As the estimate of dead bees rose to 50,000, the Oregon Department of Agriculture confirmed the insecticide Safari caused the deaths in a Wilsonville earlier this week. A landscaping company sprayed 55 linden trees in a Target parking lot to control for aphids, said Dan Hilburn, the plants division director at the department of agriculture. The bees have been dropping from trees since the spraying on Saturday.

Aphids produce honeydew, a sticky liquid that can drip off onto cars or pedestrians. A Target representative said by email that the Wilsonville store had received no customer complaints about it.

The Portland-based Xerces Society, which first reported the die off to the state Agriculture Department said aphids can be controlled without insecticides, including by spraying infested plants with soapy water.

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On Friday the Agriculture Department, the City of Wilsonville, neighboring towns and some local landscape contractors covered the sprayed trees with netting in an attempt to prevent further insect deaths.

The state is investigating any violation of pesticide laws, which could take up to four months, said Dale Mitchell of the Agriculture Department.

Mitchell said the bee deaths, the largest documented die-off of bumblebees, could prove important in determining the use and regulations of Safari and other insecticides in the United States.

Safari's main ingredient is dinotefuran, a neonicotinoid. There are two main kinds of neonicotinoids, both of which are general use insecticides. Safari is a member of the nitro-group. Research published in 2012 shows these are generally more toxic to bees than the other type. The European Union issued a temporary ban on three other kinds of nitro-group neonicotinoids, which will go into effect this December.

Elliot Associates, the land management company that rents the affected Argyle Square lot, released the following statement Thursday: "If a cause is discovered which may be attributable to our actions or those of our contractors, a proactive course of action will be taken after consulting with the DOA and other experts in this field."

Meanwhile, other reports of bees dying around Wilsonville and surrounding towns have prompted Xerces to check whether similar pesticides were used elsewhere.

"My worry is that we're going to lose sight of the real message," said Mace Vaughan of Xerces. "I think we're (using insecticides) all over the place, and people are doing it in their backyards without even knowing it."

The agriculture department and other related groups will meet Monday to discuss any further action.