With an exotic mosquito invading backyard nooks and crannies, Los Angeles County can no longer keep pace by only using pesticides or releasing mosquito-eating fish in ponds and old pools.
So the county’s pest-control district is trying squadrons of sterilized male mosquitos who will mate with females in the wild, rendering their eggs dead and hopefully lowering the female population. The females are the targets because they bite humans for the protein that is needed for egg-laying, while males do not bite.
This novel use of biological bug warfare begins on Thursday, May 16, when 60,000 irradiated male Aedes aegypti species mosquitos will be let loose every week in Sunland-Tujunga to search for mates, said Susanne Kluh, general manager of the Greater Los Angeles County Vector Control District.
The releases of irradiated male Aedes aegypti mosquitos will continue weekly through November, Kluh said. The district, which contains six million people and takes in most of the city of Los Angeles, from Santa Clarita to the San Fernando Valley to San Pedro and the southeast county cities, is launching the program in Sunland-Tujunga because there will be fewer intrusive males in the area, which is hemmed in on the north by mountains, she said.
Using sterilized males to depopulate the species is one tool in the mosquito-killing tool box of this district, which also uses hormonal-based pesticides to disrupt the life cycle of the pest. But the sterile mosquito technique is preferred.
“It is ecologically friendly. There are no other impacts,” Kluh said.
The vector control district is spending about $255,000 on the sterile release program this year but that cost could rise, she said.
Here’s how it works:
Up to 10 sterile males will be released for every one in the wild, greatly increasing the chances that females will mate with sterile mosquitos. Females can lay 150 eggs, but with sterile sperm the eggs will never hatch. The females die off after a week or so and without new babies being born, the population decreases, Kluh explained.
“And when the number of Aedes female mosquitos go down, people will go outside and not get bitten,” she said.
Mosquitos are collected and the males are put into a kind of X-ray machine that shoots them with enough radiation to make them sterile — but not enough to lessen their overall fitness. The males are then are placed in release cages that are opened by hand in neighborhoods.
Dr. Solomon Birhanie stands next to a X-ray irradiator machine where the mosquitoes are zapped at the West Valley Mosquito and Vector Control District, based in Ontario. (photo courtesy of the WVMVCD).
The L.A. County district is the second in the state to use sterile mosquitos to reduce the population of Aedes. The first group to pioneer the practice was the West Valley Mosquito and Vector Control District, which is continuing its weekly releases in Ontario, Montclair, Upland, Rancho Cucamonga, Chino and Chino Hills, said Brian Reisinger, community outreach coordinator.
The aim is to reduce the Aedes population now, before the mosquitos’ peak season in July and August, Reisinger said.
Results from a pilot project in September 2023 showed a 70% drop in the Aedes population in the targeted neighborhoods in Ontario, he said. That led to a widespread program in six cities that began in late April. “With that 70% reduction, we are excited to see what happens this year,” he said.
A technician at the West Valley Mosquito and Vector Control District gets ready to release sterile mosquitoes from a cage in a neighborhood in the Inland Empire. (photo courtesy of the WVMVCD).
Experts say the population of Aedes in Southern California has grown by four to ten times since 2017. Recent wetter years, including unusual humidity and storms in summer such as Hurricane Hilary in August 2023, have made for perfect breeding grounds.
The Aedes can do more than ruin a backyard barbecue. They carry troubling diseases including yellow fever, Chikungunya, dengue fever and Zika. This exotic mosquito variety is a mega nuisance, biting day and night. They are known as ankle-biters and leave numerous red, itchy welts each time.
Two cases of mosquito-borne dengue fever in humans last year — one in Pasadena and one in Long Beach — were the first two ever reported in California that were not brought in by someone who had traveled outside the U.S. The two illnesses were transmitted by mosquitos locally, Kluh said.
“Dengue is very painful. I heard it feels like every bone in your body is broken,” she said. Yet only 20% show symptoms. The two people recovered fully, she said.
Vector-control districts used to deal only with the Culex variety, known as the southern house mosquito. They carry West Nile Virus, but are easier to contain. They lay eggs in bunches in water-filled trash cans or wheelbarrows and they are destroyed when the vessels are overturned.
These common mosquitos only bite at night, unlike the 24-hour biting cycle of the Aedes.
“These new, invasive species (of mosquitos) bite all day long. They follow you into your house — so you can have the same problem sitting on the couch,” Kluh said.
They lay durable eggs that can hatch a year later. The eggs cling to vessels even after the water is emptied — buckets, pots, gutters, bird baths, even bottle caps — then they get transported to other areas. Because they are so resilient, they have become well-established in Southern California, she said.
“We have had in recent years a lot of trouble providing adequate controls for these Aedes mosquitos,” Kluh said.
At the West Valley district the sterile mosquito program is used in tandem with traditional control methods — for instance continuing to set traps in trees to identify the district hot spots. Then they use spot treatments with pesticides in standing water or use lures to attract and kill mosquitoes, Reisinger explained.
“Having this new tool is terrific but it won’t be a magic bullet,” he said.
The L.A. County district hopes to extend the Sunland-Tujunga program into other areas with high concentrations of Aedes, Kluh said. “If mosquito numbers (in Sunland-Tujunga) stay very low the whole summer, we hope to bring it to a larger population,” she said.
The district is also working with the Orange County Mosquito and Vector Control District, sharing materials and equipment. The Orange County district is preparing a release of sterile Aedes mosquitoes in Mission Viejo, Kluh said.
While these biological remedies proceed, summer is coming. And after a rainy winter and spring, experts are expecting more of all kinds of mosquitoes to be out looking for blood.
Tips to prevent bites and to remove larvae and adult Aedes include:
• When going outdoors, put on bug repellant that contains DEET (15% is adequate); IR3535, Picaridin or lemon oil eucalyptus.
• Wear long pants and long sleeves.
• Empty and scrub all water collection sources such as pots, bird baths, trash bins and gutters.
• Put mosquito fish in ponds, horse troughs and any permanent water sources.
Related links
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Why there are so many mosquitoes this year and what you can do about them
Health officials announce first death this year in LA County due to West Nile virus
2nd California case of local dengue fever reported in Long Beach
Pasadena announces first case of locally acquired Dengue virus in California