In the story of climate change, humans and the carbon dioxide we pump into the atmosphere are the villains of the piece. Now, it seems that we have an accomplice and a most unexpected one at that. It lives in the pine forests of North America and even though it measures just 5 millimetres in length, it is turning these woods from carbon sinks into carbon sources. It’s the mountain pine beetle.

The beetle bores into pine trees and feeds from nutrient-carrying vessels called phloem. It also lays its eggs there. Once a beetle has colonised a pine, it pumps out pheromones that attract others, which descend on the tree en masse and overwhelm its defences. The infestation damages the