ODERWOLDE, Netherlands – How far does a mosquito fly? Harry Boerema wants to know.
Boerema lives near a drainage project, where Dutch authorities are dredging a huge meter-deep (3-foot) basin in the northern rural landscape to head off flood waters and protect towns and villages from disaster.
The project threatens to inflict hordes of mosquitoes on people living around the water retention area, so scientists set out to calculate how to keep the boundaries of the ditch far enough from human habitation to protect residents from pest infestation.
The question they needed to find out: How far does a common European human-biting mosquito fly?
What they found surprised them: A hungry female looking for a “host” will fly at least 150 metres (yards), three times farther than previously thought, said Piet Verdonschot, who conducted the research.
The 1,700 hectare (4,200-acre) basin, begun in 2003, is designed to collect heavy rainwater that will slowly be channeled to the North Sea. But frequent wet-dry cycles will be perfect breeding grounds for mosquitoes.
Buzzing pests are nothing new for Boerema, a retired professor of architectural history who has lived for 36 years in his quiet cottage set amid dairy farms.
“I don’t mind them to a certain extent. But not in surplus,” he says. “I’m a nature lover, and mosquitoes are part of nature – although not the most likable ones.”
Not everyone took the prospect of living on water’s edge with such equanimity, and local complaints led authorities to commission the mosquito research, said project manager John Tukker.
At the outset, Verdonschot believed mosquitoes stay within about 50 metres (yards) of their breeding ground. The biggest nuisance for humans often originates in flower pots, buckets of collected rainwater or any kind of water left stagnant in the back garden or barnyard.
“The assumption in the literature is that people who suffer bites have bred their own specimens in their own gardens,” he said.
Hundreds of mosquito species exist around the world – 36 in the Netherlands alone – but Verdonschot concentrated on the two species most common in the Dutch climate: the culex pipiens, which prefers birds to people but will still keep you awake at night during the summer, and the Culiseta annulata, larger, more aggressive insects active year-round. Neither normally carries dangerous diseases.
Verdonschot, an aquatic ecologist working for the private environmental research institute Alterra, hatched 40,000 mosquitoes in large tents in a grassy field. The tents were surrounded by concentric circles of traps set at 50 metres, 100 metres and 150 metres. Around the edges of the field were ditches with tall reeds and wild grasses on the banks.
The traps drew mosquitoes into smoke from dry ice then instantly froze them. At the end of each day researchers collected the corpses and counted them one-by-one, using tweezers under a microscope.
Verdonschot expected most mosquitoes to be caught in the closest traps. Instead, about 80 per cent were found in the farthest, meaning most flew at least 150 metres from the tent where they were hatched.
Verdonschot then refined his experiment, placing evergreen shrubs within the inner circle of traps. The numbers caught in the closest ring of traps shot up by one-third. The bushes offered both shelter from predators and moisture evaporating from the leaves.
That discovery led Tukker, working in the north, to create small raised islands of vegetation in the middle of the retention area, which becomes a swamp after a heavy rain. Those islands deflect mosquitoes from nearby farms.
The experiments produced a few other surprises, too.
Mosquitoes are mostly quiet during the day, preferring to concentrate on the edge of a body of water. When females hunt for blood – necessary for reproduction – they move for about an hour at dusk or at dawn, staying close to the ground.
“They move differently than we thought, they move farther than we thought,” Verdonschot said.
Verdonschot believes his team’s research adds to scientific knowledge about mosquitoes. Tomes have been written about mosquito bites and the effects on human health, but little research has been done on their habits, he said.
Verdonschot’s simple experiments this summer have value for others building catchment areas around Europe and for housing developers.
“The whole northwestern European climate is becoming more dynamic because of climate change, because of wetter summers. And all this urban infrastructure has to be protected from water excess,” he said.
Boerema also has a mosquito trap in the hedge around his cottage, helping to keep track of the mosquito population during wet and dry periods. He is anxious to see the water storage project completed, recalling that he was ordered to evacuate his home during a 1998 flood.
“I think it will ease the danger,” he said, even though he’s likely to have more mosquitoes.
“We’ve always been bitten. I don’t react very much, but my wife hates them – but not to the extent that we find it unbearable to live here.”