Bitter problems in ecological feeding experiments: Commercial tannin preparations and common methods for tannin quantifications

In the field of plant–herbivore interactions, research methods where plant secondary metabolites are manipulated are becoming more and more popular. Among the most commonly used is tannic acid. However, recent studies have shown that different tannic acid preparations are not comparable in their tannin structures. While tannic acids are meant to contain only gallotannins (GTs), some commercial preparations compose mainly of more simple galloylglucoses (that have, e.g. much lower protein precipitation capacity than GTs) or even of gallic acid (the hydrolysis product of GTs). Another group of tannins used in feeding trials is condensed tannins (CTs), usually in the form of quebracho tannin. Quebracho, however, contains different CT structures than, e.g. leaves of many deciduous trees. Additionally, when analysed with the common acid-butanol assay for total CTs, quebracho tannins give even 30-fold lower absorbance than the CTs of those deciduous trees. In addition to above problems it has been shown that different tannins can give different response even within the same herbivore species, and that the same tannin structure can cause different response in different herbivores. Below we review these problems, as well as some means to deal with them.