Should All Plants Recruit Bodyguards? Conditions for a Polymorphic ESS of Synomone Production in Plants

Recent experiments showed that plants start releasing a specific set of volatile chemicals after being attacked by phytophagous arthropods and that these chemicals are used by predators (including parasitoids) to find phytophages. In this way plants exploit the natural enemies as part of their battery of defence. The evolution of this defence strategy is not a simple, self-explanatory process, for two reasons. Firstly, the odour cannot be produced without some costs and even when released in trace quantities its volatile nature causes the benefit to be of short duration so that overall costs may be considerable. Secondly, plants that do not produce odour in response to attack by phytophages, save energy to be spent in other ways of improving fitness and they may gain the same advantage of attracting predators with higher chances when settled close to an odour-producing plant. Based on these realistic and general features a Nicholson-Bailey type model of tritrophic interactions is constructed. Analysis of the model showed that under a wide range of conditions plant populations are expected to be polymorphic, when the costs of producing odour may be offset by saving the energy for other fitness-related purposes and by the probability of settlement close to an odour producing plant. These predictions have far-reaching consequences for experiments on breeding for plant resistance and for olfactometer studies of searching behaviour of predators.