Corinne Fletcher stepped outside her apartment building Friday morning to find a pollinator genocide in the park that serves as her backyard.

Dead and dying bumblebees littered the sidewalk near the Market Street entrance to downtown Portland's Pettygrove Park. The carcasses were so thick, the Lewis & Clark College law student said, "you had to step carefully to not step on any bees."

The fifth mass bee death in Portland in the past several days has state investigators on the hunt for a cause.

Lab results from the first four die-offs are expected next week, said Mike Odenthal, lead pesticide investigator for the Oregon Department of Agriculture.

It's too early to say what killed the bees, but all of the die-offs share a similarity: The bees had been feeding on linden tree blossoms.

"We're trying to find out whether this is something humans did to the bees - a pesticide or pollutant or something - or is there something weird going on with the trees?" Odenthal said.

Every major bee die-off reported to the Oregon Department of Agriculture in the past few years has taken place near the trees, which are often sprayed with chemicals to control aphids.

"They don't harm the tree, but they secrete a honeydew that's considered a nuisance," said Aimee Code, pesticide program coordinator for the Xerces Society for Invertebrate Conservation.

A high profile poisoning in 2013 of more than 50,000 bees in a Wilsonville Target superstore parking lot raised public awareness about bee deaths. A class of insecticides known as neonicotinoids were at fault in the Wilsonville incident and several subsequent Oregon bee die-offs.

The incidents spurred state and local restrictions on the use of neonicotinoids. A statewide ban on the use of four types of neonicotinoids on linden trees and related species took effect in February. Portland followed suit in April with a ban on the use of all neonicotinoids on city lands.

After reaching out to Portland Parks and the Xerces Society, Fletcher took Code's advice to return to the park and collect bees for state testing.

By the time she returned, she said, "a lot of them were gone."

A worker in one of the adjacent buildings told her bees in the park have been dropping dead daily for more than a month.

Mark Ross, spokesman for Portland Parks & Recreation, said the agency does not spray insecticides at Pettygrove Park.

Most of the dead bees found Friday appeared to be concentrated on the sidewalk just outside the park, where bees were swarming several large linden trees in full bloom.

There's little chance they died of natural causes, Code said. Mass mortality events aren't part of the bumblebee life cycle.

But it's not certain insecticides or other chemical applications are to blame, either. When stressed, lindens sometimes produce a sugar that can be dangerous to bees.

If Fletcher collects enough bees to conduct testing, lab analysts will look for substances that could have killed them. If any banned neonicotinoids show up, investigators will have to ask another question: Were the chemicals applied before or after the state's ban took effect in February?

Neonicotinoids can remain present at levels lethal to bees for months after application on a tree, Odenthal said.

"It's possible we may have had a legal application of a now-banned product before we banned it, that's causing the activity now," he said.

What to do if you spot a bee die-off?

Call The Oregon Department of Agriculture, take a photo and ask whether you should collect a sample. As Fletcher experienced in Pettygrove Park, the bodies are often swept or blown away before state investigators arrive.

Code offers another piece of advice.

"Create a habitat in your yard that is pesticide-free with many options for foraging and food sources for bees," she said.

-- Kelly House

khouse@oregonian.com

503-221-8178

@Kelly_M_House