the sum concentration of 900 mM. The cryoprotective function of accumulated metabolites is probably based on increasing the osmolality and viscosity of supercooled body fluids and decreasing the relative proportion of water molecules available for lethal formation of ice nuclei. No activity of thermal hysteresis factors (stabilizers of supercooled state) was detected in hemolymph. Lethal times for 50% mortality (Lts50) in the supercooled state at −5, −10 or −15 °C are weeks (autumn, spring) or even months (winter), suggesting relatively little mortality caused by chill injury. Lts50 at −15 °C are significantly shorter in moist (6.9 days) than in dry (>42 days) microenvironment because there is higher probability of external ice nucleation and occurrence of lethal freezing in the moist situation.