It's not just the vagaries of stifling heat and unexpectedly torrential rains that have Canadians on edge this summer. The mosquitoes have been popping up in odd places, too.

West Nile virus

Municipalities and public health authorities who track mosquitoes distinguish between those that merely bite, known as nuisance mosquitoes, and those that have the potential to spread diseases such as West Nile virus  and eastern equine encephalitis virus (EEEV).

According to the Public Health Agency of Canada, fewer than one per cent of mosquitoes in any given part of the country are infected with West Nile virus. There has only been one human case of the virus reported in Canada this year — contracted by a resident of Ontario while travelling in India and China. The virus, however, has been found in five mosquito pools since the start of the season — all of them in Ontario. 

The first human case of West Nile reported in Canada was in 2002 and, to date, most infections in Canada have occurred in August. No locally acquired human or animal cases of EEEV have ever been reported.

Normally bug-infested Winnipeg was bracing for an onslaught because of the extensive flooding in the spring, which many assumed would boost the mosquito population. But the skeeters never showed.

Meanwhile in Edmonton, the local CFL team was forced to move its football practice indoors to avoid the pesky bloodsuckers , which hadn't been seen in that part of Alberta in significant number for years.

Heavy snowfall this winter and a wet spring and summer have provided ideal habitat conditions for mosquito populations in some parts of the country.

"This is definitely the largest number of mosquitoes we've seen in many years here in Edmonton," said Mike Jenkins, a biological sciences technician in the Alberta capital.

At its peak, in the week of July 12-18, the average number of female nuisance mosquitoes caught in one of Edmonton's four light traps, the instruments used to measure mosquito populations, was 1,124. That was the week the Edmonton Eskimos moved their football practice at Commonwealth Stadium from the field to the field house because the bugs were proving to be too much of a distraction to the players.

By comparison, the average trap count over the last decade for the same seven-day period was 82, the 20-year average was 198, and the average count in 2010 was a mere 4.5.

Since that week, the trap count has gone down and is now much closer to the 20-year average.

The mosquito problem was a novel phenomenon for a city that had gotten used to its dry summers, says Doug Costigan, the director of forestry and environmental services for Edmonton's River Valley, a large stretch of urban parkland along the North Saskatchewan River.


Edmonton Eskimos take on the Hamilton Tiger-Cats during a CFL game on July 9 in Edmonton. The team was forced to move its practice indoors because of the city's mosquito problem. (John Ulan/Canadian Press)

"We've really had 10 years of dry weather, and people have forgotten a little bit about mosquitoes," Costigan said. "We've had record years of low mosquito numbers, so this was a shock for a lot of residents this year when we got back to some normal mosquito numbers for a wet season."

It was clear from January that mosquito populations would thrive this spring, given the amount of snow on the ground, said Jenkins. That snow melted in the spring, filling the ditches and low-lying areas where the larvae of the types of mosquitoes known as spring floodwater or snowmelt mosquitoes are found.

"When we first took a few flights in the helicopter back in the beginning of spring, we saw water on the ground in places where we haven't seen water probably for 10 years or more," he said.

Thunderstorms bring different species

A second batch of mosquitoes arrived in the summer, when heavy rains began falling. These, too, thrived, as June and July rainfall far exceeded that of previous years.


Municipal workers use either chemical pesticides or bacterial pathogens to kill mosquito larvae that accummulate in ditches and pools of stagnant water like this one in Orlando, Fla. (Joe Skipper/Reuters)

The type of mosquitoes that emerge at this time of year are known as summer floodwater mosquitoes. They deposit their eggs often in the same low-lying habitats as snowmelt mosquitoes but need to be covered by the warmer waters of summer thunderstorms – rather than the spring runoff – in order to hatch.

They can also produce several generations in one season – unlike the snowmelt mosquitoes, which produce only one – turning from egg to adult in a matter of days in the warm summer temperatures.

"So, every time you get a big thunderstorm, you get more eggs of those species hatching," says Terry Galloway, a veterinary entomologist at the University of Manitoba who has studied mosquitoes and other biting insects.

The eggs of both types of mosquitoes lie dormant through the winter — and, in the case of summer floodwater mosquitoes, can do so for years if they don't get covered by water.

"Especially our species on the Prairies, they're really well adapted to extended periods of drought. They just wait," said Galloway.

Pest control

Once covered with water, and given the right temperature conditions, the eggs develop into larvae, and it's at this stage that municipal mosquito-control programs try to eradicate them.

To kill the larvae, city pest-control workers use either chemical insecticides, which wipe out everything in their path, or more targeted biological larvicides that are specific to mosquitoes and related aquatic flies. They are designed not to harm other animals, fish or plant life.

The biological larvicides introduce bacteria into the water that kill the larvae when they feed on it.

Biological larvicides have to have the right temperature conditions to work and can be trickier to apply since they can be dispersed by wind, can't easily be applied by air and don't penetrate vegetation like insecticides do, says Jenkins. Larvae also have to eat enough of them for them to have an effect.

Nevertheless, more and more cities are using these biological methods of mosquito control in response to growing public pressure to find alternatives to harsh pesticides.

"The direction now is to develop effective control strategies that are also less harmful on the environment," Galloway says.

More cities using biological larvicides

Winnipeg this year boosted its mosquito-control budget by $1 million  in part to fund a switch toward using more biological larvicides. Its program used 60 per cent biological larvicides this year as opposed to only 35 per cent last year, said city entomologist Taz Stuart.

Stuart credits the city's larviciding program — and favourable weather conditions — with keeping the mosquito numbers in Winnipeg down this year.

"The spring was cool, which was key in our program," he said "We could get out and hit all those sites where, normally, as it gets hotter, those sites can start producing adults in as little as three to five days."


A portrait of an Aedes stimulans mosquito larva, a common pest mosquito that develops in spring floodwater pools. (iStock)

So far this year, Winnipeg's mosquito population has been significantly lower than normal, and on Aug. 4, the city released numbers  showing trap counts were the lowest they've been since 1980. The average number of female nuisance mosquitoes caught at the city's 24 traps in the last week of July was five while the average for the same period over the past 10 years has been 74. The average in the first week of August was three. Usual averages over the past three decades have been between 40 and 70.

"The big question in Manitoba this year has been, 'Where are all the mosquitoes?''' Galloway said. "We've had this extensive flooding, and people were really bracing themselves and it just didn't happen in a lot of areas."

That wasn't the case farther east in a small southwestern Ontario town called Park Hill, which falls under the municipality of North Middlesex and is about 50 kilometres northwest of London. There, a seemingly record number of mosquitoes this year drove local officials to hold a public meeting on the matter that was attended by more than 400 people.

"A message that we're hearing again and again is, 'I've never seen it this bad,'" said Tim Cumming, communications specialist for the Ausable Bayfield Conservation Authority, which works with Parkhill and other communities in the region to maintain watersheds and helped organize the meeting.

The Middlesex-London Health Unit, which counts mosquitoes in the region as part of its monitoring of West Nile virus, counted 87,801 male and female mosquitoes over seven weeks between June 7 and July 20 at the one trap location it maintains in Park Hill. That's more than twice the 36,133 adult mosquitoes it counted at the same trap over the full 20-week mosquito season last year.

"We know that there was certainly a spike in adult mosquito populations at the beginning of the season due to snow melt and heavy spring rains," said Elizabeth Milne, a disease technician with the health unit.

Ont. town known for attracting mosquitoes 

The number of mosquitoes in Parkhill has traditionally been higher than in other parts of the Middlesex-London region, Milne noted. In the same seven-week period, the mosquito trap in Strathroy, 30 km south, for example, caught 2,586 mosquitoes and the two traps in London had a total of 4,535.

"In other areas where numbers are lower, a spike may not create a huge impact on quality of living, whereas in Parkhill there has been a major impact," said Cumming.

The mosquito problem prompted a number of complaints from Parkhill residents, and following the July 19 public meeting, the municipality of North Middlesex, the conservation authority and the health unit launched a series of field assessments  to determine what could be done to reduce the number of mosquito breeding grounds in the region.

"There could be areas where there's stagnant water, water that's not flowing, obstructions, log jams," Cumming said.

Cummings said elevation surveys will be carried out to identify ruts and other low-lying areas where water has been pooling and to determine how to improve drainage of such areas.

Authorities have already identified a 25-hectare area downstream of the Parkhill Dam as an area with a significant amount of standing water where flow and drainage need to be improved. Measures to reduce mosquito habitats will also take place upstream of the dam, adjacent to sewage ponds, at catch basins on private property, at the local landfill site and along some disused railway tracks, the conservation authority said.

Slow start to larviciding in Regina

Drainage is a problem the city of Regina will also have to contend with following an unusually wet mosquito season.

"We have lakes around the city that weren't there last summer," said Wade Morrow, the city's supervisor of pest management. "There are sites out there that in all likelihood will take years to disappear."

That accumulation of water in fields, parks and ditches created ideal conditions for mosquito development in and around the Saskatchewan capital.

"We had a very high spring runoff and a lot of rain early in the season," said Morrow. "As a result, we saw mosquito numbers above our 10-year average."

The city caught an average of 1,162 adult mosquitoes at its 10 light traps in the first week of July, compared with the 10-year average for that period of 562.

The fact that Regina initially lost some of the provincial funding  it had been getting for mosquito control hurt the city's larviciding efforts this year. By the time money was restored to the program in July, the rain and spring runoff had already done their damage, boosting mosquito numbers substantially.

"We didn't have the same levels [of larviciding] out there earlier in the year, and with the extra water and things, it was a difficult situation," Morrow said.

Regina is one city that has found success with biological larvicides. It uses only bacterial pathogens to control its mosquito population and as a result has seen a large emergence of natural predators such as dragon- and damselflies, Morrow said.

"One of the goals of our mosquito-management program is we want those beneficial predators helping us with our program as well," he said.