As hot weather looms, insect control crews work OT to kill as many mosquito larvae as possible

Winnipeg insect-control crews are working overtime in an effort to kill as many mosquito larvae as they can before high temperatures speed up the development of the blood-sucking bugs.

Manitoba's capital has not had to fog for nuisance mosquitoes since 2020, thanks to dry conditions last year, though the province did require the city to fog for the species that carries West Nile virus for a brief period in 2021.

Heavy rains this week, coupled with the prospect of 38 C heat this weekend, has led the Insect Control Branch to send all four of its helicopters into the air for as many hours as the weather and air-safety rules permit in order to apply larvicides to stretches of standing water that are difficult to reach on foot, the branch's new superintendent, David Wade, said Tuesday.

Staff who work on foot to deploy larvicides are also putting in extra hours, to the point where as many 80 workers are out in the field at any given time, including overnight.

Together, the helicopters and workers on foot target 5,000 known standing-water sites that cover 20,000 hectares of the city.

"We have staff working around the clock, seven days a week," said Wade, a 19-year Insect Control Branch employee who completed his masters degree at the University of Manitoba.

The reason for the urgency is simple: Mosquito larvae, which do not bother people at all when they bob around in stagnant water, develop into blood-sucking adults far more quickly when weather gets very warm.

Relatively cool temperatures this spring have allowed the city's Insect Control Branch to limit the development of adult mosquitoes to the point where the city-wide average trap count for adult female nuisance skeeters was only nine on Tuesday.

Winnipeg does not fog until it experiences two straight days with a city-wide trap count of 25 or more and at least one quadrant of the city with a trap count of 100 or more.

Wade said a massive emergence of adult mosquitoes is not inevitable, even with all the water this week and warmer weather on the way.

He said his crews are doing everything they can to avoid a fogging program.

"My goal of course is not to allow it to get to that point," he said. "We'll see how the weather plays out this weekend, but I'm confident we can stay on top of it and not have to do it."

Bacterial and chemical warfare

Winnipeg deploys two types of larvicides to kill mosquitoes before they reach the adult stage: one bacterial and one chemical.

The bacterial agent is Bti, or Bacillus thuringiensis israelensis, which damages the guts of mosquito larvae and other aquatic organisms, including blackfly larvae, to the point where they can not feed.

The chemical larvicide is methoprene, which mimics the hormones present in juvenile insects and thus doesn't allow them to become adults.

Both larvicides can kill species other than their intended targets, so the city aims to deploy them only in bodies of standing water where mosquitoes lay their eggs — and in relatively small doses, especially when it comes to methoprene.

"It is targeted to organisms the size of mosquito larvae," Wade said. "Larger aquatic organisms like dragonfly or damselfly larvae wouldn't be affected at the same rate."

Deltamethrin, for adults only

Rob Anderson, a University of Winnipeg biologist who has been studying mosquitoes for nearly 40 years, said a large body of science shows the collateral damage of both Bti and methoprene is limited if they are deployed in a targeted manner.

"For instance, you can't put methoprene into flowing rivers or a large aquatic habitat where there are other invertebrates, because one would get a big kill in that situation," he said.

The science is less clear about the chemical Winnipeg now uses to fog for adult mosquitoes. The city has been using deltamethrin, a synthetic version of pyrethroid insecticides that are naturally present in chrysanthemum plants, since 2019.

"It has broad-insecticidal-spectrum properties. That means it's not just a mosquito killer; it will kill other insects," Anderson said, adding that's why the city has to be very careful to apply deltamethrin in the precise dose and concentration prescribed by its label.

Not all bloodsuckers are the same

While there are dozens of mosquito species in southern Manitoba, the main variety the city is trying to control is Aedes vexans, unofficially known as the summer nuisance mosquito. This is the one most likely to give you a nasty, itchy bite.

Aedes vexans, however, is relatively harmless, as it is not known to transmit illness to people very often. Another species, Culex tarsalis, is a stealthier biter, known to transmit the West Nile virus to people and horses.

Both Anderson and Wade said the best thing Winnipeggers can do to help the city slow the development of mosquitoes is to dump out any standing water on their properties.

Once the weather gets warm, it only takes a week for a mosquito to go from egg to adult, Wade said.

In the meantime, Wade's crews are still out on foot and in the air. 

He demurred when asked whether his pilots have a fondness for playing Wagner's Ride of the Valkyries as they take to the sky, the way the music was used to accompany a helicopter squadron in the film Apocalypse Now.

"I don't talk to the pilots," Wade said. "So I don't know … what their interests are."