The Oregon Department of Agriculture received reports of bees and other insects falling out of 55 blooming European linden trees onto a Target store parking lot beginning Monday, The (Portland) Oregonian reported Friday.
In addition to the bumblebees, some honeybees and ladybugs were also found dead throughout the week, officials said.
"I've never encountered anything quite like it in 30 years in the business," Dan Hilburn, director of plant programs at the state Agriculture Department, said.
While the trees had been sprayed Saturday with an insecticide called Safari, tests to determine exactly what killed the bees will take at least two or three days, department officials said.
If tests show the pesticide is the cause of the insect deaths, the department will assess if the company responsible for the spraying violated any state or federal laws, officials said.
Fines for pesticide regulation infractions can range from $1,000 to $10,000, The Oregonian reported.