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1. The mean supercooling points of first instar and adult M y z w persicue (Sulzer) maintained at 20°C and cooled at 1°C min-' were -26.6 and -25.0"C respectively.2. The LTso (temperature) of the same age groups drawn from the same population and cooled at the same rate were -8.1 and -6.9"C, indicating extensive pre-freeze mortality in M.persicue under laboratory conditions.3. Acclimation at 10 and 5°C did not affect supercooling but depressed the LTso of both first instars and adult aphids.4. Freezing of leaves during feeding did not increase mortality above that expected from the direct effects of low temperature.5. The level of cold in different winters can be expressed in terms of the total number of frost days, and the frequency of abnormally cold days. Winter temperatures differ markedly in a vertical profile from the soil to the soil or grass surface, and then to the air (and foliage) above.6. The time of the first record of M.persicue in suction trap samples is correlated with January and February temperatures except in the west of England and Wales. Further north December and January temperatures are relatively more important.7. Winter temperatures and the resultant aphid mortality is a primary determinant of the timing of the spring migration.

The LT,! (lethal temperature) of first instar and adult stages of the peachpotato aphid Myzuspersicae was lowered following long term acclimation at low temperatures.First instars consistently showed greater cold hardiness than adult stages at each acclimation temperature, with the differential increasing as the temperature was lowered. When maintained at 5°C (the lowest acclimation regime) nymphs and adults had LT,,s 8.3"C and 4.7"C respectively lower than those for nonacclimated individuals.When 10°C acclimated adults were returned to 20"C, the acclimation effect was retained in full for 6 days but complete deacclimation occurred by day 10. In contrast the LT,, of their progeny increased gradually from the first day of adult deacclimation towards the level of the unacclimated control over a period of 10 days.A change in cold hardiness was observed in first instars according to their position in the birth sequence. The LT,, of first-born nymphs (day 1 of reproduction) from 20°C parents was -15.9"C rising to -8.3"C by day 4 and remaining at this level until the end of the reproductive period.The differential mortality between nymphs and adults observed in the laboratory was supported by the results of a field experiment. Adult aphids kept in clip-cages on a crop of oilseed rape showed greater mortality compared with those introduced as nymphs when the minimum temperature fell below -4°C for the first time in winter. At -10°C mortality of aphids introduced as adults approached 100% whereas more than 50% of those introduced as nymphs were still alive at this temperature.

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