Scientists are breeding millions of beetles for a battle against a weed that threatens to choke rivers across China and bring one of the country's great engineering achievements, the Three Gorges Dam, to a halt.
Alligator weed is native to South America but was introduced to the Yangtze River in 1937 by the occupying Japanese as a feed crop for their horses. It was not until the 1980s that researchers realised what people living along the rivers knew - the prolific alligator weed was a menace.
After the second world war, it was grown across southern China for animal feed, garden greening and herbal medicine. That growth came at a price. Other crops such as rice could not compete with the weed for nutrition, sunlight and space, and it began to choke the life out of fish and other aquatic species.
Farmers attacked the plant with sickles and bulldozers, while others tried herbicides. Physical and chemical measures were costly, damaging to the environment, and they could not kill the alligator weed.
Engineers said alligator weed could block filters at infrastructure like the Three Gorges Dam, reducing the amount of water to turn the turbines that create electricity and disrupting river traffic.
In the early 1990s, Professor Zhang Guoliang and colleagues at the Chinese Academy of Agricultural Sciences brought alligator weed-eating leaf beetles from the United States after learning of their successful use in California and other states in the south.
They are one of the fastest breeding insects on the planet. A female produces 1,000 eggs in her six-week lifespan, and the scientists planned to use black-and-yellow Agasicles hygrophila to slow the spread of alligator weed.
Once introduced, the beetle adapted well to China and gradually wiped out the weed in warmer areas such as Guangdong and Jiangxi provinces and the Guangxi Zhuang autonomous region.
But in the Yangtze region, where temperatures in winter often drop to below freezing, the beetles and their eggs struggled to make it through the cold, according to scientists on Zhang's team.
Meanwhile, in Sichuan, Hubei, Jiangsu, Zhejiang and other provinces along the biggest river in China and the world's largest artificial reservoir created by the Three Gorges Dam, the alligator weed problem spread and became worse despite the more than 1 billion yuan (US$141 million) spent annually on machines and chemicals to remove it from farms and waterways.
"In the worst period, the weed could cover the surface of an entire lake," one of the scientists said. "Some branches of the Yangtze were blocked, with water spilling over the banks. The sight of it was terrifying."
A decade after the beetles arrived in China, Zhang proposed a bold idea. He suggested building polytunnel greenhouses to give the insects a winter shelter, and when warmer weather returned, new generations of beetles could be released into the wild to feed.
The method was challenging because the beetles ate voraciously and no one had succeeded in cooping a large colony in limited space for such a long time.
The team spent nearly a decade fighting failures and setbacks. "We almost ran out of hope and confidence," the scientist said.
Eventually, they found a way to achieve and maintain a delicate balance between the insect and plant in the greenhouse. Since 2012, 15 beetle farms have been built along the Yangtze. Together, they produce more than 90 million beetles a year.
The insects would be released into an area covering 250 sq km (96 square miles). From there, they would spread out and feed on the alligator weeds in an area five times the size of the release site.
Xiong Jialin, manager of a beetle farm in Yichang, central Hubei province - home to the Three Gorges Dam - said farmers were at first sceptical, if not resistant, to the release of alien beetles.
"They worried the insect would pray on other economic plants," he said.
Scientists tested the beetle's appetite for more than 100 native plants species and found they ate none of them. The farmers were really won over after they saw how the beetles eliminated more than 80 per cent of the alligator weed in their ponds and fields.
According to an official document produced by the Hubei provincial government this year, clearing 1 hectare (2.5 acres) of alligator weed with beetles costs 750 yuan (US$106), a third of the price of chemical sprays and a tenth of the cost of using machines.
Zhang Boting, senior scientist with the China Institute of Water Resources and Hydropower Research in Beijing, said alligator weed was flourishing in the Three Gorges reservoir in large part because of excess nutrients flowing in from waste water generated by towns and cities around the dam.
"Targeting the plant alone would not solve the problem," he said.
In recent years, more than 150 waste water treatment facilities have been built in the region and they have significantly reduced the pollutants entering the environment.
"The insect plays an important role in the fight against alligator weed," Zhang said. "But it is just part of a big, systematic campaign."
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