As if Ash die back (Chalara Fraxinea) wasn't already a worrying enough threat to Britain's Ash (Fraxinus Excelsior) trees, there is now possibly another. The Emerald Ash Borer (EAB). This little creature feeds on the layers of ash. And according to this link, is estimated it can travel up to 24 miles in its lifetime and can lay around 200 eggs in its lifetime.
In its native land (Asia) the ash there are a slightly different species and so have co-evolved and strong enough resistance to it. However our European ash does not have that resistance. America already has this pest and it is estimated to have cost america around 10 billion dollars already. Now it has been discovered as close as Moscow. This article has more detail.