Pollution suppresses delayed inducible resistance in boreal willow Salix borealis

Abstract

We studied the effects of aerial pollution on the expression of Delayed Inducible Resistance (DIR) against herbivores in the boreal willow Salix borealis, growing naturally around the nickel‐copper smelter at Monchegorsk (NW Russia). In slightly polluted and clean sites, the manual defoliation (simulated insect herbivory) of willows resulted in the development of DIR, which was expressed in the post‐treatment year as (i) a decrease in leaf damage, (ii) the dispersion of grazing damage within a plant, and (iii) a decrease in foliar quality, detected by the adverse effects on the larval relative growth rate of the autumnal moth, Epirrita autumnata. At the same time, in heavily and moderately polluted sites these effects were not observed, suggesting that DIR in plants growing under pollution impact was not expressed. Since DIR may be one of the important feedbacks regulating the population dynamics of herbivores, the disturbance of this feedback by pollution‐induced environmental deterioration may contribute to the high densities of many herbivorous insects in polluted areas.