They Released Thousands of Mosquitoes, Then Waited

Officials are seeing promising signs that efforts to combat mosquitoes are working in Southern California as the season of peak activity begins. A local agency known as a vector control district (VCD) began releasing sterile male Aedes aegypti mosquitoes in 2023, followed by a second district the following year. The goal was to have the male mosquitoes, which don't bite, mate with biting females, resulting in eggs that don't hatch. The effort appears to have been effective, reports the Los Angeles Times. The Greater Los Angeles County VCD, whose pilot program is operating in Los Angeles' Sunland-Tujunga neighborhood at an annual cost of $350,000, has reported an 82% reduction in invasive mosquitoes, compared with a control area.

The West Valley Mosquito and Vector Control District covering the southwestern corner of San Bernardino County, which launched its pilot program in 2023, reported a 44% average decrease in mosquitoes across several heavily infested sites, compared with pre-intervention levels. The overall count of invasive mosquitoes dropped 33% across the district, "marking the first time in roughly eight years that the population went down instead of up," per the Times. A rep notes there were also "a lot less calls" from locals complaining about the pests, which have been linked to the spread of dengue fever.

Thousands of sterilized males are released every week or two from May to October, with the goal to outnumber wild males by 10 to 1 for the LA County VCD, or by 100 to 1 for the West Valley VCD. The latter is increasing the number of sites treated this year, while an Orange County district plans to launch its own program, per the Orange County Register. "Many medium to smaller districts are now interested to use our approach," a West Valley VCD official tells the Times. But expanding control efforts to large population centers is rife with challenges. "Scaling the intervention to the level needed to make a dent in the vast region served by the LA County district ... would potentially require its homeowners to pay up to $20 in an annual property tax assessment," notes the Times. (More mosquitoes stories.)