Carlos Bustos-Segura, Maximilien A. C. Cuny & Betty Benrey
Parasitoids of herbivores almost always kill their host. By negatively affecting herbivores, it is often assumed that parasitoids are beneficial for the plant. However, direct evidence on the positive effects of parasitoids on plant fitness (i.e. seed production) is still scarce. Parasitized herbivores may show altered behavior and physiology that can modify the responses of plants to herbivore damage and in turn, influence other herbivores feeding on the same plant. These are known as indirect plant mediated effects and can also have consequences for plant fitness and for the whole community of herbivores and predators associated with the plant.
We tested the effects of the third trophic level (i.e. parasitoids) on plant fitness by taking into account indirect interactions between multiple herbivores. We exposed lima bean plants to leaf-eating caterpillars that were or were not parasitized by parasitic wasps whose larvae kill the host after several days. Then, we studied their effect on plant damage and seed production. We also tested if the caterpillar treatments affected attack by seed associated insects, including the beetles that feed on seeds and their parasitoids.
We found that when plants were infested with parasitized caterpillars, leaf damage was lower than when infested by unparasitized caterpillars, while seed production was similar to that of undamaged plants. Seeds produced by plants damaged by unparasitized caterpillars were less attacked by seed beetles. Interestingly, parasitoids did not change the effects of caterpillars on seed beetle attack. In addition, we found no differences in the parasitism of seed beetles among treatments.
Thus, globally, the parasitoids of caterpillars improved plant fitness. While they restored seed production, they did not impair the caterpillar-induced defense against seed predators. Our results suggest that parasitoids can be considered as mutualists of plants. They not only kill the herbivores that feed on the plants and reduce plant damage, but by doing so, they also lessen the loss of seeds with improved defenses. This is particularly important in agricultural systems, since pesticide use affects enormously the organisms of the third trophic level, reducing their beneficial effects on plants.