Silicon as a plant defence against insect herbivory: response to Massey, Ennos and Hartley

Silicon (Si) has received increased attention as a nutrient capable of providing some measure of defence for plants against fungal pathogens, and insect and mammalian herbivores. On the basis of a study including two generalist insect folivores and a phloem feeder, Massey, Ennos & Hartley (2006; Journal of Animal Ecology, 75, 595-603) have drawn attention to a putative distinction between the effects of plant Si in defending against folivorous and phloem-feeding insects. On the basis of their results they imply that phloem feeders are less likely to be adversely affected by increased plant Si than folivores. However, in making this suggestion, Massey et al. have ignored many previous studies demonstrating a clear effect of plant Si on a range of phloem-feeding and some xylem-feeding insects, and that this effect stems not only from leaf mechanical properties based on opaline silica, but also from induced chemical defences seemingly mediated by soluble Si. Furthermore, Massey et al. cannot claim that their study was the first demonstration of a direct effect of Si on insect herbivore preference and performance; there have been numerous earlier studies demonstrating this from folivores, stem borers, and phloem and xylem feeders. We contend that current evidence indicates that Si is likely to be involved to a similar extent in enhancing resistance to all four insect feeding guilds and that any conclusion to the contrary is, at this stage, premature.