Gypsy moth caterpillar feeding has only a marginal impact on phenolic compounds in old-growth black poplar

Species of the Salicaceae produce phenolic compounds that may function as anti-herbivore defenses. Levels of these compounds have been reported to increase upon herbivory, but only rarely have these changes in phenolics been studied under natural conditions. We profiled the phenolics of old-growth black poplar (Populus nigra L.) and studied the response to gypsy moth (Lymantria dispar L.) herbivory in two separate field experiments. In a first experiment, foliar phenolics of 20 trees were monitored over 4 weeks after caterpillar infestation, and in a second experiment the bark and foliar phenolics of a single tree were measured over a week. Of the major groups of phenolics, salicinoids (phenolic glycosides) showed no short term response to caterpillar feeding, but after 4 weeks they declined up to 40 % in herbivore damaged and adjacent undamaged leaves on the same branch when compared to leaves of control branches. Flavonol glycosides, low molecular weight flavan-3-ols, and condensed tannins were not affected by herbivory in the first experiment. However, in the single-tree experiment, foliar condensed tannins increased by 10-20 % after herbivory, and low molecular weight flavan-3-ols decreased by 10 % in the leaves but increased by 10 % in the bark. Despite 15 % experimental leaf area loss followed by a 5-fold increase in foliar jasmonate defense hormones, we found no evidence for substantial induction of phenolic defense compounds in old growth black poplar trees growing in a native stand. Thus, if phenolics in these trees function as defenses against herbivory, our results suggest that they act mainly as constitutive defenses.