Over 150,000 rampaging wild boars in a province in eastern China are giving sleepless nights to farmers who have been forced to maintain round-the-clock vigil to save their crops.

The boars have trampled standing crops in many counties in Zhejiang province, and the crop yield this year has been reduced by one-third, China Daily reported.

This has resulted in a rise in the number of rats which feed on the fallen grain.

According to a latest survey, the number of wild boars was only 29,000 in 2000. The number jumped to 100,000 in 2006, almost tripling within six years.

In Chun'an county, more than 100,000 residents are working as migrant workers in other places. As fewer people work in the fields in the county, wild boars have acquired a larger living area, an official at the forestry administration office said.

Farmers have now resorted to using traps and electric fences to ward off the boars.

'Hunters are usually allowed to shoot boars during eight months of the year, but with the World Expo taking place in Shanghai, restrictions were placed on guns,' the official said.