Survival of sub-zero temperatures by two South Georgian beetles (Coleoptera,Perimylopidae)

Summary The ability of adults and larvae of two species of perimylopid beetles (Hydromedion sparsutum, Perimylops antarcticus) to survive sub-zero temperatures was studied at Husvik, South Georgia in summer during October–December 1990. Experiments determined their survival at constant sub-zero temperatures, their lower lethal temperatures and individual supercooling points. The effects of cooling rates (0.015°, 0.5° and 2.0°C min^–1) and starvation on survival were also assessed. Mean supercooling points of field-collected individuals of both species were in the range -3.0° to -5.4°C with Perimylops having a deeper capacity (ca. 1.5°C) for supercooling relative to Hydromedion. The former species also survived freezing temperatures significantly better than the latter and its mean lower lethal temperature was 2.5°C lower. At a constant temperature of -8.5°C, the median survival times for Perimylops adults and larvae were 19 and 26 h respectively, whilst both stages of Hydromedion died within 3 h. The three cooling rates resulted in significantly different median survival temperatures for adult Hydromedion with 0.5°C min^–1 producing maximum survival. Prior starvation did not have a significant influence on the survival of either species at sub-zero temperatures although both adults survived less well. The results support field observations on the habitats and distribution of these insects, and suggest differing degrees of freezing tolerance.