What is killing Britain's honey bees?

With British honey bee numbers halving in the past 25 years, scientists have been using trackers to monitor the insects to find out why so many are dying.

The research is the subject of a special episode of Horizon presented by BBC Breakfasts' Bill Turnbull.

Stephan Wolf from the Rothamsted Research Institute explains how the process works.

What's Killing Our Bees - A Horizon Special, BBC Two, 9pm