Migrant Bee

Migrant Bees were a sapient insectoid species from Melissa Majoria which for a time resided on Earth among that planet's non-sentient bee population. Their decision to leave Earth and return home caused scientific concern on Earth about why the bee population had rapidly collapsed in the early 21st century. As the Tenth Doctor discovered, though, their departure was precipitated by sensing the oncoming danger of Davros moving the Earth to the Medusa Cascade.

History
At some point in Earth's history, a population of Bees from Melissa Majoria came to Earth, joining the native species. (TV: The Stolen Earth)

In 2009, the Bees detected a disturbance in the Tandocca Scale caused by the New Dalek Empire. They left Earth for Melissa Majoria, causing a major loss in bee populations. (TV: Partners in Crime, Planet of the Ood, The Unicorn and the Wasp, The Stolen Earth)

When Donna Noble was considering the extent of the human race during the 42nd century she mentioned the migrant bees. The Tenth Doctor commented that it was indeed "odd".

When the Tenth Doctor was investigating the loss of Earth, Donna pointed out the disappearance of the Bees. The Doctor was able to work out what that meant and tracked the disturbances to the Medusa Cascade. (TV: The Stolen Earth)

Behind the scenes

 * The name "Migrant Bee" is derived from the script of The Stolen Earth. Though the dialogue is actually said on screen, as well, the script makes it clearer that "Migrant Bee" is a name, rather like "Praying Mantis".
 * This fall in bee populations is mirrored in real life, and possibly inspired by the, for which the driving forces are still largely unknown. An edition of BBC Radio 4's science magazine, "The Material World", broadcast on 3rd July 2008 discussed this issue and included clips from the relevant episodes of Doctor Who.