First came the floods. The waters swamped bean and corn fields and created a breeding ground for a swarm of desert locusts the size of Manhattan that fanned out and destroyed a swath of farmland across eight East African nations as large as Oklahoma earlier this year.

Now their offspring are threatening a historic infestation—a second wave of locusts, 20 times as large as the first, that the U.N. warns could chew their way through 2 million square miles of pastureland, farms and gardens, around half the size of Western Europe.

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