WASHINGTON (AFP) - The White House unveiled Tuesday a plan to reverse an alarming decline in the populations of bees and other pollinators that play a critical role in agriculture and the environment.

Honey bee pollination alone adds US$15 billion (S$20.4 billion) in value to US crops each year, wrote John Holdren, one of President Barack Obama's main science advisors.

Bees and other pollinators are responsible for pollinating more than a third of the US food supply, according to the Natural Resources Defence Council, a conservation group.

But pollinators are struggling for a variety of reasons, and over the past 12 months beekeepers lost 42 per cent of their honey bee colonies mostly in winter, the US Department of Agriculture estimated last week.

That loss marked the second worst year on record for bee mortality in the United States, according to the USDA. The worst was the 2012-13 season, with the loss of 45 percent of colonies.

This mysterious phenomenon has been observed since 2006, mainly in North America but also in Europe, and is known as "colony collapse disorder" - the more or less sudden death of millions of adult insects in beehives.

Scientists point to a series of factors: sickness, parasites, dwindling food sources and harmful pesticides.

The new US plan also seeks to rebuild populations of Monarch butterflies, other pollinators that are also in sharp decline.

Over the past two decades, the number of Monarchs migrating south, mainly to Mexico, in winter to escape the cold has dropped by 90 per cent.

To address the problem, the White House aims to limit bee mortality in winter to a maximum of 15 per cent over 10 years.

It also aims to restore 2.8 million hectares of habitat for these insects over the next five years through federal intervention and partnerships between the public and private sectors.

As for the Monarchs, the plan is designed to boost their population over the next five years in a 15-acre span of forest in Mexico, in collaboration with that government.

Many US government agencies will be called on to find ways to grow, on federal land, plants that are more varied and better suited to bees and other pollinators.

Scientists say growing just one crop on a large stretch of land denies bees a source of food.

The plan is based on an "all hands on deck" approach including engagement of citizens and communities and the forging of public-private partnerships.

The measures are the culmination of an appeal launched by Obama in June 2014 to develop a federal a strategy to save the bees.

"Increasing the quantity and quality of habitat for pollinators is a major part of this effort - with actions ranging from the construction of pollinator gardens at Federal buildings to the restoration of millions of acres of Federally managed lands and similar actions on private lands," Holdren wrote.

But the White House was more measured in its references to the impact of insecticides. It said they played an important role in agriculture.

"Mitigating the effects of pesticides on bees is a priority for the Federal government, as both bee pollination and insect control are essential to the success of agriculture," said Jeff Pettis, a senior entomologist at the USDA's Agricultural Research Service.

In April, the Environmental Protection Agency declared a moratorium on the use of pesticides called neonicotinoids until their risk to bees can be assessed fully.

The European Union has banned three major classes of neonicotinoids, which allegedly kill bees.

Environmental groups welcomed the White House plan but said it did not go far enough, especially with regard to cutting pesticide use.

"The president is right to elevate the urgency of this matter. While the task force's recommendations are a good first step toward saving them, more urgent action is needed to safeguard our food supply," said Peter Lehner, executive director of the Natural Resources Defence Council.

"We applaud the president's commitment to accelerate the review of several neonicotinoid pesticides. But to truly save bees and other pollinators, we must drastically cut down on today's pervasive use of neonicotinoids and other pesticides."