Bumblebees, honeybees and ladybugs took a hard hit on Sunday when tens of thousands of them were discovered dead in a Target parking lot in Oregon.

Bumblebees fared the worst, with an estimated 25,000 dead and 150 colonies lost. And this at the beginning of National Pollinator Week, no less. Bumble bees are crucial to farming in western Oregon, where they are vital pollinators of many berry and seed crops.

“They were literally falling out of the trees," said Rich Hatfield, a conservation biologist with the nonprofit bee-advocating Xerces Society for Invertebrate Conservation. "To our knowledge this is one of the largest documented bumble bee deaths in the western U.S. It was heartbreaking to watch.”

Together, the Xerces Society and the Oregon Department of Agriculture (ODA) collected samples of the dead insects.

Xerces discovered that landscapers had sprayed 65 European linden trees on Saturday with the insecticide Safari. The insecticide is described by manufacturer Valent as a "super-systemic insecticide with quick uptake and knockdown,” reports UPI.

"[The landscapers] made a huge mistake, but unfortunately this is not that uncommon," said Scott Hoffman Black, executive director of the Xerces Society. "Evidently they didn't follow the label instructions. This should not have been applied to the trees while they're in bloom."

Yet ODA was not ready to pin the blame so quickly.

"I don’t think we’re there yet," said ODA Communications Director Bruce Pokarney. "We’re looking at any other pesticide applications that might have taken place in the area that might have come into play. Until we get all that figured out, we stop short of saying this is the culprit or the likely culprit. It’s one of the possibilities we’re looking at. A very strong possibility."

It couldn’t have come at a more poignant, or ironic, time. The United States Department of Interior designated this year’s National Pollinator Week as June 17-23, 2013. Every year, the week serves to highlight the invaluable services provided by our pollinators, including bees, birds, butterflies, bats and beetles. One out of every three bites of food we take comes from foods that are dependent on pollinators, so we really can’t afford to lose more.

Related on MNN: What a grocery store without bees looks like

25,000 bees found dead in Oregon; pesticide suspected
Experts say it is one of the largest documented bee deaths in the Western U.S.