I've been asked a couple of times recently about the long term prognosis for horse-chestnut trees. The simple answer is "in the worst case scenario, it's bleak".
Here's the background to why we think that is the case:
The horse-chestnut leaf-miner (the focus of Conker Tree Science) does not kill horse-chestnut trees. It does however affect the 'photosynthetic capacity' of the trees. Over the course of a year the 'net primary productivity' (which basically means the amount of food the tree makes for itself) declines by about one-third. (That is according to two scientific papers published in 2004 and 2011.)
At the the same time that the leaf-miner has spread, another more serious disease, called bleeding canker has also spread in our horse-chestnuts. This can cause the death of the tree, and branches that have been badly affected can be dropped by the tree. Local authorities and tree owners have been carefully checking their horse-chestnut trees, and trees have been managed appropriately and in some cases they have been felled.
It is possible that the spread of the leaf miner and the effect it has on the tree's photosynthetic capacity is weakening trees, so making them more susceptible to bleeding canker, though this requires further research.
So, the bleeding canker, may ultimately lead to many horse-chestnut trees being felled. That wouldn't be so bad if trees were being re-planted. However, sales of horse-chestnut trees have declined by 98% over the past 5 years.This is presumably because few people want to replant horse-chestnut trees that look so moth-eaten (quite literally!) by mid-summer.
So, even though the leaf-mining moth doesn't kill our trees, it is intrinsically linked to the possible demise of our conker trees.