Host tolerance does not impose selection on natural enemies

Coevolution between hosts and their natural enemies is believed to operate through the evolution of resistance traits. Although the importance of tolerance to natural enemies as an alternative defensive strategy has been recognized, there is still no consensus about the possible role of host tolerance in the evolutionary outcome of the interaction. Here, using bioassay experiments, we tested the hypothesis that variation in host tolerance among selected plant genotypes could impose a selection pressure upon a specialist herbivore. Tolerance did not affect herbivore larvae survival, weight gain, efficiency of food consumption, total food consumption, developmental time and adult mass. These results therefore do not support the hypothesis that host tolerance could affect natural enemy performance. However, resistance did negatively affect herbivore larva survival. Genetic variation in herbivore larva survival was detected, thus suggesting the potential for a coevolutionary response. Our results indicate that host tolerance would reduce opportunities for a coevolutionary response by the natural enemies of the host. Contrary to predictions from previous models, our results suggest that host tolerance may constitute an evolutionarily unstable defensive strategy.