A lethal combination of pesticides, parasites and disease, coupled with a shortage of flowers, has been identified as the cause of a perilous decline in honeybees that could soon threaten the nation’s food supply.

But there is insufficient evidence to single out insecticides that many beekeepers blame for the die-off, and which may be banned in Europe, federal officials said Thursday in releasing a comprehensive new report on the health of honeybees.

In a telephone news conference, they said that bees are threatened by many complicated hazards and that the costs of a pesticide ban might exceed the benefits. They called for more research on the role of agricultural chemicals, and, in the meantime, further steps to protect bees and other pollinators.

“As in most things biological, there is no smoking gun,” said Sonny Ramaswamy, director of the National Institute of Food and Agriculture at the U.S. Department of Agriculture.

The problem could be especially acute for beekeepers in Minnesota and the Dakotas, which are among the nation’s top five honey-producing states. But it’s also a problem for the nation’s food system. Bees are key to the production of $20 billion to $30 billion worth of food each year, including such crops as alfalfa, strawberries and soybeans. Fully 100 food crops rely on pollination.

Although the long-awaited report suggests possible actions, beekeepers and others said it’s unlikely to resolve the increasingly contentious debate around bees and pesticides. Beekeepers and some environmental groups last month filed suit against the U.S. Environmental Protection Agency for failing to accelerate its review of the insecticides, and have criticized the agency for failing to examine effects of the long-term, low-dose exposure that bees typically experience.

Meanwhile, the bee die-off may be worsening. Zac Browning, a North Dakota beekeeper who joined scientists and federal officials in Thursday’s call, said the problem may soon reach the crisis point.

“We are on the brink,” he said. “We are getting there very fast.”

This year, he said, beekeepers had trouble providing enough colonies to pollinate the California almond crop, and the same may be true this spring for Maine’s blueberry crop.

Colony collapse

Commercial honeybees have been in decline since 2006, when the term “colony collapse disorder” was first coined to describe the mysterious decimation of bee hives. Since then beekeepers have reported losing up to a third of their hives each year. Last year the losses were especially grim, wiping out 40 to 50 percent of the hives in some areas.

Steve Ellis, a beekeeper near Barrett, Minn., who joined the federal lawsuit, said he lost 65 percent of his hives, leaving him only 1,030 to truck to California for the almond crop. “There is no question that we are in a crisis,” he said. “The only question is whether it’s a survivable crisis.”

The federal report, a consensus of available science on the bee die-offs, came out the same week that the European Union took steps to ban a class of insecticides called neonicotinoids because of recent research linking long-term exposures to health and behavioral problems in bees.

But the U.S. scientists who compiled their report said they don’t think there is enough evidence to ban the chemicals in this country; they called for more research on the effects of long-term, low-dose exposures. Pesticide manufacturers have also said there is not enough evidence to tie their products to the decline in bees.

“There are non-trivial costs to society if we get this wrong,” said Jim Jones, the EPA’s acting assistant administrator for chemical safety and pollution prevention. The insecticides help farmers, keep food costs down and would be replaced by chemicals that could be more damaging to insects and the environment, he said.

Marla Spivak, a bee scientist at the University of Minnesota who contributed to the federal report, said Europe is more willing to ban pesticides based on perceived risk.

“The U.S. has a much stricter policy and approves pesticides until [it’s] proven that they are a problem,” she said in an e-mail.

Among the causes of colony collapse identified in the new study is a disease-carrying parasitic mite called Varroa. The report also cited studies that have found hundreds of different chemicals in hives, some of which are used by beekeepers to control the mites. It’s not clear how they interact with one another, said May Berenbaum, a bee researcher from the University of Illinois and a lead author of the new report.

That’s true, said Vera Krischik, a U of M entomologist who studies the effects of insecticides on bees. But the addition of the neonicotinoids in the mix plays a crucial role in the bees’ demise, she said. Research has shown that it affects their neurological function and their ability to forage, she said. And the pesticides are not just used in crops, she said, but are widely used in common household garden products.

“It is clear to me that there is a link,” she said.

In the meantime, the best defense for bees is finding more places to plant the flowers they need to survive — in yards, along roadsides, along trails and in public parks, researchers say.

“So they are better able to combat the effects of diseases, pests and pesticide exposure,” Spivak said.