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

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Abstract
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.