In the case of the Horse-chestnut earthdragon makes a good point regarding disease. UK Horse-chestnuts are reeling under an infestation of a small brown and white striped moth
Cameraria ohridella whose larva the Horse-chestnut Leaf Miner chews between upper and lower surfaces and prematurely kill off the leaves. Interestingly, this is a case of one alien species attacking another. Although a familiar and much loved tree the Horse-chestnut is an alien in Britain, albeit one that has been here for around 400 years. It originated in Northern Greece and, surprise surprise, that is where
Cameraria ohridella - first spotted in the UK on Wimbledon Common in 2002 - was also discovered in the 1970s. Distribution maps show it has fanned out from London into East Anglia, the West Country and as far up as North Yorkshire. It's also on the Isle of Wight - here's a picture taken a few days ago:
Rob