Can Global Warming Cause Caterpillar Outbreaks?

John Roach
for National Geographic News

November 16, 2005

Get ready—the killer caterpillars are coming.

As the weather gets wackier in response to rapid global warming, parasitism against caterpillars will decrease, biologists warn in a new study. This will free the caterpillars to devour agricultural fields and strip leafy forests bare.

"That's really of great concern, especially to agricultural systems where we rely on several of these parasites. They keep outbreaks from occurring," said Lee Dyer, a biologist at Tulane University in New Orleans, Louisiana.

Climate scientists have long predicted that variable and unpredictable weather—for example, intense storms one year, drought the next—will increase as the Earth warms.

How that variability will affect ecosystem functions, like the checks and balances in the food chain, has proven difficult to test.

In search of an answer, Dyer and his colleagues examined data on parasitism in caterpillars from 15 caterpillar-rearing programs in different regions.

Some of the programs are in regions with a highly variable climate, while others are in regions that experience the same weather year round.

The biologists found that parasitism, the greatest control for insect pests such as caterpillars, decreases as climate variability increases.

Dyer and his colleagues report their finding in an upcoming edition of the Proceedings of the National Academy of Sciences. An online early edition of the paper was posted Monday on the journal's Web site.

Caterpillar Killers

Terry Root, an ecologist at Stanford University in California who was not part of the research team, said the study issues a serious warning about the vulnerability of human food supplies to global warming.

"The work [my research group has] been doing shows how some of the bird predators are moving north but the caterpillars aren't," she said

"So if you have parasites [disrupted] and the higher predators like birds moving out, then the unchecked increase in the caterpillar population could be quite devastating [to crops]."

"So what you need to do then is resort to pesticides, which is expensive, which is hard on the environment, which is hard on people," Root added.

Certain types of parasitic insects, most commonly flies and wasps, thrive on other insect hosts. These so-called parasitoids develop from egg to adult by feeding on the internal tissues and fluids of the host creature.

"Parasitoids are the most important source of biological mortality for most caterpillars," Dyer said. "Most people think of birds eating caterpillars, but they are nothing compared to the amount of death inflicted by wasps and flies."

To determine the level of parasitism against caterpillars, researchers with the caterpillar-rearing programs collected young caterpillars, placed them in captivity, and watched.

If the caterpillar had been parasitized, a fly or wasp would emerge. If the caterpillar was parasitoid free, it would become a moth or butterfly.

Dyer and his colleagues scoured the records on annual parasitism levels at each of the sites and compared them with annual rainfall records. Regions with greater variation in precipitation had less parasitism.

The finding is particularly strong when only parasitoid wasps are included in the analysis, the researchers note.

Parasitoid flies are generalists—when one insect host is unavailable, they can readily switch to another host.

Parasitoid wasps, however, are specialized—they depend on a specific species of caterpillar for their survival. If a caterpillar host is not around when a wasp is ready to lay eggs, the parasite is out of luck.

Out of the Loop

Dyer and his colleagues are uncertain as to the exact mechanisms that cause parasitism in caterpillars to decrease as climate variability increases.

But they suspect the shifting weather patterns make it harder for specialized parasitoids to track their hosts.

For example, if a particularly wet, warm winter causes leaves to unfold on trees earlier than usual, the caterpillars could hatch early and the parasitoid wasps will be out of the loop. With no wasps to keep the caterpillars in check, an outbreak can occur, Dyer explained.

"The thing that's really important in the whole paper is that it shows how a predator-prey interaction can be disrupted," Root said. "And there's a high probability that the disruptions will continue all over as the globe continues to warm rapidly."

Future studies and detailed monitoring will look more closely at the link between decreased parasitism and increased climate variability. Researchers also hope to refine their forecasts for regional precipitation patterns.

"The results of such detailed monitoring are likely to provide additional incentives to slow our anthropogenic contributions to global climate change," the biologists conclude in their paper.

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