A fogging truck sprays malathion on a Winnipeg street. (Canadian Press)

A combination of heavy rainfall and recent hot weather has Winnipeg mosquitoes buzzing — and the city is preparing to roll out the fogging trucks to control the bloodsuckers.

The city gave 48 hours' notice Wednesday afternoon that it would begin a program to fog with malathion to kill nuisance mosquitoes on Friday evening.

Larviciding and other treatments have not succeeded in controlling the mosquito population, the city's insect control department said.

"Due to environmental conditions, these measures alone are no longer able to control current adult nuisance mosquito populations, and adult mosquitoes have become intolerable," the department said in a release.

"Therefore an adult nuisance mosquito fogging program is now being implemented."

A week ago, the city was counting no mosquitoes in city traps. This week, trap counts averaged 342.

Until further notice, fogging will be carried out on a daily basis between the hours of 9:30 p.m. and 6:30 a.m., starting with areas with the highest mosquito populations.

A public service announcement will be issued no later than 1:30 p.m. daily indicating which residential areas will be fogged that night.

Buffer zones

Buffer zones around the homes of people who register their opposition to the fogging program will be respected, city officials said. Crews will turn off the insecticide sprayers for 100 metres on each side of a registered property.

Residents who wish to register for buffer zones can do so by letter, e-mail or fax, or through a form on the city's website. Up to 72 hours may be required for processing, officials said.

In order for a nuisance mosquito-fogging program to be carried out, conditions in Winnipeg must meet three requirements:

• A minimum of 25 female mosquitoes must be caught in city traps for three consecutive nights.
• One or more of the quadrants of the city should have trap counts in the range of 100 female mosquitoes.
• The city-wide adulticiding factor analysis (AFA) rating must be high, indicating significant populations of adult mosquitoes are present and signs point to those conditions continuing.

The city raised the AFA rating to high from medium on Wednesday. The other two conditions have also been met, city officials said.

Chemical controversy

Fogging involves "ultra-low-volume" applications of malathion, which has been approved for use in Canada by Health Canada's pest management regulatory agency.

However, the insecticide's use is controversial in the city. Critics question its effectiveness in killing adult mosquitoes, and some have raised concerns about possible health effects.

Malathion use has prompted protests and demonstrations in the city in recent years — generally in cases when the city has stopped respecting the fog-free buffer zones around the properties of people who have asked to be left out of the program.

In past years, the city ignored buffer zones when the provincial government issued a health order to fog to limit the risk of West Nile virus.

Provincial officials said Wednesday the risk of contracting West Nile virus in the province is currently considered low, but is expected to increase with the warm weather.