Effects of different types of damage on the chemistry of birch foliage, and the responses of birch feeding insects.

1. We collected insect-grazed, mined, and holepunched leaves of Betula pendula Roth, and assessed their palatability to four species of birch-feeding lepidopteran caterpillars (Apocheima pilosaria D. & S., Erranis defolaria Clerck, Epirrita dilutata D. & S., and Euproctis similis Fuessly) in laboratory preference tests. The palatability of hole-punched leaves of different ages was also determined, using Apocheima pilosaria only. 2. The total phenolic content and protein-precipitating ability of undamaged and all three types of damaged leaves was measured, together with the water content of mined, insect-grazed and undamaged leaves. 3. Only the mined leaves were consistently avoided in the feeding trials; the other sorts of damage were often preferred by the caterpillars, even though phenolic levels increased in all the damaged leaves. The insects appeared either to be indifferent to changes in the protein-precipitating ability of leaves, or actually preferred leaves showing the largest increase. 4. The results show clear qualitative as well as quantitative differences in birch's response to different types of damage. They also show that herbivore preferences depend upon both the damage type and the species of insect being tested. Preferences are difficult or impossible to relate to changes in phenolic levels, or to the protein-precipitating ability of leaves. The possible consequences of these results for 'induced defense' theory are discussed.