Eco-concerns as Guangzhou tackles mosquitoes in dengue fight

"Cherish lives, including those of mosquitoes," Wu Mingliang, chief engineer of Guangzhou Huicheng Pest Control Company, told the Global Times.

With 20 years of experience in controlling mosquito populations, Wu has bemoaned the overuse of pesticides for an outbreak of dengue fever in South China's Guangdong Province in September.

A total of 41,827 dengue cases were reported in Guangdong as of Wednesday, an increase of around 20 times over the previous year. More than 80 percent of all cases were detected in Guangzhou, the provincial capital.

This year's outbreak is believed to be the worst in two decades, the Xinhua News Agency reported on October 6. Six deaths were reported in Guangdong.

Dengue, also referred to as "breakbone fever" for its characteristic symptoms of severe joint and muscle pain, is a mosquito-borne disease.

With mosquito populations reaching five times normal levels, authorities in Guangzhou, the hardest-hit city in the province, have doused the area with mosquitocide and have released mosquito-eating fish into lakes and reservoirs - moves that have triggered grave concerns among experts over their ecological impact.

Something's fishy

The Guangzhou Water Bureau announced on October 10 that mosquitofish, capable of consuming more than 200 larvae a day, would be released in a bid to control the dengue fever outbreak, Xinhua reported on October 16.

Some experts have warned that the fish, a universally acknowledged invasive species, may create some unexpected ecological consequences.

Previously released in Hong Kong and Taiwan, the fish decimated the habitats of the White Cloud Mountain Minnow native to those regions, the Guangzhou Daily reported.

But Guangzhou water authorities explained that mosquitofish have lived in Guangdong waters for decades, assuring that releasing limited numbers will not have a negative impact on the environment.

Over 20 million "native species fish" have recently been introduced to more than 2 million square meters of lakes and reservoirs, Wu Renchu, head of Guangzhou Patriotic Health Campaign Committee Office, told the Global Times. He said the number includes not only mosquitofish, but other mosquito-consuming species.

Nevertheless, Yi Zusheng, an associate professor at the School of Life Sciences at Guangzhou University, argued that "although introduced to China as early as the 1930s, mosquitofish is still an exotic and invasive species that poses a threat to the survival of native aquatic life."

Mosquitofish, which breed rapidly, compete for habitat with native species like White Cloud Mountain Minnow and Chinese rice fish, Yi told the Global Times, which are becoming an increasingly rare sight.

Since mosquitofish already populate the Pearl River Delta, releasing more is unlikely to help curb the spread of dengue fever, Yi said.

The water bureau argued that the release of about 5,000 mosquitofish in city parks in 2006 yielded positive results.

Wu Renchu, however, admitted there was no follow-up conducted after the introduction. Likewise, no projections for the possible impact on the ecology have been calculated this time around.

"In fact, the dace, a native species, is also a great mosquito eater," an unnamed official with Guangzhou's Bureau of Ocean and Fisheries told the Guangzhou Daily.

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.

Pyrethrin, a major organic chemical compound, is a broad-spectrum mosquitocide, killing both mosquitoes and their natural enemies such as dragonflies, spiders and lizards.

A lack of natural predators increases our dependence on mosquitocide, creating a vicious cycle, he said.

While Wu Mingliang encouraged the use of bio-mosquitocide which specifically targets mosquitoes and not their predators, Chen argues only a few have been developed and their effects are "undesirable."

Both Chen and Lu agreed that the fundamental measure to combat mosquitoes is to eliminate ponding, or stagnant waters where mosquitoes breed, through improved infrastructure and city planning.

Chen cited the cavernous sewers in most US cities as an example, which guarantee smooth flow of water and prevent the formation of ponds. China, however, tends to cap its narrower drainage canals with manhole and drain covers to prevent mosquitoes from laying eggs in the subterranean water collections.

Such methods are short-term solutions that fail to eliminate the root of the problem, Chen said.