Bee

Bees are a species of insect native to Earth.

In 2009, while researching strange occurrences, Donna Noble noticed that the bees were disappearing. When she told the Doctor about this (on two separate occasions), he agreed that this was odd. (DW: Partners in Crime, Planet of the Ood) It was also mentioned in Turn Left

Some fans have speculated that disappearing bees will be part of an overarching arc for the Fourth series, similar to Bad Wolf and Mr. Saxon previously. It may also be related to the real-life fall in bee populations due to Colony Collapse Disorder.