About 20,000 heads of livestock and 50 families of herdsmen have been forced this week to leave a grassland in Usu, northwest China's Xinjiang Uygur Autonomous Region, after an invasion of unknown worms.

The worms, with a density of up to 3,000 per square meter, consumed the grassland like mowers, leaving only the brown soil behind them, said a Xinhua correspondent at the scene.

Zhang Xishan, an official in charge of grassland management, told Xinhua Friday that the worms have damaged 8,000 hectares of pasture, 4,670 hectares of them totally ruined.

"The way they eat grass is like rolling a carpet," Zhang said.

It was the worst plague of worms over the past three decades in the city that is 280 km west of the regional capital of Urumqi, said Zhang.

However, local experts failed to identify the 2-cm thorny green worm with black stripes. Worm samples were sent to Xinjiang Agricultural University on Friday to find out what they are.

"The pasture was green a week ago. But now the worms are creeping around, and they even come into my house. I have to sweep them out several times an hour," said a herdsman named Bixian.

Bixian said he had never seen the worms before.

Xinjiang has a history of using chickens, ducks and birds to fight locusts, which are also a menace to grassland. However, these fowls showed no interests in the thorny worms in Usu.

The Usu municipal government has announced a grazing ban on affected grassland, which starts Saturday and lasts at least a week.

The Xinjiang regional headquarters of locust and rodent control sent two spray vehicles along with 5 tonnes of pesticide to Usu to fight the worm plague. Each vehicle was spraying pesticide on between 400 and 530 hectares per day and the operation could last 12 days.

Mu Chen, a headquarters official, said the worms might be moth larvae. He suggested the warm winter and plenty of rain this spring might have helped produce the army of worms.

(Xinhua News Agency May 9, 2009)