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SGV Mosquito District says mid‑winter black‑fly surge prompted early river treatments and new alerts
Summary
The San Gabriel Valley Mosquito & Vector Control District told the Duarte City Council that an unusually warm February and higher river flows triggered a black‑fly surge; the district began weekly Bti treatments early, expanded larval surveillance and launched activity alerts for residents.
Anais Medina Diaz, director of communications for the San Gabriel Valley Mosquito & Vector Control District, told the Duarte City Council the district saw a sharp increase in biting black flies in mid‑February after unusually warm weather and higher river flows created new breeding sites.
The district began black‑fly treatments “four weeks ahead of our actual routine operations,” Medina Diaz said, applying Bacillus thuringiensis israelensis (Bti) at the highest application rate allowed by law at known breeding sites along the river. She said Bti is a naturally occurring bacterium that “is not harmful to humans, wildlife, or birds” and targets mosquito and black‑fly larvae.
Why it matters: the council heard that black flies are a painful nuisance in foothill communities and can cause significant quality‑of‑life impacts when they surge outside the typical summer season. Medina Diaz said a single adult can lay 300–500 eggs and adults may live three to five weeks, and that black flies can travel several miles, complicating adult‑stage control efforts.
The district described steps it has taken and recommended public precautions. Medina Diaz said staff increased weekly treatments and launched expanded larval surveillance and a black‑fly activity alert system so residents can sign up for warnings and recommended steps such as using DEET‑containing repellents, avoiding peak activity times and using nets or face coverings for outdoor work.
Council members asked technical questions about life span and whether only females bite; Medina Diaz confirmed females require blood to produce eggs and said adult black flies are painful right away, unlike mosquitoes, which often produce delayed itching.
Medina Diaz also urged cooperation with other agencies that manage water releases and riparian vegetation, saying the district must coordinate with those stakeholders to make treatments more effective in upstream or adjacent jurisdictions. She said residents can report abnormal activity to help the district target surveillance and treatments.
The council did not take formal action on the presentation; staff and the district said they will continue outreach and share social‑media graphics and alert resources with the city.
