Vector control officials brief Duarte on black fly activity and local control steps

The San Gabriel Valley Mosquito and Vector Control District on Monday told Duarte officials that recurring black fly activity in foothill communities is tied to breeding in fast‑moving water and to regional water releases, and that control options are limited because adult black flies can fly miles from breeding sites.

Anais Medina Diaz, the district’s director of communications, said black flies breed in fast‑moving streams and rivers and can travel up to about five miles from emergence sites. She told the council the district uses larval surveillance strips along riverbanks and adult traps to detect activity and treats known breeding sites with Bacillus thuringiensis israelensis (Bti), a bacterial larvicide, at the maximum label rate during peak season.

“Black flies are a particular concern for foothill communities,” Medina Diaz said. “They typically target the face, particularly the eyes and the neckline, and their presence can significantly impact quality of life.” She added the district is strengthening larval surveillance, expanding monitoring at sites such as Tall Pines near Duarte and consulting with national programs to refine best practices.

The district recommended prevention measures residents can take: wear DEET‑based repellents (the district said DEET is the only repellent recommended for black flies), use a netted hat for face protection, wear long sleeves and report abnormal activity to the district’s hotline. Medina Diaz also said the district has launched a black‑fly alert system and encouraged residents to sign up.

The district described constraints on control: the river terrain is hard to access for treatments, dam water releases can trigger mass emergences and aerial or wide‑area adulticide treatments are impractical because black flies disperse broadly. The district said it treats larval sites weekly at label‑allowed rates and continues to evaluate new tools for long‑term control.