Peteros and organizers with Hayes Valley Farm believe household pesticide was sprayed into vents on the sides of the hives.
"You could smell it distinctly," Peteros said. "We noticed a liquid dripping on the side also, and the dead bees smelled like household pesticide."
The dead bees were discovered late Tuesday afternoon. Each hive had between 60,000 and 100,000 each.
A third smaller hive was also sprayed with pesticide, and Peteros said about 60 to 70 percent of those bees died. She has been unable to find the queen. The smaller hive has smaller vent openings, which probably prevented more pesticide from being sprayed inside.
Peteros, who runs the nonprofit San Francisco Bee-Cause, placed the hives on the farm in partnership with Hayes Valley Farm and tends the hives herself.
A police report was filed, but Peteros believes there is little they can do to find the culprit.
"There's not much we can do about it except use the opportunity to make the point that bees are not harmful. They are only interested in flowers. They are beneficial insects," she said.
Peteros said honeybees do not pose a danger to humans and only sting people when provoked. Typically the stinging insects that attack people include wasps or yellow jackets, Peteros said.
"A honeybee, she does not want to sting. She stings, she dies," Peteros said.
The worst thing someone can do is thrash their arms or swat at the bee, Peteros said. Instead it's best to either slowly walk away from the area or gently direct the bee away.
The bees killed this week were brought in to pollinate the urban farm, serve as an education tool to teach others about urban farming and beekeeping, and provide honey to sell, the proceeds of which are returned to the organization for education.
Organizers of the farm believe the attack was not the result of teenagers or vandals but instead an adult who lives nearby. There was no evidence of vandalism on the farm.
The contents of the pesticide-sprayed hives must be discarded, and the boxes themselves will not be able to be used anytime soon because the toxicity of the pesticide lingers, though Peteros said she is unsure how long.
Even with a thorough cleansing, she said she would not want to take the chance of placing new bees in the hive for fear of killing them too.
The hives were kept in an open dirt area near the farm, which opened in January, but not directly within it.
The space is an old off-ramp to a freeway near the intersection of Laguna and Fell streets. It is being used as a farm through an interim-use agreement with San Francisco's Office of Economic and Workforce Development.
The city is working with farms and art projects to fill vacant lots as part of a "green development agreement," and the project is part of Mayor Gavin Newsom's Healthy and Sustainable Food for San Francisco directive.
Volunteer community members who work to develop sustainable urban agricultural methods and provide food to the community tend Hayes Valley Farm.
Start-up grant money from the city and fiscal sponsorship from the San Francisco Parks Trust got the farm going.
The organization holds classes and workshops on topics such as urban gardening basics, beekeeping, sustainable landscape design and more. It also holds yoga classes, outdoor film screenings and photo clubs.