EVOLUTION OF SIGNAL PERSISTENCE UNDER PREDATOR EXPLOITATION

We investigated the problem of the optimal persistence of a chemical s ignal as an evolutionary game between conspecifics when such signals c an be exploited by natural enemies. We used a plant-herbivore-parasito id 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 op timal decay rates of marking pheromones. As long as the mark had no as sociated costs, the population evolved towards the lowest possible dec ay 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 incr easing levels of parasitoid attack. However, the degree of change depe nded on competition for, and the probability of re-encounters with, ex ploited oviposition sites. Additionally, we tested the effect of the t ime of onset of parasitoid attack (representing early instar to late i nstar parasitoids) on the evolution of optimal decay rates and found d ecay rates to decrease in a non-linear fashion with increasing time la g between herbivore egg deposition and parasitoid attack. We discuss o ur findings with respect to chemical communication and signals in gene ral that might be exploited by natural enemies, and stress the importa nce of an evolutionary ecological perspective towards an understanding of signal quality.