Levels of damage of Scots pine and Norway spruce caused by needle miners along a SO2gradient

Needle damages caused by mining insects on Scots pine and Norway spruce were studied in the vicinity of a pulp mill. The abundance of needles mined by the pine bud moth Exoteleia dodecella (Lepidoptera: Gelechiidae) on Scots pine Pinus sylvestris and the spruce needle miner Epinotia tedella (Lepidoptera: Tortricidae) on Norway spruce Picea ahies had a significantly peaked response curve on logarithmic distance scale. For pine bud moth, the maximum expected population density was estimated to be at the distance of 1.35 km from the factory. The maximum expected population density for the spruce needle miner was at the distance of 1.53 km from the factory. However, for both species the curves were significantly different among transects. Both species had a peaked and significant response to sulphur level in needles as well. The maximum expected density in pine was at 1270 ppm, and in spruce at 1070 ppm sulphur concentration in pine needles. The results are consistent with earlier reports demonstrating that these mining insects frequently attack trees suffering from air pollution. The nonlinear response of both species to distance from the pulp mill suggests that E. dodecella on pine and E. tedella on spruce are rather indicators of the zone of intermediate air pollution than of strongly polluted or nearly unpolluted sites. This also agrees with the plant stress-insect performance hypothesis indicating that insect response varies with the magnitude of stress, and at very high stress levels a tree no longer provides the insects with relevant food.