Thursday, March 19, 2026

Traps Go Up Across San Gabriel Valley as Mosquito Surveillance Season Begins

[Photo courtesy of San Gabriel Valley Mosquito & Vector Control District]

The agency that monitors disease-carrying mosquitoes in Pasadena and Altadena launches its annual early warning system with heightened vigilance after the Eaton Fire

The traps are back.

Across the San Gabriel Valley, technicians from the region’s mosquito control district have begun setting surveillance traps to capture, identify, and test mosquitoes for the viruses they carry — the annual opening move in a public health effort that this year carries particular urgency for Pasadena and Altadena.

The San Gabriel Valley Mosquito & Vector Control District launched its routine seasonal surveillance program in March, marking the shift from winter’s low-activity baseline to the full monitoring schedule that will guide mosquito control operations and public health alerts through the peak summer months.

The District serves more than 1.5 million residents across 26 cities and unincorporated communities in the San Gabriel Valley, including Pasadena and Altadena, according to the agency’s website.

The stakes for the region are not abstract. In October 2023, Pasadena became the site of California’s first confirmed locally acquired dengue infection — a case identified by the Pasadena Public Health Department in coordination with SGVMVCD that required hospitalization and prompted neighborhood-wide mosquito control operations, according to a report published in the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report. In 2024, Los Angeles County recorded 14 locally acquired dengue cases, and 11 of them fell within the SGVMVCD’s service area, according to the Los Angeles Times.

West Nile virus, which has been endemic to LA County for more than two decades, remains the most persistent threat. In 2025, California reported 113 human West Nile virus cases statewide and 11 deaths, according to the California Department of Public Health. LA County has averaged 56 West Nile virus cases annually over the past five years, according to the county Department of Public Health.

The seasonal surveillance functions as an early warning system, according to the District’s website. Technicians deploy specialized traps to measure mosquito abundance and identify which species are active as temperatures rise. Mosquitoes captured in those traps, along with dead birds collected through reporting programs, are tested in the District’s laboratory for West Nile virus, St. Louis encephalitis, western equine encephalomyelitis, and Aedes-borne viruses including dengue.

The weekly results determine where and when the District deploys targeted control operations — which can range from larvicide treatments in storm drains and neglected pools to, in severe cases, truck-mounted adulticide applications — and when to issue public health advisories.

This year’s surveillance season begins against the backdrop of the Eaton Fire, which swept through Altadena on January 7, 2025, leaving behind what District Manager Jason Farned called “thousands of unmaintained swimming pools, damaged septic systems and debris-filled areas — ideal conditions for mosquitoes to grow,” in a statement reported by the Pasadena Star-News.

The District identified nearly 5,000 pools in and around the burn area and has been conducting treatment operations since March 2025. By mid-May of that year, 1,475 pools in the burn zone remained nonfunctional and filled with stagnant water, according to the Los Angeles Times. Farned has noted that a single unmaintained pool can produce up to a million mosquitoes in a month.

The District emphasizes a three-part prevention message for residents: tip out standing water from containers around the home, toss unused items that collect water, and protect against bites by using insect repellent with CDC-recommended active ingredients including DEET, picaridin, oil of lemon eucalyptus, or IR3535.

Mosquitoes can lay eggs in a container as small as a bottle cap and complete their life cycle in as few as seven days, according to the District.

The District, formed in 1989 under California Health and Safety Code, is one of five vector control districts in Los Angeles County. It follows an integrated vector management approach that treats pesticide application for adult mosquitoes as a last resort, prioritizing education, biological controls such as mosquito fish, and larvicide treatment.

The agency is headquartered at 1145 N. Azusa Canyon Road in West Covina. Residents can report mosquito problems, request inspections, or submit tips by calling (626) 814-9466 or visiting sgvmosquito.org.