Acclimatisation in the green spruce aphid, Elatobium abietinum

Thin layer chromatography separation of 80% ethanol extracts of adult Elatobium abietinum revealed the presence of the polyhydric alcohol mannitol in aphids overwintering outdoors but not in aphids kept permanently indoors at 15°C. After 3 days at 15°C no traces of mannitol were left in overwintering aphids. Mean freezing temperatures of outdoor, unfed instar I nymphs were about 4°C lower than those of unfed instar I nymphs produced at 15°C. Mean freezing temperatures of overwintering adults were considerably higher than those of unfed instar I nymphs and showed no changes associated with time at 15°C following transference indoors. Similarly, mean freezing temperatures of Sitka spruce needles transferred to 15°C did not change. It was concluded that, although freezing was mainly avoided by supercooling, the presence of mannitol lowered the true freezing temperature of aphid haemolymph and, consequently, the actual freezing temperatures of nymphs produced under cold conditions. However, the considerable increase in freezing point temperatures caused by imbibition of plant sap masked these acclimatisation changes in feeding aphids.