The relationship between nutrition of the host tree, Abies amabilis (Dougl.) Forbes, as influenced by nitrogen fertilizers, and the growth of populations of the balsam woolly aphid, Adelges piceae (Ratz.), was investigated. Sixty trees, growing in a greenhouse in two soil types, a nutrient-deficient mineral soil and an enriched humic soil, were infested with woolly aphid larvae. Subsequently, groups of 10 trees received foliar treatments of various concentrations of ammonium nitrate and urea. The following year, 32 young trees in Seymour Valley, B.C., were studied to determine the effect of fertilization of uninfested trees on establishment of aphid larvae.
In the greenhouse, foliar sprays of 1% ammonium nitrate solution resulted in a 23% decrease in population in 10 weeks, as contrasted with a 31% increase in the control population. In the field, larval establishment was 31% to 37% lower on the ammonium nitrate-treated trees than on the control trees. Inasmuch as there was no evidence of increased mortality of aphids in situ, we infer that the fertilizer acts primarily by inhibiting initial settling of larvae on the host trees.