Supercooling and the low-temperature survival of the green spruce aphid Elatobium abietinum

Laboratory investigations into the low-temperature tolerance of the green spruce aphid, Elatobium abietinum, revealed that the insect was killed by freezing. Aphids and host Sitka spruce needles showed similar seasonal changes in supercooling ability. A noticeable increase in this ability occurred between June and October. Aphids were more susceptible to low temperatures when attached to the plant. It is suggested that mortality resulted from ice which formed in the sap of the host needles and spread into the feeding aphids via their mouthparts. Neither the chlorotic banding of needles, caused by aphid feeding, nor needle length affected needle supercooling. Increased duration of exposure increased the probability of freezing of supercooled needles at low temperatures. A small percentage of first-instar nymphs supercooled to much lower temperatures than the remainder of the population. These were newly born nymphs whose high supercooling ability markedly decreased when they began to feed.