ADDIS ABABA, Ethiopia (AP) — The most serious outbreak of locusts in 25 years is spreading across East Africa and posing an unprecedented threat to food security in some of the worldís most vulnerable countries, authorities say. Unusual climate conditions are partly to blame.

The locust swarms hang like shimmering dark clouds on the horizon in some places. Roughly the length of a finger, the insects fly together by the millions and are devouring crops and forcing people in some areas to bodily wade through them. Near the Kenyan town of Isiolo on Thursday, one young camel herder swung a stick at them, with little effect. Others tried to shout them away.

An “extremely dangerous increase” in locust swarm activity has been reported in Kenya, the East African regional body reported this week. One swarm measured 37 miles long by 25 miles wide in the country’s northeast, the Intergovernmental Authority on Development said in a statement.

“A typical desert locust swarm can contain up to 150 million locusts per square kilometer,” it said. “Swarms migrate with the wind and can cover 100 to 150 kilometers in a day. An average swarm can destroy as much food crops in a day as is sufficient to feed 2,500 people.”