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WILSONVILLE, Ore. Scientists investigating a bee die-off in Wilsonville are calling it the largest mass bumblebee death on record, with more than 50,000 reportedly dying.

The Oregon Department of Agriculture determined the pesticide Safari is responsible for the death of the bumblebees in a shopping center parking lot this week, but it said the death toll is closer to 25,000.

The Xerces Society for Invertebrate Conservation has been investigating the bumblebee die-off since last weekend when the bees started falling from a cluster of 65 European Linden trees near a plaza just off Interstate 5. It said the bees likely represented more than 300 wild colonies.

Each of those colonies could have produced multiple new queens that would have gone on to establish new colonies next year. This makes the event particularly catastrophic, said Rich Hatfield, a biologist with the Xerces Society in a press release.

More: State investigating death of 25,000 bumblebees

Public and private partners were putting up bee-proof netting to prevent more bees from dying. The pesticide was reportedly applied to the trees on June 15 to control aphids. Safari s active ingredient is dinotefuran, part of a group of insecticides known as neonicotinoids.

The ODA is continuing an investigation to determine if the pesticide was in violation of state and federal pesticide regulations.

The ODA also thought the trees may have been responsible for the dying bees because European Linden trees have been known to kill bees before. But after collecting samples earlier in the week, the ODA received laboratory analysis that confirmed the pesticide was killing the bees.

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