This moth Cydalima perspectalis, pyrale du buis in French, and its caterpillars came to Europe from Asia accidentally around 10 years ago and can cause widespread problems to gardens – especially as they eat box hedge, which is notably used in traditional jardins à la française.
The 4cm green caterpillars can eat their way through the leaves of a box hedge or topiary work in a day and kill a large garden in a week, often leaving the desiccated wood covered in webbing.
The caterpillars go into their pupal stage over winter and hatch into moths in spring and summer. Young caterpillars are greenish-yellow with black heads, while older ones also have thick black and thin white stripes along the body.
The first thing that is recommended to do is to buy a specialised pheromone trap – piège à pyrale du buis. You can find them online or at garden centres. They use female pheromones to attract male moths and have a dual function in that they get rid of some male moths and stop them reproducing, but also the numbers caught in the trap give an indication of how big a problem there is in your garden in any given year.
Traps should be set up from April to October, suspended in or near to your box shrubs. They do not use any noxious chemical substances.
If necessary then you can buy chemical sprays to use in the case of infestation but otherwise an effective natural remedy is to soak rhubarb leaves (not stalks) in water (500g in three litres) and then spray that liquid onto the plants.
If you add liquid savon noir and colza oil it can make it more effective and help the substance stick to the leaves.