Does your Box ( Buxus ) Plant looks like in this image?
Covered with Webbing and leaves are eaten of the Topiary/ Box (Buxus ) plant means that your plant has been attacked by BOX TREE CATERPILLAR ( Cydalima perspectalis ).
This Caterpillar is the larvae of a moth and it feeds on Box ( Buxus ) plants. If this caterpillar not controlled immediately then it will completely defoliate your box plants like in this image below.
The more your plant is damaged the more caterpillars are present, so that could be up to hundred in one box plant as I personally collected and counted approximately 51 Box tree caterpillars as in images below.
How to eliminate Box Tree Caterpillar?
Best way is to remove them by hand as I do and the rest leave it to the natural predators insects such as the common Wasps which I found the most incredible predator which eats all the plants eater insects. I actually filmed it eating caterpillar, CLICK HERE to watch video. I also upload this video on my twitter @inspiringhobby.