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Coexistence of Plant and Insect Species Image 7
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Coexistence of Plant and Insect Species (Image 7)

July 11, 2011
A predatory ladybug (Hippodamia variegata) feeds on an aphid (Aphis nerii), while resting on a milkweed plant (Asclepias spp.). To take maximum advantage of predators, plants have evolved chemical signals to "call in" these insect-eating bodyguards. Scientists are studying these organisms as part of their research on the community and evolutionary ecology of interactions between the milkweed plant and the insect herbivores that prey upon it. [Image 7 of 7 related images. Back to Image 1.]

More about this Image Kailen Mooney, an assistant professor of ecology and evolutionary biology at the University of California, Irvine, and colleagues Anurag Agrawal, Rayko Halitschke and Andre Kessler of Cornell University, examined 16 species of milkweed, a flowering plant found throughout the Western Hemisphere. Specifically, they wanted to determine the relationship among plant growth; the methods by which plants defend themselves against plant-eaters (ex., thorns and toxins); and what type of protection plants receive from predator insects that eat the herbivore insects. Herbivores, such as aphids (which Mooney used in her study), damage plants, while predator insects, such as ladybugs, help the plants by eating the herbivores.

The researchers wondered if plants were capable of thriving, while having to defend themselves against herbivores, and at the same time, still attract protection from ladybugs and other predator species.

Milkweed species that grow quickly are more vulnerable to herbivore insects eating them because of poor defenses. This makes them more dependent on predator insects for survival. Basically, milkweed falls into one of two categories: hard-to-eat, slow-growing plants that don't need the help of predator insects; or prone-to-attack, fast-growing plants that rely on the help of predators.

"We can breed plants to grow rapidly, but it appears that when we do, we're weakening the plants' immunity to herbivores, rendering them more needful of protection from potentially unreliable predators," says Mooney. "Milkweed has been evolving for as many as 20 million years. Natural selection favors faster-growing plants and those that easily fight off insects. If nature hasn't found a way to combine these traits, perhaps it's something that cannot be done."