DIVERSITY OF STRUCTURE AND ANTIHERBIVORE ACTIVITY IN CONDENSED TANNINS

Abstract

We characterized the structure of condensed tannins from 16 woody plant species (seven genera, six families) and determined their effects on six herbivorous insect species (four genera, two families). There were major differences in tannin structure, even between congeneric plant species. Condensed tannins differed markedly in their antiherbivore activity, averaged over these herbivores, and the herbivores differed in their sensitivity, averaged over these tannins. Furthermore, the same tannin can have different effects on different herbivores, presumably because of interactions between tannin structure and gut physiology. Results challenge the view that tannins provide an evolutionarily stable plant defense because of their uniform chemical properties. Condensed tannin can sometimes impact herbivore fitness through effects on survival and growth, but the largest effects in 45 insect–tannin combinations were less than that of many other plant metabolites at lower doses. Even at high doses, condensed tannins frequently had no strong antiherbivore activity, even against insects with no evolutionary history of encountering the tannin (<10% reduction in growth rate in 24 of 45 experiments). Most condensed tannins apparently do not have broad-spectrum antiherbivore activity. We doubt that selective pressures from folivorous insects can be the main explanation for the diversion of so much carbon, in so many plant species, into the synthesis of condensed tannins.