Box tree moths are an invasive and destructive alien species that have become widespread in Europe over the last few years. The caterpillars have already killed countless box - Buxus sempervirens - trees.
Luckily, a biological control is now widely available. I started using it in 2021 on our precious box topiary. It's easy and quick to use and has proved completely effective.
Note:
This is not an adverstisement. I have purchased my 'TopBuxus XenTari' just like anyone else - in my case from Amazon. This provides 5 sachets which is sufficient in our garden for a year. For Amazon link please scroll down to Update March 2024.
The control works by coating the box leaves with a bacteria which is harmful to the caterpillars when they eat the leaves. The bacteria mucks up their digestive system and they die within a few days. Other insects and animals are therefore completely unaffected. Three or four repeat sprays are needed through the summer to kill successive generations. I inspect every week or so to check whether I need to spray again. There is no point in spraying if there are no active caterpillars.
The effects of box moth caterpillars (which are easy to find if you look closely) should not be confused with box blight which is a fungal disease and has been around for many years especially in the warmer wetter parts of this country.

Update 27th May 2022
Our topiary has looked perfect this spring, just like it used to, before box moths. Over the last week I've noticed just a few trouble spots here and there so I have just applied another spray. The weather is quite windy at the moment so got up early and sprayed at 6 am when the air was still.

Update April 2023
Our box topiary is looking almost perfect. Last year was very hot and dry and I only sprayed it three times.
Last week I found a couple of active small caterpillars (which had survived the winter even though we had some exceptionally cold periods) and so I did the first spray of the year on a dry day. When spraying remember to try and direct the spray up onto the underside of the foiliage as far as possible where it will be protected from rain.
Elsewhere in our neighbourhood box has been almost wiped out unless it has been sprayed. I'm guessing that with the depletion of their foodsource the local population of box moths will also decline and it may become less of a problem in future.

Update May 2023
Found a few caterpillars had escaped the first spray this year - (it was applied when it was still a bit cold). But after a second spray, a few weeks later, all the remaining caterpillars were dead the following day. Interestingly, for the sprays so far this year, I have been using sachets left over from last year. I was concerned they might be out of date but they were totally effective. I read somewhere that if stored appropriately they can be okay for two years.

Happy spraying!

Update August 2023
I've just sprayed our box for the fourth time this year. Earlier on there seemed to be fewer caterpillars in our locality than previous years but things seem to have picked up again recently. Also, 2023 has brought a lot of rain which, I suspect, has tended to wash the bacteria off the foliage more than usual. I have therefore tried to direct the spray up onto the underside of the leaves where it is less prone to being washed off.

But overall, all our box is looking great. I'm glad my video has now helped so many people with the problem of box moth damage. And many thanks to everyone who has liked the video or left a comment.

Update March 2024
Our box topiary is in perfect shape but the caterpillars will soon be active again. Keep a close eye on your plants for the first signs of damage. As an experiment I sprayed ours at the end of March using a sachet left over from the previous year and possibly even older but it still seems to have worked as there were lots of dead caterpillars after a few days so don't automatically throw away old sachets.
For this year I have added the following link to buy Top Buxus ZenTari from Amazon if you wish.
https://amzn.to/4a3BJRk

Update July 2024
Our box topiary has never looked better. Amazingly, I have only sprayed once this spring when I saw a few overwintered caterpillars emerging. Since then there have no signs of caterpillars.
All around us in Cambridge most box trees have been completely killed. From asking around almost the only ones that have survived are those owned by people who have used the bacterial spray. It is logical to assume that the local population of box moths has therefore crashed, because there is no more food, and the threat from box moths is greatly reduced, at least for now!

Update August 2024
Noticed a few patches of box moth damage just a few days ago. A spray quickly put a stop to that. But overall, box moths have been vastly less of a problem this year.