Depression of a Coccinellid Functional Response to an Aphid by Sequestered Plant Chemical Defenses
Blaauw, Brett R.
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The three trophic levels of plants, herbivores, and predators have complex effects on one another. Both "bottom/up" processes generated by plant chemical defenses and "top/down" effects mediated by predator foraging behaviors are important and herbivores act as the conduit for variable dynamics among these interactions between defense and foraging. For example, some specialized herbivores, such as oleander aphids, Aphis nerii, are able to sequester plant defense chemicals for their own defense against predators. A. nerii sequesters cardenolides that are produced by the milkweed host plants and use these chemicals in their own defense against predators, such as coccinellids. I investigated whether aphids possess the ability to defend against coccinellid predators when feeding from the milkweed Asclepias curassavica that contains inducible levels of cardenolides. To answer this question, I observed the functional response of Harmonia axyridis, know as the Asian lady bird beetle, as it interacted with two different aphid species: the ardenolide-sequestering A. nerii and Myzus persicae taken from a cardenolide-deficient plant as a control. A. nerii showed significant defense against the predator through the expression of a depressed type II predator functional response in comparison to that generated by the palatable M. persicae. Handling time of A. nerii was longer, and H. axyridis ate fewer of these aphids, further indicating that the sequestered chemical defense of A. nerii is effective against aphidophagous predators.