Bumblebees were the species hardest hit, with an estimated 25,000 dead and 150 colonies lost.
“They were literally falling out of the trees," said Rich Hatfield, a conservation biologist with the nonprofit 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.”
The Xerces Society contacted the Oregon Department of Agriculture (ODA) and together they collected samples of the dead insects.
Xerces concluded 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."
But state investigators say they won’t blame the landscapers until they have investigated other pesticide applications in the area.
"[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."
"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."
The Xerces Society started getting calls about dead and dying bees on Monday and Black said more bees are still dying. "It's still going on," said Black. "My staff is out there right now and there are still dead and dying bees."
ODA is considering pruning the flowers off of the linden trees that were sprayed, to prevent the trees from attracting more insects that will end up dying from pesticide poisoning.
National Pollinator Week is an annual event meant to draw attention to the role of beneficial pollinating insects in ecosystems and agriculture. The event is sponsored by several pesticide manufacturers, including Valent.