Aphids have strong direct and indirect effects on many members of natural and agricultural food webs. Aphid density has direct negative affects on plants by phloem feeding. However, through indirect interactions, aphids have the potential to return a net benefit to plants. Aphids can benefit plants through two primary mechanisms. First, phloem feeding by aphids induces changes in plant chemistry which can make plants less suitable for other, more damaging herbivores. Second, aphid density can increase the abundance and foraging efficiency of natural enemies with negative consequences for herbivores on the same plant. We predict that cotton aphids (Aphis gossypii) will promote indirect effects on the diverse arthropod community of cotton (Gossypium hirsutum) via aphid-induced changes in plant chemistry and aphid-induced changes in the abundance and behavior of predators. We further predict that the net effect of cotton aphids on cotton yield will be positive. These hypotheses were tested by conducting a series of factorial laboratory and field experiments.
Results/Conclusions
In laboratory experiments we demonstrate that aphids increase the density of pigment glands on cotton leaves. These glands contain the terpenoid aldehyde gossypol which is toxic to caterpillars. As such, oviposition by beet armyworm moths (Spodoptera exigua) is reduced when aphids are present. Further, growth and survival of beet armyworm caterpillars is reduced as aphid density increases. Cotton aphids also have negative indirect effects on beet armyworm caterpillars via shared natural enemies. Ladybeetles and other generalist predators spend more time on plants with aphids than those without. This translates into significantly greater predation of beet armyworm caterpillars. Finally, the results of a factorial field cage experiment indicate that reduced caterpillar load on plants and increased predator abundance results in synergy between induced defenses and apparent competition. Ultimately this synergy reduces caterpillar abundance and damage to levels below what would be predicted based on each mechanism alone. This research describes a novel set of interactions that alter the strength of trophic cascades thus promoting a positive plant-herbivore interaction. In this system, induced plant defenses and apparent competition work in concert to yield a net benefit for cotton plants of hosting aphids.