Though it happened over eight years ago, researchers are still interested in studying mass bee casualties that occurred at an Argyle Square parking lot in Wilsonville.
While the Xerces Society for Invertebrate Conservation initially identified the root cause of the 2013 event — which resulted in the death of thousands of bees — to be the severe overapplication of a pesticide, Oregon State University professor Sujaya Rao published a study in 2019 suggesting other possible factors at play such as memory- and learning-diluting properties in linden tree nectars, lower temperatures, the day of the event being late in the blooming season and a lack of nectar to sustain energy.
Recently, researchers including Rich Hatfield and Sarina Jepsen from the Xerces Society, Jonathan B. Koch from the U.S. Department of Agriculture, Isaak Stapleton from the Oregon Department of Agriculture and James P. Strange from the Department of Entomology at The Ohio State University released their own study that essentially confirmed the initial findings.
"Our work stands on its own and shows that this was a pesticide kill that had population-level effects on bumblebees in the Willamette Valley of Oregon," Hatfield said.
Hatfield, for his part, actually visited the parking lot two days after the first bees fell from the linden trees to the pavement in 2013.
"I was struck by the number of dead animals in the parking lot. It was shocking," he said. "You could literally hear them dropping to the pavement as you walked around the parking lot. That was my initial impression."
But what led him to want to publish the study, he said, was that some were blaming the bee casualties on a nectar called manos deriving from linden trees. The analysis included collecting many of the dead bees and analyzing them for pesticide concentration, as well as assessing how many bee colonies were affected. It estimated the death toll to be between 45,000 and 108,000, most of which were a single species (B. vosensenski).
"When we looked at the tissues of the bees themselves, the concentration of insecticides in these bees was well beyond what was known to be a lethal dose for similar bees," Hatfield said. "The bees and that level of dose in their bodies themselves, and the concentration of the nectar they were drinking from the analysis the ODA (Oregon Department of Agriculture) did, was at times 800% greater than what is considered a lethal dose."
Hatfield added that the pesticide dinotefuran was applied for the first time that day and had not been applied previously or thereafter.
"That's pretty clear evidence that insecticide is the major culprit in this," he said.
The study, however, doesn't discount that the nectar played some role in the casualties. The theory is that the alkaloid trigonelline in the linden nectar causes bees to attempt to collect nectar from trees that already have been exhausted of it.
One surprise of Hatfield's study, he said, was the high volume of colonies that were logged to have been affected in the event. The estimate, according to the study, was between 289 and 596.
"It suggests there's a fairly high concentration of nests along this area or these (bees) were flying fairly long distances to get there. That was a somewhat surprising result," Hatfield said.
Following the event, the city of Wilsonville created an Integrated Pest Management Plan and altered its use of pesticides. The state of Oregon, meanwhile, banned the use of dinotefuran on linden trees. Hatfield felt the state's measure didn't go far enough and should have applied to more trees that are attractive to pollinators.
"While I think that's a meaningful step to take forward and will likely prevent this event from happening in the future, I think it was too narrow of a ruling," Hatfield said. "I think they should have thought about more trees."
To read the study in its entirety, visit https://academic.oup.com/ee/advance-article/doi/10.1093/ee/nvab059/6305931.
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