This helicopter, owned by Hummingbirds Helicopters Inc. of Valley Center, was recently conducting an aerial assault on mosquito larvae in ponds along the San Diego River.
This particular chopper was spraying larvicide onto the edges of the pond at Lakeside River Park. They target the shallow edges of lakes and ponds, where larvae live. The larvae eat the red flaky looking poison (which is allegedly harmless to humans) and die before they mature.
With the threat of West Nile virus, you may see this type of aerial application more often along standing bodies of water, ponds, lakes, and creeks in San Diego County.