Much of the decline, ranging up to 85 percent in some areas, is taking place in the industrialised northern hemisphere due to more than a dozen factors, according to a report by the UN's environmental agency.

They include pesticides, air pollution, a lethal pinhead-sized parasite that only affects bee species in the northern hemisphere, mismanagement of the countryside, the loss of flowering plants and a decline in beekeepers in Europe.

"The way humanity manages or mismanages its nature-based assets, including pollinators, will in part define our collective future in the 21st century," said UNEP executive director Achim Steiner.

"The fact is that of the 100 crop species that provide 90 percent of the world's food, over 70 are pollinated by bees," he added.

Wild bees and especially honey bee colonies from hives are regarded as the most prolific pollinators of large fields or crops.

Overall, pollinators are estimated to contribute 153 billion euros ($212 billion) worldwide or 9.5 percent of the total value of food production, especially fruit and vegetables, according to the report.

Honey bee colony declines in recent years have reached 10 to 30 percent in Europe, 30 percent in the United States,and up to 85 percent in Middle East, said scientist Peter Neumann, one of the authors of the first ever UN report on the issue.

But in South America, Africa and Australia there were no reports of high losses.

"It is a very complex issue. There are a lot of interactive factors and one country alone is not able to solve the problem, that's for sure. We need to have an international network, global approaches," added Neumann of the Swiss government's Bee Research Centre.

Some of the mechanisms behind the four-decades-old trend, which appears to have intensified in the late 1990s, are not understood. UNEP warned that the broad issue of countryside management and conservation was involved.

"The bees will get the headlines in this story," UNEP spokesman Nick Nuttall told journalists.

"But in a sense they are an indicator of the wider changes that are happening in the countryside but also urban environments, in terms of whether nature can continue to provide the services as it has been doing for thousands or millions of years in the face of acute environmental change," he added.

Nonetheless, scientists have been unable so far to quantify the direct impact of bee decline on crops or plants, and Neumann insisted that some of the impact was qualitative.

Citing British research, the report estimated that pollination by managed honey bees is worth 22.8 billion to 57 billion euros in terms of crop yields, and that some fruit, seed and nut crops would decrease by more than 90 percent without them.

One key driving force behind bee destruction in Europe and North America has been a type of mite, the varroa destructor pest, which attacks bees and that beekeepers struggle to control, Neumann said.

"It's quite shocking how little we know about this essential pest of honey bees although it has caused havoc in agriculture for more than 20 years."

"African bees are tolerant, we don't know why," he added.

Meanwhile, frequent changes in land use, degradation and fragmentation of fields, trade carrying hostile species such as the Asian hornet into France or virulent fungi, chemical spraying and gardening insecticides as well as changing seasons due to climate change have added to the hostile environment for bees.

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