Thousands of dead bees in a parking lot in Oregon are prompting an investigation. About 25,000 bumblebees were found dead in a "colony collapse disorder".

The dead bees were found in one of the largest mass deaths of bumblebees to date in the western US. They were found in Wilsonville, Oregon.

The Xeres Society, an Oregon environmental organization, says the 25,000 dead bees were found underneat blooming European linden trees in a Target parking lot.

"I've never seen an incident on this scale," said Mace Vaughan, pollinator conservation program director for The Xerces Society. ,

"When I was here Monday, it was even more dramatic than it is today," Hatfield told press. "There were bees raining out of trees."

The bumblebees' mass death may be due to a pesticide used on the trees, or the tree species may be toxic to the bees..

Bees have been dying rapidly in the U.S. since 2005, and researchers are not sure why.  It may be due to a new kind of insecticide that reduces bees' ability to learn scents, making it hard for them to find food.

About a third of beekeepers in the U.S. lost their colonies last year alone in the epidemic, dubbed "colony collapse disorder".

Colony collapse disorder means there are many less bees to pollinate crops nationwide. Bees are critical to agriculture.

"[The dead bees found in the parking lot] brings it home," Hatfield said, "that we've got a lot of work to do to learn how to not harm these insects that are critical to our food supply."

The Xeres Society and the Oregon Department of Agriculture is currently working to determine the cause of the bees' death.