When Donna Noble was considering the extent of the human race during the 42nd century she mentioned the migrant bees...
Donna: Back home, the papers and the telly, they keep saying we haven’t got long to live. Global warming. Flooding. All the bees disappearing. The Doctor: Yeah. That thing about the bees is odd.
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The Doctor: Donna, come on, think: Earth, there must have been some sort of warning. Was there anything happening back in your day, like... electrical storms, freak weather, patterns in the... sky? Donna Noble: Well, how should I know? Um, no. I don't- I don't think so. No. The Doctor: [disappointed] Oh, OK, nevermind. Donna Noble: Although, there were the bees disappearing. The Doctor: [dismissive] The bees disappearing. The Doctor: [sarcastic] The *bees* disappearing. The Doctor: [revelational] The bees disappearing!