A Target parking lot in Oregon became a mass grave for about 25,000 bumble bees which were found dead there earlier this week.

The incident, which took place in Wilsonville, Ore., southwest of Portland, is being called the largest mass death of bumble bees in regional history.

"There were literally falling out of the trees," Rich Hatfield, a conservation biologist with the Portland-based Xerces Society of Invertebrate Conservation, told local news station KPTV. "To our knowledge, this is one of the largest documented bumble bee deaths in the Western U.S. It was heartbreaking to watch."

Hatfield said the bees were found beneath blooming European linden tress. While the cause of the bees' death is unconfirmed, the Xerces group said it suspects acute pesticide poisoning due to a substance sprayed on the trees for cosmetic reasons or simply because the particular species of the European linden tree is toxic to bees.

KPTV reported that at least 150 bumble bee colonies were lost. Honey bees, lady bugs and other insects were also found dead at the site.

"If the trees are indeed toxic they should be cut down and replaced by something that will provide non-toxic pollen and nectar for bees," Scott Hoffman Black, Executive Director at the Xerces Society, said in a statement. "On the other hand, if pesticides are the cause, we need to spotlight this as a real-world lesson in the harm these toxic chemicals are causing to beneficial insects. It would be especially alarming to find out whether pesticides are the cause in this case because the linden trees are not even an agricultural crop. Any spraying that happened would have been done for purely cosmetic reasons."

Dying bee colonies have made headlines recently, renewing public awareness of the importance of bees to the food chain and growing seasons.

Known as "colony collapse disorder" the epidemic caused beekeepers in the U.S. to lose about one-third of their colonies last year. A possible cause of the endemic is the heightened use of a relatively new class of insecticides called neonicotinoids, which reduce bees' ability to learn scents, which hinders the insects' ability to find food.

Photos of the dead bees can be seen here.