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Choking on chemicals

When cases of dengue infection reached 20,000 on October 4, the Guangzhou government issued a circular to mobilize all schools, subdistricts and communities to join the fight by spraying mosquitocide.

According to Wu Renchu's estimates, some 12 tons of the chemical spray are used in the Guangzhou metropolitan area on a daily basis.

One community in South China Normal University has employed three full-time sprayers to hit known breeding areas, such as manhole covers, sewers and garages, the Guangzhou-based Nanfang Daily reported October 5.

Even on October 25, when for the first time less than 200 new cases were reported for a single day in October, about 19.5 tons of mosquitocide were sprayed, the New Express reported.

"But mosquito density at more than half of all observation points in Guangzhou is still high and not meeting sanitary standards," said Wu Mingliang.

Although Guangzhou has sprayed an average 1,000 tons of chemical insect killer every year for 30 years, it has been increasingly difficult to control the mosquito population, he said, adding that dosages are estimated to be 100 times more than necessary.

As a result, mosquitoes are beginning to develop a resistance to the chemicals, he explained.

Such large doses not only make the spray less effective, but also temporarily contaminate the water, soil and air, Lu Yongyue, an entomology professor at South China Agricultural University, told the Global Times.

But Chen Xiaoguang, deputy head of the School of Public Health and Tropical Medicine at Southern Medical University, said that there is no solid evidence showing the mosquito vector in Guangzhou has generated a significant resistance to insecticide.

Furthermore, there are no specific standards to assess whether mosquitocides are being overused, Chen added.

Wu Mingliang also points out the largely diminished number of natural predators of mosquitoes as a result.