Indirect interactions between herbivores may affect natural enemies and plant defense traits involved in the suppression of herbivores. Moderate densities of the aphid Aphis gossypii, an herbivore of cotton (Gossypium hirsutum), do not reduce plant fitness. Aphids induce defenses in plants that reduce subsequent herbivory and are a food source for generalist predators. Thus, aphids may indirectly suppress 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, this effect may be density dependent. At high aphid densities, increases in plant defense compounds could become costly or aphids could directly reduce plant fitness to a greater extent. Natural enemies may also switch to consuming more aphids at high aphid densities, reducing predation of more damaging herbivores. Using a factorial design in greenhouse microcosms, we manipulated aphid density (0, 12, 50, or 100 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. Analyses were conducted on leaf tissue exposed to increasing aphid density treatments for concentrations of trypsin inhibitor, peroxidase and chitinase.
Results/Conclusions
Aphid-induction of plants decreased S. exigua survival, but increasing aphid density did not result in added S. exigua suppression. This was surprising because increasing concentrations of defensive compounds in leaf tissue were found with increasing aphid density. H. convergens also suppressed S. exigua, and combined effects of predation and aphid-induction were additive at moderate aphid densities. Yet, at low aphid densities, predation of S. exigua was low, perhaps because lady beetle predation of aphids is often density dependent. Other experiments conducted by our lab reveal that higher aphid densities (>100 aphids per leaf) may also decrease herbivore suppression and negatively impact plant fitness. Overall our results support a net benefit of aphids to plants via suppression of more damaging herbivores as a result of aphid-induced defenses and apparent competition. However, an optimal aphid density may exist where plant defenses and natural enemies complement one another in the suppression of more damaging herbivores.
