WILSONVILLE -- It's a mystery that has prompted an investigation by the State of Oregon. Thousands of dead bumblebees are blanketing a parking lot in Wilsonville.

The plaza, just off Interstate 5 that houses Costco, Target, a Sprint store and Panda Express to name a few, has about 65 European Linden trees.

Since the weekend, dead bumblebees have been falling from the trees. Experts estimate there have been more than 25,000 dead bees. On Sunday, the bees started falling from the trees until shoppers reported them to the various stores.

They were just evenly spread out over the entire parking lot, said Mace Vaughan, Pollinator Conservation Program Director with the Xerces Society for Invertebrate Conservation.

Monday, the calls flooded The Xerces Society, a nonprofit organization that protects wildlife through the conservation of invertebrates and their habitat.

(The were) just telling us that there seemed to be an incredible, awful bumblebee die-off and could we get out here to look and see what was going on, said Vaughan.

Wednesday, KGW found even more dead or dying bees falling from the trees.

I've never seen any sort of a die-off of bumblebees on this scale, said Vaughan.

The group called in the Oregon Department of Agriculture who opened an investigation. They said they're looking into whether a pesticide on the trees killed the bees.

We're aware of a pesticide application in the vicinity, but have not yet identified the active ingredient. We are in the process of interviewing parties that may have applied the pesticide, said Dale Mitchell with the Oregon Department of Agriculture.

But, they're also looking into whether the trees played a part. European Linden trees have been known to kill bees in Europe.

Vaughan and his staff have collected their own samples and they're sending them to a lab in North Carolina that specifically tests for insecticides in bees and flowers. Vaughan said this is devastating because bumblebees are incredibly important to us all.

The Willamette Valley is the heart of blueberry country, raspberry country, blackberry country. We are the biggest seed producing state in the union. All of those crops, bumblebees are critically important, probably the most important pollinator, he added.

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