Inoculative freezing and thermal hysteresis in the adult beetles Ips acuminatus and Rhagium inquisitor

Abstract

Thermal hysteresis producing antifreeze proteins provided protection against inoculative freezing in the freeze-susceptible bark beetle, Ips acuminatus and the cerambycid beetle, R. inquisitor held in close contact with freezing water. Specimens were invariably killed by freezing and the level of inhibition provided by antifreeze proteins differed greatly between the two species. In I. acuminatus, the level was much greater than the measured hysteresis of about 2°C. In fact, evidence for inoculation of external ice was not provided in beetles displaying thermal hysteresis activity since the ability to supercool remained unaltered between dry- and wet-surface individuals. Specimens depleted of antifreeze proteins, however, froze by inoculation following supercooling to temperatures at about 9–10°C below mean haemolymph melting points. In R. inquisitor, the capacity to inhibit inoculative freezing provided by antifreeze proteins was much smaller. The thermal hysteresis activity ranged from 2.6 to 6.2°C in individuals maintained in continuous darkness and between 1.8–5.8°C among specimens treated in continuous light. Wet-surface cerambycid beetles froze following supercooling to temperatures at 1.3 and 2.7°C below mean hysteresis-freezing points. In contrast, specimens with a dry-surface supercooled to temperatures about 14 to 17°C below mean hysteresis-freezing points.