Evolution of signal emission by uninfested plants to help nearby infested relatives

Herbivory by arthropods often induces the emission of plant volatiles, which attract natural enemies of the herbivores. This induced emission of volatiles is considered to be a strategy of plants to effectively defend against herbivores by employing bodyguards. Recent empirical research has revealed that these volatiles can also affect neighboring undamaged plants and cause them to emit volatiles secondarily. Provided that signal emission imposes some cost on plants, the evolutionary advantage to undamaged plants in the emission of such secondary signals is unclear. We hypothesized that plants have evolved to emit a secondary signal to help nearby relatives by promoting the recruitment of natural enemies, whereby they increase inclusive fitness. We constructed a simulation model to evaluate this hypothesis. Our simulations suggest that a secondary signal evolves if the following five conditions are met: the cost of the signal is low; the potential risk of infestation is high; the attractiveness of the signal to natural enemies is highly positively correlated with the local density of the signal chemical; dispersal of offspring is spatially restricted, causing population viscosity, and; sufficient vacant space is available, allowing the population to be elastic.