Fogging describes 'ultra-low-volume' spraying applications of malathion. The chemical has been approved for such use by Health Canada, but the practice has sparked controversy in previous years. (Canadian Press)

Fogging trucks will once again hit Winnipeg streets Thursday night as the city's annual summer battle against mosquitoes continues.

The city's insect control department announced Thursday that spraying with malathion to kill adult, flying mosquitoes would begin Thursday evening.

Starting at 9:30 p.m. CT, trucks will be in more than a dozen areas, mainly in the eastern and western quadrants of the city.

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

The insect-control department said requirements for a fogging program for nuisance mosquitoes had been met:

• 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 is 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 Thursday, saying the citywide average trap count for July 31, 2008 was 62.

Fogging will continue daily between 9:30 p.m. and 6:30 a.m., for as long as mosquito numbers are high enough to warrant it and as long as weather permits. Fogging is not carried out in rain or strong winds or when temperatures are below 13 C.

All of the city's neighbourhoods — excluding buffer-zone areas — were fogged with malathion at least once during a control campaign earlier this summer.

The city parked the trucks July 3 when mosquito numbers were deemed too low to warrant the program's continuation.