DIVERSITY OF STRUCTURE AND ANTIHERBIVORE ACTIVITY IN CONDENSED TANNINS

We characterized the structure of condensed tannins from 16 woody plan t species (seven genera, six families) and determined their effects on six herbivorous insect species (four genera, two families). There wer e major differences in tannin structure, even between congeneric plant species. Condensed tannins differed markedly in their antiherbivore a ctivity, averaged over these herbivores, and the herbivores differed i n their sensitivity, averaged over these tannins. Furthermore, the sam e tannin can have different effects on different herbivores, presumabl y because of interactions between tannin structure and gut physiology. Results challenge the view that tannins provide an evolutionarily sta ble plant defense because of their uniform chemical properties. Conden sed tannin can sometimes impact herbivore fitness through effects on s urvival and growth, but the largest effects in 45 insect-tannin combin ations were less than that of many other plant metabolites at lower do ses. Even at high doses, condensed tannins frequently had no strong an tiherbivore activity, even against insects with no evolutionary histor y of encountering the tannin (<10% reduction in growth rate in 24 of 4 5 experiments). Most condensed tannins apparently do not have broad-sp ectrum 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 condense d tannins.