Predators Maintain the Balance

When the same kind of animal comes in different colors, the reason is usually the environment. But red and green pea aphids live side by side in the same alfalfa fields. Why doesn't natural selection weed out one of the hues? According to a report in this week's issue of Nature, it's because the aphids' two major predators have different tastes.

To figure out why both red and green pea aphids survive, John Losey, an entomologist at the University of Wisconsin, Madison, looked at how the two main enemies of pea aphids--ladybugs and wasps--affect the population. Last summer, Losey and his colleagues sampled 100 alfalfa stems in eight similar locations every 6 days. They counted the number of red and green aphids, ladybugs, and the mummified remains of aphids parasitized by wasp larvae. They discovered that the predators had distinct preferences--ladybugs being partial to red aphids, and wasps to green aphids.

The result is that the two predators keep the colors balanced, says Losey. With ladybugs eating the red aphids and wasp larvae parasitizing the green, neither form dominates the population and both survive. Losey says the finding supports the idea of conserving whole ecosystems--including predators--rather than just single species. David Pimentel, an entomologist at Cornell University, agrees. "There is a balance between parasites and their host and predators and prey," he says. "They keep each other from destroying the ecosystem."