Studies on the cold hardiness of the peach-potato aphid Myzus persicae (Sulzer)
A system incorporating a prototype automatic thermoelectric cooling method with computer-based recording of aphid supercooling points was developed and formed the basis of cold hardiness assessment. Under laboratory culture all developmental stages of Myzus persicae had a mean inherent supercooling potential below -20°C, with first instar nymphs the most cold hardy. When maintained at 5°C, younger instars demonstrated acclimation ability unlike adult aphids, and in an insecticide resistant strain, adults lost cold hardiness. When in contact with surface moisture, the majority of aphids did not experience inoculative nucleation. Sexual morphs of M. persicae possessed supercooling ability comparable with laboratory maintained parthenogenetic morphs; eggs supercooled to below -30°C. Seasonal studies of supercooling ability demonstrated that all aphid stages were most cold hardy in summer. Younger instars showed natural acclimatisation and were cold hardy throughout the year. Overall adults lost cold hardiness as winter progressed, exhibiting bimodal supercooling point distributions in two winters, with distinct high (HG) and low (LG) groups and mean supercooling points of approximately -20°C and -10 oe respectively. Clonal differences and adult age did not account for this pattern. Following experimental starvation at 5°C, first instars of M. persicae maintained extensive supercooling potential but adults exhibited losses of cold hardiness comparable with those in natural overwintering populations, suggesting that feeding may be necessary to maintain adult cold hardiness levels during winter. Subsequent starvation experiments did not reproduce the dramatic losses of cold hardiness implying that the feeding influence was more complex than the availability of food per se. In a series of host transfer experiments the mean supercooling point of Aphis fabae adults could be shifted by over 10°C, increasing when they fed on beans and reduced when transferred back to spindle; the LG (spindle/poor supercoolers) to HG (bean/good supercoolers) shift was more difficult to achieve and suggested a nucleating agent in spindle sap. Trimethylsilyl derivatised carbohydrate extracts of M. persicae and A. fabae were analysed by capillary gas-liquid chromatography. Glucose, glycerol, fructose, mannitol, sucrose, and trehalose were detected in samples of both species, together with trace amounts of unidentified carbohydrates in M. persicae samples. Dulcitol was present in spindle-fed A. fabae only. There was no obvious correlation between carbohydrate content and supercooling ability but high total percentage body carbohydrate levels were revealed and may have a solute effect, enhancing inherent supercooling potential and dependent on carbohydrate-rich sap intake. Laboratory cultured A. fabae were capable of extensive supercooling, as were individuals collected from summer herbaceous hosts; first instars were the most cold hardy. When associated with the primary host, spindle, all aphids showed poor supercooling potential, less than -15°C; overwintering eggs were capable of supercooling to below -30°C and acclimatised in winter. Eggs and oviparae were not subject to inoculative nucleation. Preliminary experiments to relate supercooling ability to mortality at sub-zero temperatures proved inconclusive and were terminated when temperature shock and/or desiccation were thought to have induced premature mortality. The results demonstrate that the cold hardiness characteristics of M. persicae are atypical of those observed in other freezing-susceptible arthropods. It is proposed that continued feeding during mild winters maintains cold hardiness levels in adult M. persicae and this influence may provide a possible explanation for the successful anholocyclic overwintering of this aphid during such winters. Avenues of research to further investigate this proposal are suggested.