The "pollination deficit" could see a dramatic reduction in the yield from crops.
The research, carried out in the Rocky Mountains, Colorado, is the first to show that the effect is real and serves as a "warning" to Britain which if anything has seen an even greater decline in bees and pollinators.
"This serves as a warning to other countries," said Professor James Thomson at the University of Toronto, who carried out the research.
"For quite some time people have been suggesting that pollinators are in decline and that this could have an effect on pollination.
"I believe that this is the first real demonstration that pollination levels are getting worse. I believe it is a significant decline. I believe the pollination levels have dropped by as much as 50 per cent.