Effects of summer frost exposures on the cold tolerance strategy of a sub-Antarctic beetle

The sub-Antarctic beetle Hydromedion sparsutum (Coleoptera, Perimylopidae) is common locally on the island of South Georgia where sub-zero temperature s can be experienced in any month of the year. Larvae were known to be weak ly freeze tolerant in summer with a mean supercooling point (SCP) around -4 degreesC and a lower lethal temperature of -10 degreesC (15 min exposure). This study investigated the effects of successive freezing exposures on th e SCP and subsequent survival of summer acclimatised larvae. The mean SCP o f field fresh larvae was -4.2 +/- 0.2 degreesC with a range from -1.0 to -6 .1 degreesC. When larvae were cooled to -6.5 degreesC on 10 occasions at in tervals of 30 min and one and four days, survival was 44, 70 and 68%, respe ctively. The 'end of experiment' SCP of larvae surviving 10 exposures at -6 .5 degreesC showed distinct changes and patterns from the original field po pulation depending on the interval between exposure. In the 30 min interval group, most larvae froze between -6 and -8 degreesC, a depression of up to 6 degreesC from the original sample; all larvae were dead when cooling was continued below the SCP to -12 degreesC. In the one and four day interval groups, most larvae froze above -6 degreesC, showing no change as a result of the 10 exposures at -6.5 degreesC. As with the 30 min interval group, so me larvae froze below -6 degreesC, but with a wider range, and again, all w ere dead when cooled to -12 degreesC. However, in the one and four day inte rval groups, some larvae remained unfrozen when cooled to -12 degreesC, a d epression of their individual SCP of at least 6 degreesC, and were alive 24 h after cooling. In a further experiment, larvae were cooled to their indi vidual SCP temperature at daily intervals on 10 occasions to ensure that ev ery larva froze every day. Most larvae which showed a depression of their S CP of 2-4 degreesC from their day one value became moribund or died after s ix or seven freezing events. Survival was highest in larvae with SCPs of -2 to -3 degreesC on day one and which froze at this level on all 10 occasion s. The results indicate that in larvae in which the SCP is lowered followin g sub-zero exposure, the depression of the SCP is greatest in individuals t hat do not actually freeze. Further, the data suggest that after successive frost exposures in early winter the larval population may become segregate d into two sub-populations with different overwintering strategies. One gro up consists of larvae that freeze consistently in the temperature range fro m -1 to -3 degreesC and can survive multiple freeze-thaw cycles. A second g roup with lower initial SCPs (around -6 degreesC), or which fall to this le vel or lower (down to -12 degreesC) after freezing on one or more occasions , are less likely to freeze through extended supercooling, but more likely to die if freezing occurs. (C) 2001 Elsevier Science Ltd. All rights reserv ed.