THE SQUIRMING PESTILENCE

Townspeople in Belefanai, Liberia reported a noise like the sound of heavy rain cascading down through the leaves. It was caterpillar droppings. Millions upon millions of the creatures were writhing around in the trees. In early January, when the long, black caterpillars reached the creeks that serve as the main water sources for the town of Belefanai in north-central Liberia, the creatures' faeces instantly turned streams dark and undrinkable.

Moving through the forest canopy on webs, devouring the leaves as they went, the caterpillars advanced like nothing the townspeople had ever seen. They ate food and cash crops—coffee, cocoa, citrus, plantain, banana, and cassava. They took over homes and people fled. Venturing into the forest meant being hit by a wave of caterpillars that appeared to be moving forward about as fast as the average person walks.

"The worms would drop on you from all angles," said Moses M. Kolinmore, a mason who arrived in Belefanai just as townspeople realized they had to get word to the government. "They would cover the whole ground—thousands upon thousands of thick, strong, stubborn worms. It was fearful, very fearful."

The outbreak, which began in December in a remote, forested region of Guinea just over the border from Belefanai, has affected an estimated half million people in more than a hundred towns and villages, prompting the Liberian government to declare a state of emergency.

This week entomologists identified the insect as a moth with a name only scientists had heard before: Achaea catocaloides. Government officials and international experts are searching for a strategy to control the outbreak, which threatens to spread because first -generation Achaea caterpillars are dropping to the ground and turning into pupae, then moths.

The moths can fly long distances—up to 620 miles (1,000 kilometers) if the winds are right—and then lay eggs that will hatch into still more caterpillars. Each moth not killed will lay 500 to 1,000 eggs and start the cycle again. Johnson Nyelartah, a teacher in Belefanai, said townspeople are bracing for another worm invasion, "It is our fear that these butterflies will multiply into more than we had before," he said.

One cause for optimism: The caterpillars and moths are dry-season pests, and Liberia's dry season should last only another month or so, experts say. "We are hoping the rains will end the outbreak," said J. Qwelibo Subah, director general of Liberia's Central Agricultural Research Institute (CARI), who is coordinating the response to the infestation.

In the interim, the Ministry of Agriculture is scrambling for solutions.

Fifty-five field staff are scouting for concentrations of the caterpillars and moths, spraying them with pesticide from the ground and collecting and burning the pupae in their cocoons. Farmers have been setting slow-burning fires in leaf litter to smoke caterpillars out of trees and incinerate them. Because the caterpillar infests wide areas so rarely, scientists have not researched a way to safely and selectively kill large numbers of Achaea catocaloides.

If all other measures fail, Subah said, he would not rule out aerial spraying, even though UN Food and Agriculture Organization (FAO) officials have advised against the technique, because it would kill many other species and further contaminate open water supplies. "We are considering all options," Subah said. No one is sure what turned Achaea catocaloides—one of the most common moths across central and West Africa—into a nightmarish plague in Liberia's forest.

It could have been the weather, experts say. During the dry season, a strong wind called the harmattan blows from the Sahara across West Africa. Perhaps in late 2008, this wind picked up Achaea catocaloides moths and then deposited a large concentration along the Guinea-Liberia border. Or perhaps there was an unnoticed local increase in Achaea catocaloides numbers and, when the temperature was right, the population exploded, said Gregory Tarplah, an entomologist with Liberia's Ministry of Agriculture.

Whatever happened, once the caterpillars emerged in great numbers they needed an expanded food supply. They turned from eating their favorite food, the Dahoma tree, to eating most any tree in their path. And they began to migrate in search of more.