How valuable are bees? In the UK, about £1.8bn a year, according to new research on the cost of hand-pollinating the many crops bees service for free. If that sounds a far-fetched scenario, consider two facts.

First, bees are in severe decline. Half the UK's honey bees kept in managed hives have gone, wild honey bees are close to extinction and solitary bees are declining in more than half the place they have been studied.

Second, hand-pollination is already necessary in some places, such as pear orchards in China, and bees are routinely trucked around the US to compensate for the loss of their wild cousins.

The new figure comes from scientists at the Reading University and was released by Friends of the Earth to launch their new campaign, Bee Cause. Paul de Zylva, FoE nature campaigner, said: "Unless we halt the decline in British bees our farmers will have to rely on hand-pollination, sending food prices rocketing."

So what's the problem? The losses of flowery meadows that feed wild bees is a factor, as are the parasites and diseases that can kills hives. But a third factor has now moved to the centre of the debate: pesticides called neonicotinoids. The insect nerve-agents are used as seed dressings, which means they end up in every part of the crop they protect, including pollen and nectar.

Two landmark studies, conducted in field conditions, published in Science in March clearly implicated sub-lethal doses of the pesticides with increases in disappeared bees and crashes in the number of queens produced by colonies. Then on 5 April, another study was released, showing the pesticides can cause colony collapse disorder (CCD), the name given to the ghostly hives from which bees have vanished.

"The data, both ours and others, right now merits a global ban," said Chensheng Lu, in the department of environmental health at Harvard University, and who led the the CCD study. "I would suggest removing all neonicotinoids from use globally for a period of five to six years. If the bee population is going back up during the after the ban, I think we will have the answer."

Lu told me he was in no doubt about the result of his work, which tested the effect of a very widely used neonicotinoid called imidacloprid, which is registered for use on over 140 crops in 120 countries. "Our study clearly demonstrated that imidacloprid is responsible for causing CCD, and the survival of the control hives that we set up side-by-side to the pesticide-treated hives augments this conclusion." He said the hives were initially healthy, were placed in a natural foraging environment and that the doses of the pesticide the bees were exposed to were realistic.

After 12-weeks of dosing, all the bees were alive, but after 23 weeks, 15 of the 16 treated hives had died - but none of the untreated control hives. Lu said the dead hives were virtually empty, as is seen with CCD, and in contrast to the impact of parasites or disease, which leave hives littered with dead bees.

The leader of one of the Science studies, Mickaël Henry, at INRA in Avignon, France, agreed with Lu that action is urgently needed on neonicotinoids. "We now have enough data to say authorisation processes must take into account not only the lethal effects, but also the effects of non-lethal doses." In other words, testing whether the pesticide use kills bees stone dead immediately is no longer good enough, given the hard evidence now available that sub-lethal doses cause serious harm.

So what does the UK government have to say? To date, it has agreed with the neonicotinoid manufacturers that there is no evidence that the pesticides used at normal levels cause harm. A statement on Wednesday from an environment department spokeswoman suggests no change: "The UK has a robust system for assessing risks from pesticides. We keep all the science under review and we will not hesitate to act if we need to."

What more does it need? The new data makes it impossible to maintain this position, whatever vested interests are at stake. It is 50 years since Rachel Carson published Silent Spring, which documented the devastation wrought by pesticides in the US. What better time to act?