CHANGSHA, July 9 (Xinhua) -- An estimated 2 billion field mice are chomping their way hungrily through crops in 22 counties around the Dongting Lake in central China's Hunan Province after their homes on islands in the lake were flooded.

    Local authorities in Yiyang, Yuanjiang, Junshan and Huarong are rushing to build walls and dig ditches to keep the mice away from flood-control dikes and cropland.

    The mice burrow through the dikes and spread out into cropland, eating crops and posing a threat to human health. They could contribute to flooding because it is the rainy season in the area, said experts with the Hunan provincial office of plant protection and quarantine.

    In Yiyang, to the north of Dongting Lake, a long wall and a 30-60 cm deep ditch have been built on the lakeside to resist the mice invasion. On Monday the ditch was alive with mice, and a thick, dark wedge of mice jostled and heaved at the foot of the wall.

    Local residents used clubs and shovels to kill the mice by the score. Some lowered down fishing nets to catch the mice alive, catching several kilograms of mice at each attempt.

    More than 2.25 million mice -- about 90 tons of rodents -- have been killed since June 21, local authorities said.

    The massive invasion of field mice began on June 23 when the Yangtze River flooded, raising the water level in Dongting Lake and submerging mouse holes on islands in the lake, said Hunan plant protection experts.

    In Yuanjiang, Junshan and Huarong, where local people were slower to build walls and ditches, mice have already damaged flood-control dikes and ruined crops.

    The mice threat is not over with more floods forecast in the upper reaches of the Yangtze River and Dongting Lake, said experts.