A jump in mosquito samples carrying West Nile virus has led to ground and aerial insecticide spraying in 10 areas of San Joaquin County, including Southside Tracy and the San Joaquin River Club.

Today between 3:30 and 6 a.m., San Joaquin County Mosquito & Vector Control District crews on the ground will spray Pyrocide 7067 to kill adult mosquitoes in an area bordered by the Union Pacific railroad tracks to the north, Schulte Road to the south, MacArthur Drive to the east and Tracy Boulevard to the west. If weather conditions, such as high winds, prevent the spraying, they will try again Saturday morning at the same time.

An area of south San Joaquin County near Kasson Road, including the River Club, was scheduled for aerial spraying of Trumpet EC on Thursday between 8:30 and 11 p.m. If weather prevents the spraying, crews will try again Saturday evening.

The spraying followed new warnings from the vector control district about the prevalence of West Nile virus as the San Joaquin Valley heats up.

Aaron Devencenzi, a public information officer for the vector control district, said the number of mosquito samples testing positive for the virus had increased by more than 10 times within the past three weeks

“When we were looking at the week of June 24 through the 30th, we only had eight mosquito samples positive for West Nile virus (since Jan. 1), and as of now — we collected last week — we’re at 89 samples that are positive, so we have had substantial amount showing positive in that three-week period,” Devencenzi said.

He said the rise in temperatures was the likely cause of the increased mosquito numbers.

“We had a fairly cool spring and our populations were probably below average, and then we started hitting this hot weather over multiple days, so we are seeing an increase above average of what our mosquito populations are,” Devencenzi said.

The vector control district places more than 70 traps throughout the county and collects them twice a week. Each sample includes up to 50 mosquitoes, which are crushed and then tested for the virus.

In addition to the 89 positive mosquito samples, two dead birds have tested positive for West Nile virus this year.

On July 17, the San Joaquin County Public Health Department reported that a 51-year-old Stockton man had contracted West Nile virus, the first confirmed human case in the county this year.

Most people infected with the virus do not become ill, and about 20 percent have mild to severe flu-like symptoms. In rare cases — about one in 150 — the virus causes encephalitis or meningitis, which may lead to death.

The vector control district is reminding residents to be cautious of mosquitoes when they are outdoors at dawn and dusk, the times when mosquitoes that carry West Nile virus are most active.

“Most importantly is to use EPA-registered repellents,” Devencenzi said. “Wear loose-fitting clothing with long sleeves and pants.”

The vector control district recommends emptying any standing water in cans, buckets or other containers in the yard. Window screens should fit tightly and have no holes.

Mosquitofish can be placed in ponds or unused pools to eat immature mosquitoes. The fish are still available free from the vector control district and can be delivered by request to homes around the county or picked up at the vector control office at 7759 S. Airport Way in Stockton.

The district will continue trapping, testing and spraying to kill mosquitoes throughout the summer. Mosquito activity diminishes as the weather grows colder.

“We are usually seeing virus activity well through October and into November,” Devencenzi said.

Residents can report significant mosquito problems, including mosquitoes that bite during the day, to the vector control district at 982-4675 or 800-300-4675.

Dead birds can be reported to 877-968-2473 or at www.westnile.ca.gov.

For more information, to request service or to request free mosquitofish, visit www.sjmosquito.org.

Contact the Tracy Press at tpews@tracypress.com or 835-3030.

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