Evolution of signal persistence under predator exploitation

Abstract

We investigated the problem of the optimal persistence of a chemical signal as an evolutionary game between conspecifics when such signals can be exploited by natural enemies. We used a plant-herbivore-parasitoid system as a model to determine the optimal decay rate of a contact pheromone used by a herbivore to signal the presence of an herbivore’s egg to the marking female and conspecifics. These pheromones are also exploited by a specialized parasitoid of the herbivore offspring. We used a series of models with increasing complexity to elucidate the optimal decay rates of marking pheromones. As long as the mark had no associated costs, the population evolved towards the lowest possible decay rates, independent of herbivore oviposition response towards marked sites. When marking behaviour had some associated cost, increasingly higher decay rates evolved with physiological cost of marking and increasing levels of parasitoid attack. However, the degree of change depended on competition for, and the probability of re-encounters with, exploited oviposition sites. Additionally, we tested the effect of the time of onset of parasitoid attack (representing early instar to late instar parasitoids) on the evolution of optimal decay rates and found decay rates to decrease in a non-linear fashion with increasing time lag between herbivore egg deposition and parasitoid attack. We discuss our findings with respect to chemical communication and signals in general that might be exploited by natural enemies, and stress the importance of an evolutionary ecological perspective towards an understanding of signal quality.