Birch leaves as a resource for herbivores: Seasonal occurrence of increased resistance in foliage after mechanical damage of adjacent leaves.

Seasonal occurrence of such wound-induced reaction in birch foliage which deteriorates the quality of nearby leaves for herbivores was tested by means of bioassays. Length of the larval period was protracted in two early and mid-summer (larval period!) lepidopteran species as well as in two mid-summer hymenopteran species when larvae were reared on birch leaves whose adjacent leaves had earlier been damaged mechanically. This response was not found for two late-summer hymenopteran species. In a lepidopteran species whose larval period lasts through the whole season, retardation in growth was significant in the beginning of August but notl later. Hence such response of leaves, interpreted as defensive on the part of the birch, was not efficient after leaves had gained their final size. The potential consequences of wound-induced responses of leaves for herbivores are discussed.