Aphids as Beneficial Insects? Effects of Aphid-Induced Plant Defenses and Aphid Natural Enemies on Other Cotton Pests

Aphids have direct, negative effects on cotton plants via phloem feeding. Aphids, however, may have the potential to return a net benefit to plants via indirect effects on other pests. Aphids can benefit plants through two primary mechanisms. First, feeding by aphids induces changes in plant chemistry that can make plants less suitable for other, more damaging herbivores. Second, aphid density can increase the abundance of natural enemies that consume other herbivores as well as aphids. We predict that cotton aphids (Aphis gossypii) will have negatively affect other cotton via aphid-induced changes in plant chemistry and aphid-induced changes in the abundance and behavior of predators. We tested these hypotheses by conducting a series of factorial laboratory and field experiments. 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. We also found that aphid feeding increased trypsin inhibitors, polyphenal oxidase, chitanase, and other defensive compounds in cotton plants. We found that oviposition by beet armyworm moths (Spodoptera exigua) was reduced when aphids were present on cotton plants and that growth and survival of beet armyworm caterpillars was reduced on plants with aphids. Ladybeetles and other generalist predators spent more time on plants with aphids than those without in our experiments and this translated into significantly greater predation of beet armyworm caterpillars on plants with aphids. Thus, induced plant defenses and apparent competition (predation by shared predators) can work in concert to yield a net benefit for cotton plants of hosting aphids.