OPPOSITE, LINEAR AND NONLINEAR EFFECTS OF PLANT STRESS ON A GALLING APHID

The impact of plant stress on the occurrence and performance of the ga ll-making aphid Sacchiphantes abietis (Homoptera: Adelgidae) was inves tigated by estimating the density and size of galls on experimentally stressed Norway spruce (Picea abies) trees. Plots with 25-yr-old spruc e trees were treated so as to create a range of stress. Trees were exp osed to an untreated control or one of the following five treatments: drought, irrigation, ammonium sulphate addition, nitrogen-free (''vita lity'') fertilization and optimum fertilization with irrigation. The o ccurrence of galls was higher on drought-stressed trees than on trees in other treatments; Gall size varied continuously among treatments, w ith 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 conclud e that plant stress has opposite effects on different life-stages of t his galling insect and possibly on other species of gall-makers as wel l.