Air Pollution Levels and Abundance of Forest Insects

Abstract

This chapter investigates and reviews the effects of industrial air pollution on forest damage through insect pests. A spatial pollution gradient (SO2, heavy metals) was utilized in southwestern Finland around a factory complex producing copper, nickel, sulphuric acid and fertilizers. Several insects associated with the Scots pine, including bark bugs, aphids, diprionids and tortricids, abounded in the moderately polluted pine stands, but were scarce (excluding aphids) in the immediate vicinity of the industrial plants probably due to the toxic effects of heavy metals. Preliminary results showed no clear patterns for parasitization of pests in relation to the pollution level. Chemical analyses of the needles showed that the concentrations of heavy metals decreased exponentially with increasing distance from the emission source. Also the concentrations of potassium, sodium as well as ash weight and air-dry weight decreased, while those of magnesium, manganese and calcium increased with increasing distance. The peaked abundance patterns in the gradient found in many species could not be completely explained by findings in the laboratory. The pupal/cocoon weight of defoliator species reared was the smaller, the nearer to the emission source their larval food originated from. A considerable amount of variation in the weight could be explained by the heavy metal concentrations in the needles. In the European pine sawfly, the number of eggs laid was positively correlated with the cocoon weight. However, smaller females produced more viable eggs, which masked the effect of female size on the number of offspring produced. Contaminated food increased the pupal mortality and decreased the developmental rate of the large pine sawfly. Although air pollution affected several essential population parameters of insect pests, the available results cannot be generalized uncritically, since causal relationships are still insufficiently known. All signs suggest that the adverse effects of forest pests increase with pollutant load, which may lead to unexpected problems in forest management.