Plant Anti-Insect Armaments: Because individual plants are unable to relocate, they are subject to extreme selection by the insects feeding upon them. One means by which plants suppress herbivory is to produce toxic compounds to deter feeding (see the Perspective by Hare ). Agrawal et al. (p. 113 ) compared pesticide–treated or untreated evening primroses. Over 5 years of pesticide treatment, the production of defensive chemicals in the fruit reduced and flowering times shifted, and the primrose's competitive ability against dandelions improved. Züst et al. (p. 116 ) examined large-scale geographic patterns in a polymorphic chemical defense locus in the model plant Arabidopsis thaliana and found that it is matched by changes in the relative abundance of two specialist aphids. Thus, herbivory has strong and immediate effects on the local genotypic composition of plants and traits associated with herbivore resistance.

Summary: 1. Tannins are one of the most studied groups of plant secondary metabolites in research related to chemical ecology. They are traditionally thought to form an important factor of plant defence against herbivorous insects. 2. For a long time, tannins’ anti‐herbivore activity was thought to derive from their protein precipitation capacity that rendered plant tissues non‐nutritious and unpalatable for herbivores. Recent evidence suggests that tannin activity cannot be explained quite this simply, as tannin oxidation should also be taken into account as a defence mechanism for plants. 3. Tannins show very high variability in their structures with several hundred unique molecules detected in plants. These molecules are unevenly distributed in the plant kingdom and only very seldom—if ever—do two plant species share the same tannin pool. In many cases tannin composition varies even within organs of the same plant species and individual. Still, the overall tannin composition of many plant species is as of yet unknown. 4. Chemical ecology of tannins is challenging due to its multi‐disciplinary nature. To facilitate research on tannins, we must provide ecologists and chemists with methodological and collaborative alternatives that enable the true and holistic investigation of all important questions that may arise from the field. So far this has not been possible with the tannin oxidation hypothesis, since simple and widely usable methods have not been available. 5. The aim of this review is to give a clear but detailed view of the chemical ecology of tannins and the methodology used to study them. In addition, we introduce a new method to estimate the oxidative activity of all types of tannins and other phenolics that might cause oxidative stress to herbivores. Hopefully our arguments and method will lead to clear changes in the approaches we take to tannins and their exciting biological activities, and we will witness a new era of flourishing and productive research in the chemical ecology of tannins.