I noticed some brown leaves on my box plants and thought it may be blight. A week later I was watering close to the biggest, 7feet topiary trimmed, specimen when I noticed a rotten farmyard odour. On inspection I noticed an infestation of caterpillars. I understand there is no treatment for this problem. I am in the process of digging up all of my box plants, some almost forty years old. Is t worth keeping the cutting which I took some weeks ago or is the problem with me forever. I have destroyed many of the caterpillars but have noticed the moths flying about so it looks like other local box plants are in for an infestation.
You don't say where you are in the world, and that's a consideration when trying to give a sensible answer. That said, if its Box Tree Caterpillar, you should have noticed plenty of webbing over the plants, but you haven't mentioned this feature at all. There is treatment for Box Tree Caterpillar, but its true to say it's an ongoing battle, and it sounds like, as yours is advanced (if that's what it is) you may need to use a neonicotinoid spray such as Bug Clear Ultra. If you do use this, ensure you don't spray when bees are working, and that the spray does not drift onto any flowering plants in the vicinity. More information here https://www.rhs.org.uk/Advice/Profile?pID=760&cID=1247