WASHINGTON (Reuters) -- Go to work, come home. Go to work, come home. Go to work -- and vanish without a trace. Billions of bees have done just that, leaving the crop fields they are supposed to pollinate, and scientists are mystified about why. The phenomenon was first noticed late last year in the United States, where honeybees are used to pollinate $15 billion worth of fruits, nuts and other crops annually. Disappearing bees have also been reported in Europe and Brazil. Commercial beekeepers would set their bees near a crop field as usual and come back in two or...