It's one of the most perplexing environmental mysteries of recent years: Why are honeybees dying? And what can be done to stop a catastrophic disaster with far-reaching economic and environmental consequences in the United States and beyond?

Scientists don't yet have a definitive answer. But a U.S. Department of Agriculture and Environmental Protection Agency report issued Thursday suggests a complex mix of problems contributing to honeybee colony declines : parasites, disease, genetics, poor nutrition and, perhaps most immediate, pesticide exposure .

Range of the problem

Since 2006, multiple bee species have been dying off in the millions in a phenomenon known as colony collapse disorder. Experts mapped the decline in the most detail within a year, in 2007 . Here's what they estimated:

• The number of bee colonies in the U.S. has been cut in half since 1945.

• The die-off is happening in at least

half of the states, Oregon included.

• Economically, between $20 billion and $30 billion in U.S. agricultural production is at stake.

Scientists involved in the recent study said the examination of dead bees found residue of more than 100 chemicals, including insecticides and pesticides used to control parasites in beehives.

This week's report warns that even with intensive research to understand the cause of honeybee colony losses in the United States, losses continue to be high and could pose a serious threat to meeting the pollination demands for some commercial crops.

Bee economics

It's not just honey. The USDA estimates that one-third of all food and beverages are made possible by pollination, mainly by honeybees.

What's being done

The federal report came the same week that European officials took steps toward banning a class of pesticides known as neonicotinoids, derived from nicotine, that they consider a critical factor in the mass deaths of bees there. But the USDA, the EPA and others involved in the bee study here said there was not enough evidence to support such a ban , and that the costs of such action might exceed the benefits.

“There is no quick fix,” said May Berenbaum, an entomologist (insect scientist) and a study participant. “Patching one hole in a boat that leaks everywhere is not going to keep it from sinking.”

What's next

Farmers, beekeepers, pesticide manufacturers, government researchers and academics are scheduled to meet this summer to further study the bee die-off and pesticide use, and to brainstorm possible solutions . Unfortunately, this summer could be a tough one for industrial beekeepers around the world.

Sources: New York Times, McClatchy-Tribune, The Associated Press