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A bizarre swarm of caterpillars is munching its way through the forests, cocoa and coffee fields of Liberia, threatening crops and forcing thousands to leave their homes because the bugs have contaminated the drinking water from rivers and lakes.

Entomologists have identified the pests as a moth usually found in the forests of West Africa, but normally not in the huge numbers that appeared early last month in Bong County, a lush northern region of Liberia that produces cash crops on large plantations, according to the Food and Agriculture Organization of the United Nations.

The infestation threatens crucial crops in a patch of West Africa that includes several impoverished, war-torn countries. The caterpillars have already been found in Guinea, and could appear in Ivory Coast, the world’s top cocoa producer, which borders the region of the outbreak in Liberia, and Sierra Leone, U.N. officials said.

The outbreak was first reported in mid-January, when the black wormlike creatures set upon farms in northern Liberia.

The caterpillars seemed to appear out of thin air, said Winfred Hammond, representative of the Food and Agriculture Organization office in Liberia. “They were eating the forest trees, vegetation around homes and crops,” he said. “It caused a lot of panic. People were fleeing their homes.”

Initially scientists thought that the caterpillars were army worms, which are hard to differentiate from the current pest when in the caterpillar form but look different when they turn into a moth. Army worms are dreaded because they eat food crops and bury their larvae deep under the soil, which makes them tougher to attack with pesticides, U.N. officials said.

But further testing showed that the caterpillars were actually Achaea catocaloides rena, a species that only rarely appears in such great numbers. The caterpillars live in the forest, and their population is usually kept down by wasps that lay eggs on the moth’s cocoon and eat the caterpillars, Hammond said.

But the rains last year were unusual. Downpours as late as Christmas may have interrupted the reproduction cycle of the wasps that prey on the caterpillars, Hammond said.

The caterpillars eat the leaves of plants, and then the moths burrow into fruit, causing it to rot. When the caterpillars run out of leaves, they set upon food crops like millet and wheat.