A Study of the Relationship Between Defoliation of Sitka Spruce and Population Levels of Elatobium abietinum (Walker)

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Abstract
A correlation between the size of the surviving overwintering population of Elatobium abietinum and the size of the succeeding summer population peak is shown. It is also demonstrated that the loss of the needles from Sitka spruce is, in turn, related to the size of the summer population of aphids. After applying malathion in early April at the end of the overwintering period only small summer populations of aphids occurred, with consequent retention of needles. It is shown that predators, parasites, and alate migration have little effect on the population build-up and are not responsible for the breakdown of summer population levels. A diminishing rate of increase of the populations is demonstrated from April to July in all plots where no chemical control had been attempted. It is suggested that this may be due to a density-independent change in the quality of the phloem sap resulting in a drop in aphid numbers where population densities are low, and to this factor plus a density-dependent loss of feeeding sites where population densities are high.

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