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An estimated 25,000 bumblebees have been found dead in a Target parking lot in Wilsonville since Saturday, the largest known incident of bumblebee deaths in the United States, according to the

. Preliminary information suggests pesticides may be at fault.

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Update: Officials from the Oregon Department of Agriculture confirmed the active ingredient in the insecticide Safari is responsible for the bee and insect deaths, and that the insecticide was originally sprayed to control for aphids.

]

The

received reports of bees and other insects falling out of 55 blooming European linden trees Monday from the Xerces Society for Invertebrate Conservation.

The bees were still dying on Wednesday. Yellow-faced bees fell from the trees, twitching on their backs or wandering in tight circles on the asphalt. Some honeybees and ladybugs were also found dead. A few dead bumblebees even clung to linden flowers, while hundreds littered the lot.

Dan Hilburn,

at the state Agriculture Department, surveyed the damage after an earlier assessment from pesticide experts.

"I've never encountered anything quite like it in 30 years in the business," he said Wednesday outside the Argyle Square Target.

Hilburn said initial findings indicate the trees were sprayed Saturday with an insecticide called

Tests to confirm what killed the bees will take at least two or three days, department officials said. The department of agriculture is also investigating other possible culprits, which may include other pesticides used in the surrounding area.

Safari is part of the neonicotinoid pesticide family. When it is sprayed on a plant, the leaves, flowers and nectar become toxic to almost all insects. The product's label on the distributor's website warns it is "highly toxic" to bees and tells applicators not to apply it "if bees are visiting the area."

"Bumblebees are the single most important natural pollinator in Oregon," said Mace Vaughan, pollinator program director for Xerces.

They play a crucial role in pollinating berries, flowers and other plants. The decline of the honeybee, whose populations have been decimated by

, has received much attention, but bumblebee populations are decreasing as well.

Elliot Associates Inc., the company that rents and manages the Argyle Square land, did not respond to multiple calls by The Oregonian. The landscapers that care for the grounds couldn't be reached for comment.

The Agriculture Department is working with the Xerces Society to help mitigate any further insect deaths at Argyle Square. As precautionary moves, they are considering either putting up netting around the trees, stripping off flowers and leaves or finding non-toxic repellents to keep bees and insects from eating the leaves or nectar.

Dale Mitchell, pesticide compliance program manager for the state agriculture department, said if test show pesticide is the culprit, the department will assess if the company responsible violated any state or federal laws, and if so, the severity of those violations. Fines for pesticide regulation infractions can range from $1000 to $10,000.

The bumblebee deaths marked an inauspicious start to National Pollinator Week, which runs through June 23.

-- Elizabeth Case