The mystery of what killed thousands of honeybees bivouacked in two oak trees in downtown St. Paul was solved Wednesday, when an official said that fire crews had sprayed the bees with fire retardant foam in response to a police call for help.

"During the day we might have called animal control or other resources, but it was just a few minutes before midnight on a Sunday night," St. Paul Fire Marshal Steve Zaccard said. "We were trying in good faith to avoid injuries or panic."

He added: "I've never heard of this before, and I've been here 28 years."

The foam caused the bees to drop dead to the sidewalk below the trees, creating dark-colored mounds that one pedestrian the next morning likened to "small snowdrifts."

Given a recent national decline in honeybee populations associated with insecticides, pesticide investigators with the state Department of Agriculture went to the scene Monday to retrieve samples of the bees, tree soil and vegetation for clues on what happened.

They discontinued their work when learning that the bees were killed with water spray.

It was a bittersweet resolution for Marla Spivak, a University of Minnesota entomologist who feared that insecticides had been used. At least 25,000 bumblebees in linden trees near Portland, Ore., were killed last month by a pesticide used to control aphids.

Using foam on bees is "what fire departments have been instructed to do nationwide," Spivak said. "If you've got a bee problem, it's a good way and a fast way to get rid of it. It's a much better way to do things than insecticide."

Still, she said, it would have been better had officials contacted a beekeeper to move the swarm, which she estimated contained 25,000 to 30,000 bees.

Lorna Call, a Grant beekeeper, said she was called by a fire official Monday to remove a remaining football-sized cluster of bees from the trees. By then, she said, the bees had been too badly damaged to save.

Two phone calls

Sgt. Paul Paulos said that police got two calls Sunday evening about swarms of bees in two trees near the corner of Kellogg Boulevard and St. Peter Street, a busy intersection across from City Hall.

"The officers used their best common sense and notified [the fire department]," he said.

Zaccard said that two fire crews arrived shortly before midnight and sprayed the trees with foam, using a nozzle mounted on the engine's front bumper. They left about 12:30 a.m., he said.

Spivak said that night is the best time to kill bees, since they don't fly then. Also, swarms are generally docile despite the fact that they look scary, Call said.

If people are looking for a beekeeper to move a swarm, they can call Bob Sitko of the Minnesota Hobby Beekeepers Association at 651-436-7915. It may be too late for this year, since Spivak said that we're closing in on the end of swarm season.

"A colony of bees grows just like a plant, and they normally get to their maximum size in Minnesota in early June, when half would take off looking for a new place," she said.

That's likely what this swarm was doing — camping in trees while waiting to find a new home — when they were killed.

"This was an unfortunate situation," Paulos said. "Bees are cool. They help pollinate and they are a necessity to nature. But in this case, it was a public safety issue."