Warmer temperatures and humidity in San Diego County have created favorable conditions for mosquitoes to thrive this season.
“March was actually quite warm this year,” said Allison Bray, the county’s vector control program coordinator. “So we did see a slightly earlier start to … what we usually think of mosquito season is about April through October.”
Bray said she expects mosquito activity to continue into November if temperatures remain warm.
But warmer weather isn’t the only reason people may be getting bitten more. Over the past decade, Bray said San Diego County has also seen the arrival of invasive mosquitoes that bite — sometimes multiple times — during the day. She calls them ankle biters.
“It’s not necessarily that there's more mosquitoes total, but that there are specifically more of these little mosquitoes that like to live in backyards, apartment complexes, places like that,” she said.
The county is trying to reduce mosquito numbers and the risk of West Nile virus.
They monitor about 1,500 potential mosquito breeding sites.
“We routinely set mosquito traps throughout the county,” Bray said. “We're able to collect those mosquitoes and test them for various diseases in our labs.”
Crews use helicopters to drop larvicide into breeding sites, targeting mosquito larvae before they become adults. When adult mosquito numbers are high, crews also use truck-mounted treatments to reduce their numbers.
But more rain from El Niño could complicate those efforts.
Bray said more rainfall could create additional standing water, providing more places for mosquitoes to breed.
The latest treatment was in Coronado last month after the county found high mosquito counts and detections of West Nile virus.
“Leading up to the adult mosquito treatment, we were seeing pretty consistent weekly detections of West Nile virus, and average trap counts of 20 to 30 mosquitoes,” Bray said.
After the treatment, the county put traps out again.
“In our traps it went down to about four or five mosquitoes per trap,” Bray said. “We definitely consider that to be a success.”
But Bray said numbers can go back up in as little as two weeks.
“For a lot of these mosquitoes, they can grow from an egg to an adult in as little as a week or two. We do usually expect to see some increase after about that amount of time,” Bray said. “So our hope is we interrupt that cycle, and even if the mosquito numbers go back up, our goal would be to see less West Nile virus activity.”
This year the county has reported 11 human West Nile cases, compared to none last year.
“West Nile virus tends to have these kind of cycles where we won't see any significant West Nile virus activity for years,” Bray said. “We'll see a mosquito here and there, we'll get reports of birds that have died from West Nile, but it's scattered, it's not consistent.
Bray said that continued monitoring will help determine whether more treatments are needed.
“Based on our mosquito data, we do see, generally, a decrease in mosquito numbers after we canvas an area, look for breeding sources,” Bray said. “So hopefully we're achieving our goals.”
The county encourages residents to empty standing water around their homes, request mosquito-eating fish, and use mosquito repellent.
The next aerial treatment is scheduled for Thursday.
