A comparison of Elatobium abietinum populations on Sitka spruce trees differing in needle retention during aphid outbreaks.

No difference in the rate of symptom development and needle loss was evident between resistant and susceptible trees. Population growth rates, which were higher on current year needles in the upper crown, adult weights, reproductive state and instar ratios were similar for aphids from both resistant and susceptible trees. Overwintering survival rates were higher on susceptible trees and were the main cause of the higher number of aphids found on susceptible trees and on the lower crown in comparison with the resistant trees and the upper crown. The supercooling points of needles from resistant trees were higher than those from susceptible trees during the three coldest months of the year-December, January and February. As the aphid freezes at the same temperature as the needle on which it feeds differences in needle freezing temperatures can result in greater aphid mortality on the resistant trees at the temperatures prevailing in NE Scotland during the period of the study. No correlation was found between provenance latitude and supercooling ability of the needles although a very great variability was evident between individual trees. Changes in the form of the super-cooling temperature frequency curves indicated a gradual increase in the quantity of nucleators present in the spruce needles during the winter, the change being more pronounced in the resistant needles.