Predators, parasites, pests and the paradox of biological control

When a bird swoops down and grabs a caterpillar devouring your backyard garden, you might view it as a clear victory for natural pest control.

But what if that caterpillar is infected with larvae from a tiny parasitic wasp -- another agent of biological pest control. Who should you root for now, the bird or the wasp?

A new study from University of Michigan researchers suggests that the gardener should cheer for both of them or, more precisely, for the struggle between the predator and the parasite. That kind of competition -- even when it involves one creature killing and eating the other in what ecologists call intraguild predation -- strengthens and stabilizes biological control systems, the U-M scientists found.

"Competition between control agents may actually help suppress pest problems by acting as a system of checks and balances, limiting overexploitation by any one of them," said Theresa Ong, a U-M doctoral candidate and lead author of a paper scheduled for online publication Jan. 20 in the journal Nature Communications.

"Thus the coupling of two unstable systems has the counterintuitive result of creating a stable, more diverse system," Ong said. The co-author of the paper is Ong's faculty adviser, John Vandermeer, of the U-M Department of Ecology and Evolutionary Biology.

Ong and Vandermeer say the findings of their computer-modeling study have potential applications for the control of crop pests, especially in organic farming where synthetic pesticides are not allowed.

"Many traditional farmers and environmentalists subscribe to the popular idea that the natural world offers ecosystem services that contribute to the stability, productivity and sustainability of agriculture," Vandermeer said. "That is in sharp contrast to the more industrial view of a farm as a battlefield on which the enemies of production must be vanquished."

Ong and Vandermeer argue that in order to achieve effective, pesticide-free control in agriculture, "We must do away with reductionist analyses in favor of more holistic approaches that account for the complex nature of ecosystems."

For decades, farmers have attempted to control crop pests by releasing natural enemies of those pests as biological control agents. In the United States, for example, small parasitic wasps, lady beetles, lacewings and damsel bugs have all been released into alfalfa fields to attack alfalfa weevil larvae.

But biological control systems have proven difficult to stabilize. Often, specialized control agents -- which can include various predators, parasites and pathogens -- are too good at their jobs. They eliminate the pests that feed them and eventually die out, leading to a resurgence of the pest.

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Predators, parasites, pests and the paradox of biological control