Density and performance of Epirrita autumnata (Lepidoptera: Geometridae) along three air pollution gradients in northern Europe.
Larval density, size of pheromone-trapped adult males, and survival and parasitism rates in the laboratory of field-collected larvae of Epirrita autumnata were investigated along three pollution gradients: the surroundings of smelters at Monchegorsk and Nikel in north-western Russia and a factory complex at Harjavalta, south-western Finland. Along the gradients, the emissions are qualitatively similar, consisting mainly of Cu, Ni and SO2, but the amounts emitted vary. Cu and Ni concn in birch (Betula pubescens) leaves, a common diet of E. autumnata larvae, were monitored and used as general indices of pollution. High Cu and/or Ni concn in birch foliage, and/or associated changes in other pollutants, were associated with detrimental effects on E. autumnata performance. They occurred when concn of foliage Cu and/or Ni exceeded 20-30 µg/g. In the Harjavalta and Monchegorsk gradients, where these threshold values in the foliage were exceeded, larval density and survival increased significantly with distance from the smelters. No pollution-related trends with distance were found either for male size or for larval parameters in that section of the Nikel gradient where foliage heavy metals were below this threshold value. Parasitism rates were not associated with pollution, indicating no difference in sensitivity between parasitoids and their herbivorous host. No indication was found of a positive effect of moderate pollution on E. autumnata, and heavy pollution had detrimental effects. The results do not suggest that the species might become a severe pest in polluted areas outside its natural outbreak range.