"Mosquitoes can affect athletes' performance and disturb their rest."

Workers also put a foreign-brand pesticide, called "Anbei" in translated Chinese, in pools and waterways surrounding stadiums. It was recommended by the World Health Organization (WHO).

It could immediately kill mosquito larva bred in water and was safe to both fish and humans. The effect could last 15 to 20 days.

Zeng said the humid environment, affecting both the water and grass, in the public area surrounding the stadium was suitable for pests to live.

"The stadium was built on a previously-deserted place or farmland, which could breed pests more easily than age-old residential areas."

Geng Yanling, an official in charge of the environment and landscape of the Olympic Park, said the water area of 180,000 square meters and greenbelt of 500,000 square meters and sewers within the Olympic Park, adjacent to the Bird's Nest and the Water Cube, the National Swimming Center, would receive a large scale pests extermination.

The Four Harms can spread diseases such as malaria and plague and the West Nile Virus. Zeng said Beijing's major mosquito type was different from that which caused the West Nile Virus outbreak in the United States in 1999, though they were both culicine mosquitoes.

"Mosquito diseases are few in Beijing. So far, we have not found any West Nile Virus in Beijing's mosquitoes, according to monitoring results.Most worries come from annoyance and sting from the pesky insects."

The government has made a control plan of the Four Harms. Olympic stadiums and fields have to meet the highest standards in pests control. The Beijing CDC Center has employed special pest clean-up companies, ranked class-A in an official certification, for pest-killing in the venues.

One of the Four Harms, rats, was a headache for the Olympic host as their chewing of cable lines in venues had resulted in massive power failures over the Games' history. Rat's teeth, which grow all the time, are hard enough to destroy building materials. But Zeng was not worried about that, saying time-old remedies such as using rodenticide proved effective.