Rats, lots of them, have invaded farms around Dongting Lake in South China's Hunan Province. Estimates put the rat army at 2 billion as they continue to scurry their way through 20 counties of the province. The marshy banks of the country's second largest freshwater lake, with fiber-rich reeds to feed on and soft ground to burrow, provide the perfect habitat for the pests.

But rising water levels in the lake caused by flooding in the upper reaches of the Yangtze River have opened a Pandora's Box, forcing armies of the little critters out of their saturated holes on to the banks and into farms, villages and towns. The result has been devastating with the pests destroying crops on more than 1.6 million hectares.

So who do we blame for the sudden explosion in the number of rats? The floods? Or, the rich vegetation and insects they feed on? Alternately, is the pest menace sudden at all? None of these, say experts. Instead, they blame human intervention in the ecology and global warming for the menace.

The rat menace has been a long time coming, says Deng Zhi, a senior researcher with the Academy of Military Medical Sciences. The root cause of this menace is the reclamation of land from the lake. "Reclamation violates the laws of ecology. It not only facilitates floods, but also creates the ideal conditions for rapid rat reproduction."

It's true that about 110,000 hectares of farmland has been re-reclaimed, and given back to the lake. But lax management and lack of preservation has turned that land into marshy swamps.

It's also true that reeds grow on marshy land. But they are not the only vegetation. Marshlands usually have a variety of plants, shrubs and trees. So why do reeds dominate vegetation around Dongting Lake? Because paper mills in nearby counties "grow" them. "The more reeds people grow, the more serious the rat problem and pollution will become," says Yang Hualin, director of the Chinese Association for the Control of Rodents and Sanitary Insects.

Another important factor why the rats have multiplied is the dearth of natural predators, such as owls, cats, weasels and snakes. People have killed most of the owls either because of their medicinal properties - cure for ear diseases - or because they are considered symbols of bad luck. Weasels and cats have been found dead after eating rats, poisoned by farmers to save their cropland. And snakes have ended up on dining tables because some people in the south consider them to be a delicacy. In fact, hundreds of thousands of snakes have ended up in restaurants since the 1980s, says China News Service.

Hunan forestry officials deny that, though.

The solution? Li Bo, an expert with the Institute of Sub-Tropical Agriculture affiliated to the Chinese Academy of Sciences, suggests more predators such as cats be released in the areas.

Other experts have proposed several immediate and long-term solutions, including government measures to prevent rats from breeding before summer. But for that peasants would have to stop growing crops then.

Monitoring stations with an organized and trained workforce should be set up immediately because "prevention is the key to everything", says Shi Dazhao, director of Chinese Agricultural University's laboratory on the prevention and control of rodents.

There could also be a link between the rat menace and global warming. Zhang Zhibin, a renowned expert with the Animal Research Institute of Chinese Academy of Sciences, says: "Less rainfall in the north has speeded up the desertification of the grasslands, giving rise to grass mice. Warm winters, too, are responsible for the boom in rat numbers."

Zhang has warned the local government to be prepared to deal with the outbreak of a disease.

But the Hunan provincial government says it has already taken effective preventive measures. "It's very unlikely for any rat-borne disease to break out," says Chen Xiaochun, deputy director of Hunan's health department.

Local health authorities have been monitoring the situation closely. Results of their observation have been reported daily to the provincial health department and the public. The chances of a disease such as plague breaking out are remote, especially because no human has contracted a rat-borne disease in Hunan since 1944.

But in the long run, coordinated development and supervision at all levels for a national preventive program is a must, says Chen, adding that a disease-prevention fund could be the first step.

(China Daily 07/20/2007 page12)