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Ecological Society of America

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Herbivore-induced plant resistance and apparent competition are two indirect ways herbivores interact. If a less damaging herbivore indirectly suppresses the abundance of a more damaging herbivore via these mechanisms, then plants may ultimately benefit. Changes in herbivore density, however, can dictate the intensity of species interactions and may play a critical role in determining the outcome of plant- and predator-mediated herbivore interactions. We tested the effects of herbivore density on the strength of indirect interactions among phloem-feeding aphids and herbivorous caterpillars and the outcome of these interactions for their shared host plant, cotton (Gossypium hirsutum). We quantified the survival of caterpillars on host plants that were infested with varying densities of aphids in the presence and absence of predators (ladybeetles). We found that aphids induced defensive proteins in cotton plants and that caterpillar survival was negatively affected by induced resistance. Likewise, we found that the presence of aphids increased predation of caterpillars by ladybeetles, but that apparent competition between aphids and caterpillars was density dependent. Ladybeetles consumed relatively high numbers of small caterpillars at low to intermediate aphid densities, but essentially became aphid specialists at high aphid densities. Aphid induced defenses and apparent competition combined such that plant damage by caterpillars was lowest when predators were present at low aphid density (induced resistance + highest level of apparent competition). This suggests that herbivores can benefit plants, but the effect on host plants is mediated by herbivore density. Indirect herbivore-plant mutualisms may increase plant quality, plant fitness, and yield of crop plants and these interactions need to be considered in ecologically based pest management plans. In addition, these interactions likely alter arthropod community structure and natural selection on anti-herbivore defense traits in plants in natural systems.

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