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.

Biology
Migrant Bees were similar to the bees native to Earth, except that they were intelligent and used the Tandocca Scale as a carrier signal. (TV: The Stolen Earth)

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)

When the Tenth Doctor was investigating the loss of Earth, Donna Noble 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) It is unknown whether the bees of Melissa Majoria returned to Earth after the Daleks were defeated.

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 species name, rather like "Preying Mantis".
 * This fall in bee populations is mirrored in real life, and possibly inspired by the Colony Collapse Disorder, 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".