About 40 percent of Erhai Lake's pollution came from agricultural activities, according to a 2015 news report by China Central Television.

To tackle the problem, Yunnan province's Dali Bai autonomous prefecture has dispatched 16 teams to towns along the lake since 2017. The main focus of their work is to implement a ban on the cultivation of garlic and promote crops that use less water and fertilizer.

Yang Ping is the leader of the team sent to the town of Dengchuan, located near the source of Erhai. He arrived in the town with the nine members of his team in March last year.

"We are under a lot of pressure because water quality at the source has a large effect on Erhai," he said. "We inspect all four villages every day to find out whether water quality and sewage outlets meet standards."

Yang said it was extremely hard to persuade villagers not to plant garlic.

"Garlic had been the major source of income in the area as the suitable climate there ensured it could be sold for a good price," he said.

The town, listed as a poverty-stricken area for decades, was lifted out poverty in September 2018 thanks to garlic cultivation.

However, the prefecture government said that same month that it planned to completely ban the growing of garlic by the end of this year because its cultivation required large amounts of fertilizer containing nitrogen and phosphorus as well as many pesticides.

Yang said the price of garlic was far higher than that for any replacement crops, so villagers' incomes dropped sharply.

"A farmer I know signed the contract for the garlic ban. Both of his sons were university students. In the past, he could earn about 60,000 yuan ($8,772) a year, but now he only earns about 4,000 yuan," Yang said.

Since March last year, the team has visited about 3,000 households at least seven times.

"In some extreme conditions, villagers even threw us out of their homes and insulted us, saying that we were destroying their livelihood," Yang said. "But if Erhai was polluted, their hometown would be destroyed, let alone their livelihood."

Zhao Bili, a villager from Jiuzhou village in Dengchuan, signed the contract earlier than his neighbors. He said he had noticed the pollution.

"People here like to eat fried grasshoppers. However, I spotted that there was so much fertilizer and pesticide that all the grasshoppers were killed last year," he said. "I've realized since then that the soil and lake pollution was severe."

He had grown garlic on 2,700 square meters of farmland, but in accordance with the contract he switched to growing vegetables including celtuce, resulting in a loss of about 50,000 yuan a year.

"Despite the lower income, I am doing good for Erhai and I have confidence that my life can become better even without garlic," he said.

Forest-based economic activity, such as the picking and selling of flowers, mushrooms and pine nuts or raising bees, has become an alternative to growing garlic, and about 90 percent of villagers have already signed contracts agreeing not to grow it.