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Green spruce aphid Elatobium abietinum is an important defoliating pest of Sitka spruce Picea sitchensis that may become more damaging in the future as a result of climate change. Mild winters and higher spring temperatures increase E. abietinum population densities, although this positive response to warmer conditions might be over-ridden if E. abietinum is adversely affected by exceptionally high summer temperatures. In laboratory experiments, aphids exposed to temperatures above 25 degrees C became more restless and were more likely to drop from the shoot, and subsequently showed a reduction in mean relative growth rate. In contrast, survival was not reduced significantly unless temperatures exceeded 35 degrees C. Field data from Radnor Forest in Wales indicated that E. abietinum populations in the autumn were inversely correlated with mean and maximum summer temperatures, at temperatures below those that had a negative impact in the laboratory. The greater sensitivity of populations in the field to high summer temperatures may reflect higher temperatures at the needle surface compared with the temperature of the surrounding air, or associated increases in natural enemy activity. The results from the present study indicate that high temperatures during mid-summer have a significant impact on E. abietinum individuals and populations and, ultimately, may influence how the aphid responds to climate change.