Hundreds of Tracy residents were startled by the appearance of a twin-engine airplane circling low above their neighborhoods Monday night.
Many people thought the plane was in distress as it passed over rooftops and fields. Some tracked sightings of the plane on mobile apps and social media. A handful of people even called the police to report the aircraft’s low altitude.
What most people didn’t know was that the plane was circling town as part of a fight against mosquitoes that carry West Nile virus.
Aaron Devencenzi, spokesman for the San Joaquin County Mosquito & Vector Control District, said the pilot was spraying a chemical to kill mosquitoes over Banta and a large section of south Tracy because the virus had been found at significant levels in those areas. (Scroll to the end of this story for maps.)
“It’s important not to be alarmed,” he said. “It’s part of our normal operations. We do that in some parts of the county more than others depending on the mosquito population and the amount of virus we find in them.”
The district sets up mosquito traps throughout the county, which are collected twice a week and taken to a lab where male and female mosquitoes are separated out. The females of two different species are sorted and then tested in groups of 50 to see if they are carrying West Nile virus.
“When they are showing West Nile virus within that group of mosquitoes collected, that’s when we know based on the population we are going to need to spray that area to protect the public from those diseases,” Devencenzi said.
So far this season, 40 mosquito samples throughout the county have tested positive for the virus, with last week’s traps showing a number of positive results in Tracy and Banta along with Manteca and Ripon.
Devencenzi said the number of mosquitoes in the county wasn’t unusually high.
“It’s not anything that’s spiking or so forth like we had in the previous two years,” he said. “I think we’re just a little bit below our five-year average of the amount of mosquitoes infected with the virus. That could change with these next few days of hot weather, but people still need to take precautions on West Nile virus, and we have certainly found West Nile virus in the Tracy area and we want people to be aware of that.”
Several times in the past few months, the district has sent trucks to Tracy to spray for mosquitoes in the early morning hours, but Monday night’s aerial spraying was the first over Tracy in several years and caught many residents off guard.
“The decision to go aerial over ground is simply based on the amount of acreage we have to cover,” Devencenzi said. “Smaller acres of land, we can use a truck or several trucks to cover that area, but when we start looking at the much larger acreage, the plane is much more efficient and gets the job done quicker for us.”
The aerial spraying was done by a firm that uses a Cessna 402, a twin-engine plane that can seat six to 10 people but is often modified for non-passenger uses.
“They are planes, they make noise, they fly about 300 feet above the ground, which is lower than most people expect,” Devencenzi said. “They will be making multiple passes in a general area until the entire area is completed.”
Sometimes, the pilot has to fly outside of the target area to account for drift based on the wind speed and direction.
On Monday, the airplane dispersed what Devencenzi called an ultra-low-volume mist of Dibrom Concentrate — about two-thirds of an ounce per acre — over the southern half of Tracy, from the Union Pacific Railroad tracks south to Linne Road between Lammers and Chrisman roads. The Banta area — just north of 11th Street to beyond Interstate 205 between Bird Road and Deuel Vocational Institution — was also treated with Dibrom Concentrate.
“It’s done in a very fine mist,” he said. “Probably not going to see a lot of it coming out of the truck or the airplane that would be flying.”
The district selects a time to spray when mosquitoes are out and active.
“It is a contact spray, so we do it at dusk and dawn, into the evening hours, when the mosquitoes that carry West Nile virus are most active. They go through the spray, they pick up some of that mist and it kills mosquitoes,” he said. “What lands out on the ground pretty much breaks down with the sun in the next few hours after that.”
Recent ground sprayings in sections of east Tracy have used a different chemical, Evergreen 5-25. Information about each of the chemicals is available on the district’s website.
The district tries to give at least 24 hours’ notice before spraying. Devencenzi said anyone in the community can sign up at sjmosquito.org to get a free alert by email any time the district plans to send out ground crews or pilots to spray for mosquitoes. People can also call 982-4675 or 800-300-4675 to ask questions or get help signing up for alerts.