Apparently they're Simulium vittatum.
Residents in the San Gabriel Valley (SGV), to the east of Los Angeles, are battling unusually high numbers of black flies, also known as Simulium vittatum.
https://www.glamosquito.org/black-flies
Control Agents Used to Combat Black Flies
Black fly management is accomplished by applying a natural soil bacteria called Bti to water sources where black flies breed. Bti stands for Bacillus thuringiensis israeliensis. Bti is a special formulation of common soil bacteria that is highly effective in controlling black flies and mosquitoes.
https://en.wikipedia.org/wiki/Bacillus_thuringiensis_israelensis
Bacillus thuringiensis serotype israelensis (Bti) is a group of bacteria used as biological control agents for larvae stages of certain dipterans. Bti, along with other B. thuringiensis products, produces toxins lethal to various species of mosquitoes, fungus gnats, and blackflies but which have negligible effects on other organisms.