About eight years ago a new insect pest, the horse chestnut leaf miner, was found on some horse chestnut trees growing in London. Since then insect pest has spread at an alarming rate throughout the UK with an estimated 70% of all horse chestnut trees infected. Infected leaves are covered in small brown patches which spread rapidly across the entire tree, giving an autumnal appearance by mid-late July. Eventually the leaves die and fall prematurely; when new ones grow they are again infected.
Growing season long control of horse chestnut leaf miner can be obtained by a single soil drench around the base of the tree with a systemic insecticide applied either in February, March, October or November. Generally an October or November drench will provide the greatest degree of control over the following growing season. Alternately spraying the tree with an insect growth regulator will provide a high degree of control. The insect growth regulator has the advantage that there are no effects on beneficial insects such as bees, beetles and ladybirds. Just leaf mining insects are affected.