Plant Defenses and Predators: How Aphids Interact with Other Herbivores in Cotton

Indirect interactions between herbivores may affect natural enemies and plant defense traits involved in the control of herbivores. Moderate densities of the cotton aphid Aphis gossypii has little direct effect on plant fitness. These less damaging herbivores induce defenses in plants that reduce subsequent herbivory, and are a food source for generalist predators. Thus, aphids may positively impact the suppression of more damaging herbivores via the combined effect of aphid-induced plant defenses and apparent competition, whereby one herbivore negatively impacts another through a shared natural enemy. However, at high aphid densities, increases in plant defense compounds could become costly and natural enemies may switch to consuming more aphids, which could reduce any benefit of aphids. Plant defense chemical analyses were conducted on leaf tissue exposed to increasing aphid density treatments for concentrations of trypsin inhibitor, peroxidase and chitinase. Using a factorial design, in microcosms in a greenhouse, we manipulated aphid density (0, 50, 200, or 400 aphids per leaf) within the presence or absence of the predatory beetle, Hippodamia convergens. We then recorded the impact of these manipulations on the survival of a more damaging herbivore, Spodoptera exigua. Increasing concentrations of defensive compounds in leaf tissue were found with increasing aphid density, but surprisingly, increasing aphid density alone did not systematically decrease S. exigua survival. H. convergens and aphid-induced plants suppressed S. exigua, and combined effects were additive at moderate aphid densities. Yet, at high aphid density, predation of S. exigua was reduced, perhaps a result of changing predator behavior. Overall our results support a net benefit of aphids to suppress more damaging herbivores via aphid-induced defenses and apparent competition at moderate densities. However, an optimal aphid density threshold may exist for plant defenses and natural enemies to complement one another in the suppression of more damaging herbivores.