Mosquito season is not over, experts say, so don't let your guard down

Sep. 15—The war on mosquitoes in Kern County introduced a new weapon this summer.

And while early indications showed the weapon reduced mosquito populations in the areas it was tested, a key question still remains unanswered.

Was the effect only temporary?

"We don't know the answer yet," said Terry Knight, public information officer for the Kern Mosquito & Vector Control District.

Beginning in June, the district tried out the new weapon in limited areas in Bakersfield. Using a truck-mounted, gasoline-powered mist blower that generated a plume of water mist and mosquito larvicide over the front and back yards of homes in residential neighborhoods, operators hoped to be able to cover a swath of about 300 feet.

The motorized atomizer mixes water with a product called VectoBac WDG, which contains Bacillus thuringiensis israelensis, or Bti, a naturally occurring soil bacteria whose toxicity is limited to mosquitoes and related flies, according to the district.

Bti has no toxicity to people and pets, Knight said, and is certified for the treatment of organic crops.

The process is known as Wide Area Larvicide Spray, or WALS treatment.

The district chose three 1-square-mile zones, in southwest, northwest and northeast Bakersfield to spray repeatedly over the next several weeks. The study also included a control zone to monitor mosquito levels in an area untouched by the spray.

Some residents were understandably unsettled by the effort to spray over their property. Other area residents who had suffered awful bites and effects from the mosquitoes begged to be added to the spray area.

Beginning in early June, one zone received six weekly applications and the other two zones only five due to equipment failure.

In July, each zone received two applications, two weeks apart, Knight said.

Mosquito levels were continually monitored in all areas of the study in order to determine the efficacy of the product and its distribution method.

Mark Dery, the district's scientific program director, said Thursday that it's hard to say definitively at this point to what extent there was a reduction of mosquitoes in the three treatment zones, as the study is still ongoing and the data incomplete.

"It appears that there was a reduction in the populations of Aedes aegypti and Culex quinquefasciatus following the treatment applications," Dery said in an email. "However, it appears that these populations may be recovering now that we are several weeks past the final application.

"Weekly mosquito trapping in the control and three treatment zones is still ongoing to determine if there is a long-term reduction of mosquito populations in the treatment zone compared with the control," he added.

"It will ultimately take statistical analysis on the complete data set to determine what reduction was due to the larvicide applications in the treatment zones and what was due to other factors such as the weather."

In fact, Knight said, the drought may very well be having an effect on mosquito populations — although the cooling trend now in effect appears to be boosting population levels, according to as many as 150 mosquito traps the district monitors.

It's important to remember that the while the newcomer, the Aedes aegypti — also known as the "ankle-biter" — is more aggressive, bites multiple times and may be more hated than the common Culex quinquefasciatus. The Culex is capable of carrying West Nile Virus, while aegypti is not.

"In 2020 — I call it the Aedes aegypti explosion in Kern County — we had over 3,900 service requests, or calls for service, which is ... about 400 percent higher than the previous year," Knight said.

That's when news of this new mosquito had residents talking — and taking action. Knight has no proof, but he thinks it's possible that more people started using mosquito repellent and taking other actions to avoid being bitten. And as a result, West Nile Virus infections have fallen.

"In 2019, we had 32 human cases in Kern County," he said.

In 2020, there were eight; the next year, three, and so far this year, there have been four.

If Aedes aegypti had that kind of positive effect," he said, "I guess in one way, they are also a blessing."

Mosquito season ends Oct. 21, Knight said. So don't let your guard down.

Reporter Steven Mayer can be reached at 661-395-7353. Follow him on Facebook and on Twitter: @semayerTBC.