THE world's honeybees appear to be dying off in horrifying numbers, and now consensus is starting to emerge on the reason why: it seems there is no one cause. Infections, lack of food, pesticides and breeding - none catastrophic on their own - are having a synergistic effect, pushing bee survival to a lethal tipping point. A somewhat anti-climactic conclusion it may be, but appreciating this complexity - and realising there will be no magic bullet - may be the key to saving the insects.

A third of our food relies on bees for pollination. Both the US and UK report losing a third of their bees last year. Other European countries have seen major die-offs too: Italy, for example, said it lost nearly half its bees last year. The deaths are now spreading to Asia, with reports in India and suspected cases in China.

But while individual "sub-lethal stresses" such as infections are implicated, we know little about how they add together. The situation should become clearer in the next few years as the US government, the EU and others are pouring money into bee research. The UK, for example, has doubled its annual research budget, allocating £400,000 a year for the next five years.

On top of that, the UK National Bee Unit will get £2.3 million to map the problem. This money is urgently needed, says Peter Neumann of the Swiss Bee Research Centre in Berne, who runs COLLOSS, a network of researchers studying colony loss in 36 countries. "We don't have the data to assess the situation in Europe, never mind the world," he says.

The main stress facing bees is the varroa mite, a parasite from Siberia that has now spread everywhere but Australia. Mite infestations steeply reduce bees' resistance to viral infection. Worryingly, the mites are developing resistance to the pesticides used to control them, forcing beekeepers to use methods that are often less effective.

French and German beekeepers blame their losses on insecticides called neonicotinoids - but France banned them 10 years ago and its bees are still dying. Neumann suspects a wider problem, citing experiments showing that agricultural chemicals that are safe for bees when used alone are lethal in combination. "Farmers increasingly combine sprays," he says. They also leave few flowering weeds, depriving bees of essential nutrients from different kinds of pollen, he adds.

Meanwhile viruses may cause a syndrome dubbed colony collapse disorder (CCD) in the US, in which adult bees abandon their hive, leaving the healthy queen and young bees to die. Diana Cox-Foster of Penn State University in University Park, where the syndrome was first identified, says viruses, including one called IAPV, duplicate the symptoms of CCD in her greenhouse studies. There is no IAPV or CCD in the UK, says Mike Brown of the National Bee Unit, yet bees are still dying.

At the root of the vulnerability to these stresses could be the way breeding has affected the bees' genetic make-up. By being highly selected for calmness and honey production, honeybees have lost other useful characteristics, says Francis Ratnieks of the University of Sussex, UK. In research to be published in the journal Heredity, he describes a way to breed for "hygienic" bees that, unlike most commercial bees, clear out infected young and can resist varroa mites.