Opposite, linear and non-linear effects of plant stress on a galling aphid

The impact of plant stress on the occurrence and performance of the gall-making aphid Sacchiphantes abietis (Homoptera: Adelgidae) was investigated by estimating the density and size of galls on experimentally stressed Norway spruce (Picea abies) trees. Plots with 25-yr-old spruce trees were treated so as to create a range of stress. Trees were exposed to an untreated control or one of the following five treatments: drought, irrigation, ammonium sulphate addition, nitrogen-free ("vitality") fertilization and optimum fertilization with irrigation. The occurrence of galls was higher on drought-stressed trees than on trees in other treatments; Gall size varied continuously among treatments, with the largest galls being found on "vitality"-fertilized trees and the smallest on drought-stressed trees. Mechanisms that might explain the inconsistent effects of stress on aphids are discussed. I conclude that plant stress has opposite effects on different life-stages of this galling insect and possibly on other species of gall-makers as well.