Melissa Majoria was the homeworld of Migrant Bees, some of which made their way to Earth. In early 2009, the bees sensed that a catastrophe was about to befall the Earth, and began to make their way back to Melissa Majoria. Donna Noble noticed this, and remarked it to the Tenth Doctor several times. He also found it odd. (TV: Partners in Crime, Planet of the Ood)
The catastrophe was Davros moving the Earth to the Medusa Cascade. The bees' movement created a disturbance on the Tandocca Scale, which allowed the Doctor and Donna to trace their path towards the Earth. (TV: The Stolen Earth)
Behind the scenes
The phenomenon of bees disappearing is a real-life concern which has garnered some fair media attention.