FACTORS AFFECTING THE COLD HARDINESS OF THE PEACH-POTATO APHID MYZUS-PERSICAE

Supercooling point studies were used to investigate the factors influencing the cold hardiness of the peach-potato aphid Myzus persicae, a freezing-susceptible insect. Overwintering adults lost cold hardiness as winter progressed, with a variable proportion showing a marked reduction in supercooling ability. Cold hardiness increased in spring so that all individuals demonstrated extensive supercooling ability typical of aphids reared in the laboratory at 20.degree. C with a long photoperiod; these levels of cold hardiness were maintained in the field during summer and early autumn. First instar nymphs demonstrated considerable cold hardiness all year. Surface moisture caused inoculative freezing in some first instar nymphs and adults when supercooled, but the majority were unaffected. In the laboratory, adults starved for 7 days at 5.degree. C showed distinct losses of supercooling potential equivalent to those observed in the field during mid to late winter. No loss of cold hardiness was found in first instar nymphs starved under the same conditions. The results demonstrate that the cold hardiness characteristics of M. persicae are atypical of those observed in other freezing-susceptible insects and it is suggested that continued feeding during mild winter conditions allows maintenance of cold hardiness particularly in adult aphids, and provides a possible explanation for the successful anholocyclic overwintering of M. persicae during such winters.