Pre-freeze mortality in three species of aphids from sub-Antarctic Marion Island
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Understanding the mechanisms by which aphids survive low temperature is fundamental in forecasting the risk of pest outbreaks. Aphids are chill susceptible and die at a temperature close to that at which a small exothermal event is produced. This event, which can be identified using differential scanning calorimetry(DSC), normally occurs at a higher temperature than the supercooling point(SCP)and has been termed a pre-freeze event(PFE). However, it is not known what causes the PFE or whether it signifies the death of the aphid. These questions are addressed here by using a sensitive DSC to quantify the PFE and SCP and to relate these thermal events to the lower lethal temperature(LT50) of sub-Antarctic aphids acclimated to low temperatures. PFEs were observed in each of the 3 species of aphids examined. They occurred over a narrower temperature range and at a higher temperature range than the SCP (-8.2 to -13.8 and -5.6 to -29.8°C, respectively). Increased acclimation temperature resulted in increased SCPs in Myzus ascalonicus but not in Rhopalosiphum padi. The LT50 reduced by approximately 1°C from -9.3 to -10.5°C with reduced acclimation temperature(10–0°C). The LT50 was close to the temperature at which the PFE occurred but statistically significantly higher than either the PFE or the SCP. In the majority of cases the PFE exotherm occurred well before the main exotherm produced by the bulk of the insect’s body water freezing SCP). However, in a few cases it occurred at the same temperature or before the super-cooling point making the term, pre-freeze event(PFE), rather misleading. The possible origins of the PFE are discussed.