Data shows the ebb and flow of mosquito season

· The Pulse

You've probably never swatted a mosquito and stopped to ask yourself if it was a Culex or an Ochlerotatus, but that is one of the considerations guiding Edmonton's ever evolving pest control program.

There are dozens of mosquito species in the region, hatching in waves over the course of the summer. Some, like the complex of species that develop in cold-water environments in early spring, are often naturally limited to one generation a year. Others, like the Culex subspecies, are the most common carriers of the West Nile virus, though that is fortunately less of an issue this far north.

The most bloodthirsty and populous varieties that breed in the warm months and ruin your summer outings are the main concern of the city's pest department.

"When we start seeing Ochlerotatus dorsalis and Ochlerotatus spencerii, those are spring developers, and they're also quite aggressive daytime biters," said the city's pest management coordinator, Mike Jenkins. "When they show up, that gives us a warning that those species are developing. And so, we can target our programs to look for the larval development, and to concentrate our efforts on the sort of sites where those species are developing."

The pest control teams gather weekly data from sources such as light traps placed around the city, weather forecasts, and observations of larvae growing in standing water. From that, they make projections and determine their response, meaning their approach is constantly being adjusted. For the past few years, the city has also been incorporating carbon dioxide bait traps that mimic human scents to supplement the data gleaned from the light traps.

"They're more effective. They draw in all the species that we're looking for, not just the ones that get attracted to the light," Jenkins said. Unlike light traps, the BG-Sentinel carbon dioxide traps can also be used during the day, when some of the most aggressive biting species are active.

A bar chart comparing the light-trap counts of 10 species of mosquito in Edmonton, with Aedes vexans the most populous in mid-August, followed by Culiseta inornata

Mosquito species come in waves over the summer. Some you likely never notice; others, like Aedes vexans, are a months-long annoyance. This chart shows cycles in a typical year, using data from 2020 as a sample. (Brett McKay)

"The light traps also use an insecticide to kill the insects that are drawn into the light, and we're trying to get away from using that insecticide as well," Jenkins added. Eventually, the pest control program will transition exclusively to the carbon dioxide traps, but there are limitations and inconsistencies that the pest control teams are still trying to iron out. The battery-powered traps are only put out one day a week, while the light traps run continuously, and a bad storm can damage the sample and cost a full week's worth of data.

"We're still working on some of those issues," Jenkins said. "The numbers don't necessarily always jive, when we get a spike in the light traps, we don't necessarily see the same spike in the carbon dioxide traps and don't know exactly why."

The City of Edmonton recently announced how it would be replacing the aerial spraying program that was cancelled in April of this year. The $507,000 that would have been spent on aerial spraying will be divided between monitoring, bat houses, biological control programs, and a public education campaign meant to build public confidence in the city's pest control strategy.

The end of aerial spraying is probably the highest-profile change to the mosquito control program, but what the summer will look like without it still isn't clear.

"Without the aerial program, we are expecting that we are going to see more mosquitoes. How much is really hard to say," said Jenkins.

Factors like the level of precipitation and the availability of suitable breeding habitats have the largest influence on mosquito populations, and on how much the helicopter pesticide drops were even needed.

"We actually only did one program last year with the aerial program. We did one treatment at the end of May, and that was pretty much it," he said. "We had a really hot and dry summer, and we really didn't have an awful lot of development of mosquitoes in any of the habitats."