Wound induced defences in plants and their consequences for patterns of insect grazing

Three scales of wound-induced chemical responses in plants are identified: (1) highly localised chemical changes associated with disruption of cell compartmentation; (2) changes induced in cells surrounding the damaged area, forming a kind of halo around the damage, and (3) more widely-dispersed changes which may affect an entire organ, branch or plant. A brief review of the literature reveals that such chemical responses are very widespread in plants, and many of the substances formed are known to affect adversely the growth, development, or reproduction of insects. It is argued that wound-induced changes in plant chemistry represent for insects a powerful selective pressure for the dispersal of grazing. Levels and patterns of invertebrate grazing in a range of herbaceous and deciduous woody plants sampled at the end of the growing seasons were examined. Leaves of many species exhibited a strikingly evident over-dispersion of grazing initiations, and in some cases the arrangement of holes appeared close to regularity. The pattern of damage between leaves was, in most cases, heavily biased towards a large proportion of leaves receiving a low level of grazing. These highly dispersed patterns of grazing damage are consistent with the hypothesis that wound-induced responses play an important role in determining patterns of insect feeding. They have important implications for the expected levels of insect exploitation of host plants and for the advantages to the plant of distributing grazing damage evenly through the canopy.