A huge die-off of bees worldwide, a major threat to crops depending on the honey-making insects for pollination, is not due to any one single factor. The World Organisation for Animal Health (OIE) said yesterday that parasites, viral and bacterial infections, pesticides and poor nutrition resulting from the impact of human activities on the environment had all played roles in the decline. At normal times, bee communities naturally lose about 5 per cent of their numbers but with the syndrome known as colony collapse disorder, up to 90 per cent of the insects can be wiped out. In the US, government figures released last month showed a 29 per cent drop in bee numbers in beehives last year, coming on the heels of declines of 36 and 32 per cent in 2008 and 2007. The mysterious decimation of bee populations in the US, Europe, Japan and elsewhere in recent years threatens agricultural production worth tens of billions.