Effects of temperature on overwintering populations of the green spruce aphid Elatobium abietinum

Recorded minimum temperatures of – 7 ^oC or lower noticeably reduced overwintering populations of the green spruce aphid, Elatobium abietinum, in north-east Scotland. It is suggested that, at these temperatures, ice formation in the needles of the host Sitka spruce, caused attached aphids to freeze. Aphid mortality also occurred when maximum temperatures did not rise above + 6 ^oC for prolonged periods, possibly as a result of starvation following an extended chill coma. A diagrammatic representation of the main factors affecting anholocyclic populations of E. abietinum during the winter is presented to emphasize the governing role played by temperature. The balance between mortality and recruitment determines the size of the population at the end of the winter, and this in turn determines the subsequent summer infestation. It should be possible, therefore, to predict aphid outbreaks either from winter temperatures or from the number of aphids present at the end of the winter. Temperature records obtained from integrating thermometers indicated that the inside of the lower crown tended to be the warmest part of the tree during the winter, resulting in greater aphid survival in the lower branches.