CRI听力:How Far Can Mosquito Fly

In Olde in the Netherlands, authorities are dredging a huge bowl in the northern rural part of the country to head off floodwaters and protect towns and villages from disaster.

The project threatens to inflict hordes of mosquitoes from attacking people who live nearby the 1,700-hectare water retention area where heavy rainwater collects and is slowly channeled into the North Sea. The frequent wet-dry cycles are perfect breeding grounds for mosquitoes.

This is why authorities have set out to determine where they should build low dikes to hem in the meter-deep water containment area. They asked scientists to determine how far the mosquitoes should be from human habitation. In other words, they wanted to find out how far a common European human-biting mosquito could fly from its breeding ground.

Piet Verdonschot, who conducted the research, says the scientists' finding surprised them: A hungry female looking for a "host" will fly at least three times farther than the distance that researchers previously believed.

"The flight distance of the mosquito is much greater than what we expected. We thought that mosquitoes could fly between 50 and 100 meters (165 and 330 feet) maximum and not spread widely. The first result shows us that the biggest part of the mosquito population has flown more than 160 meters. Smaller populations managed to fly even 250 meters. In our area of experiment and in the open space, we concluded that mosquitoes could fly much farther than we expected."

Verdonschot, an aquatic ecologist who works for the private research company Alterra, hatched 40,000 mosquitoes in large tents in a grassy field. The tents were surrounded by concentric circles of traps every 50, 100 and 150 meters. Ditches with tall reeds and wild grasses on the banks marked the edges of the field.

The traps drew mosquitoes into a plume of dry ice and instantly froze them. At the end of each day, Verdonschot and his assistants collected the remains and counted them one-by-one, using tweezers under a microscope.

Verdonschot says he expected most mosquitoes to be caught in the closest traps. Instead, the scientists found about 80 percent in the farthest traps.

Verdonschot then refined his experiment, placing evergreen shrubs within the inner circle of traps. The number of mosquitoes caught in the closest traps rose by one-third. The bushes offered them both shelter from predators and moisture evaporating from the leaves.

"We also concluded that a good way to protect (people) from mosquitoes is to create small islands of raised vegetation that attracts mosquitoes, which offers them shelter and moisture evaporating from the leaves. Most of them will stay, and some of them will fly away. The more mosquitoes stay on such islands, the less hindrance there will be for people."

Verdonschot's simple experiments have value for others building catchment areas around Europe and for housing developers.

He says the entire northwestern European climate is becoming more dynamic because of climate change and wetter summers. And the urban infrastructure must be protected from water excess and pesky mosquitoes.

For CRI, I am Li Dong.