'Once the bees have left the earth, man will have four years left on the planet', Albert Einstein once said. It has not happened yet, but the world's bee population is in steep decline. It matters because bees pollinate the plants that produce a third of all the food mankind eats. Now there is growing evidence that certain pesticides may be contributing to the bees' decline - they are called neonicotinoids and their use is already restricted in some countries, but not the UK. Al Jazeera's Simon McGregor-Wood reports from London.