Freezing induces a loss of freeze tolerance in an overwintering insect.
Cold-hardy insects overwinter by one of two main strategies: freeze tolerance and freeze avoidance by supercooling. As a general model, many freeze-tolerant species overwinter in extreme climates, freeze above -10 degrees C via induction by ice-nucleating agents, and once frozen, can survive at temperatures of up to 40 degrees C or more below the initial freezing temperature or supercooling point (SCP). It has been assumed that the SCP of freeze-tolerant insects is unaffected by the freezing process and that the freeze-tolerant state is therefore retained in winter though successive freeze-thaw cycles of the body tissues and fluids. Studies on the freeze-tolerant larva of the hoverfly Syrphus ribesii reveal this assumption to be untrue. When a sample with a mean 'first freeze' SCP of -7.6 degrees C (range of -5 degrees C to -9.5 degrees C) were cooled, either to -10 degrees C or to their individual SCP, on five occasions, the mean SCP was significantly depressed, with some larvae subsequently freezing as low as -28 degrees C. Only larvae that froze at the same consistently high temperature above -10 degrees C were alive after being frozen five times. The wider occurrence of this phenomenon would require a fundamental reassessment of the dynamics and distinctions of the freeze-tolerant and freeze-avoiding strategies of insect overwintering.
School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.
C L
CL
Brown
J S
JS
Bale
K F A
KF
Walters
D000064:Acclimatization / Q000502:physiology*; D000704:Analysis of Variance; D000818:Animals; D004175:Diptera / Q000254:growth & development / Q000502:physiology*; D004739:England; D005615:Freezing; D007814:Larva / Q000254:growth & development / Q000502:physiology; D012621:Seasons; D013696:Temperature