Vector-control district: black flies are a nuisance tied to high river flows; weekly BtI treatment and public tips offered

Tristan Hallam, director of scientific programs for the San Gabriel Valley Mosquito and Vector Control District, briefed the Duarte City Council on Aug. 26 about rising black-fly nuisance in the Foothill area and the districts response.

The nut graf: black flies breed in fast-flowing rivers and can travel long distances; the district is applying larvicidal treatments at permitted rates, expanding surveillance and urging residents to use repellents and report unusual activity so the agency can time treatments effectively.

Hallam explained that black flies in Los Angeles County are primarily a painful nuisance rather than disease vectors locally, and that high river flows create ideal breeding conditions in the San Gabriel River. Technicians are treating the river and other breeding sources weekly with Bacillus thuringiensis israelensis (Bti) at the highest rate allowed by federal regulations and have expanded larval surveillance to detect population increases early. "Bti is a naturally occurring soil bacteria that only affects mosquitoes and blackfly larvae," Hallam said.

The district also provides prevention guidance: wear protective netting, use DEET-based repellents (which the presenter identified as the only repellent proven effective against black flies), and turn off backyard water features weekly for 24 hours. Residents were asked to report unusual activity via sgvmosquito.org or by phone. Hallam noted the district is publicly funded through local property taxes and provides these services without an additional charge.

Ending: Councilmembers asked for a future agenda presentation to share more information with the community and thanked the district for proactive treatment and outreach.