All terrestrial vascular plants probably invest in some form of toxic chemical defence that can be lethal to potential or actual herbivores. However, many insect herbivores are specialists that can handle these toxins with varying degrees of success (Bernays, 1988, 1989; Bernays & Graham, 1988) and some of the most specialized species store or sequester host plant-derived toxins and use them for their own chemical defences against natural enemies at the third trophic level (Rowell-Rahier & Pasteels, 1992). It is these sequestering specialists that pose a paradox for plants because greater investment in toxic chemical defences by a plant may lead to enhanced herbivore fitness and reduced plant fitness.