Box moth caterpillars (also known as box tree moth caterpillars) can strip all the leaves from your box plant or hedge.
And they can do two or three life cycles a year. That’s hatching from eggs into caterpillars, then into moths and laying eggs again!
In my neighbourhood many front gardens have rows of dead box hedges. All the leaves are brown and if you look carefully, you can often see the caterpillars still on them. Once they’ve eaten all the leaves, they often move onto eating bark. Your plants will never recover.
This is a relatively recent pest. Box moths (Cydalima perspectalis) are native to Eastern Asia. The caterpillars are their larvae and will themselves grow into a moth within about a month of munching your box tree.
Box moths and box moth caterpillars travelled across Europe on box plants, arriving in the UK in 2007. And the infestation is spreading fast around the world to the USA, Canada and other places.
However, it’s spreading slightly more slowly in the US. There are varieties of box native to Asia, Europe and the UK, so they grow in the wild. This means more plants for the moth to lay its eggs on! Box is not native to the US, so there is little or no box growing wild.
Last year, I discovered box moth caterpillars on all of my box topiary, including my beloved topiary spiral. This is the most expensive plant in my garden and is also a focal point.
My box topiary spiral in happier times.
How to spot box moth caterpillar damage
Box tree moth caterpillar damage starts in spring when the caterpillars appear. The moth lays its eggs on the undersides of leaves. The eggs are wrapped in silk webbing, sheltering over winter in your box plant. Even a cold winter, down to minus 30C, will not kill them off.
As your plants sprout new green growth, the caterpillars emerge. You will see darker green or brown-grey patches on the plant, where the caterpillar has damaged that new growth.
Go closer and you will see a net of white or silvery webbing with dying foliage, then a caterpillar inside. You may also see the droppings (known as frass) which will be tiny flakes.
If you get a serious infestation of box moth caterpillar, then you will lose a complete box hedge. I recently saw an open truck go by stacked high with dead box plants from a single nearby garden. If only the truck company had bagged up the plants!
Look for patches where there is no new growth (see top photo). When you look closer, you will see dead or dying leaves on these patches. You can see a larger patch in the lower photo.
What do box moth caterpillars look like?
Box moth caterpillars have citrus-green and black stripes, running the length of their body. They have black heads and are usually protected by the fine webbing around them.
The box tree moth caterpillar – it only feeds on Box (Buxus) so if you find caterpillars on other plants, it’s not box moth caterpillar.
What eats box moth caterpillars?
We’re all being encouraged to have wildlife friendly gardens, so that predators can eat up our pests. It makes for easier gardening and is better for the environment.
I recently interviewed Helen Bostock, Senior Wildlife Advisor for the RHS, on gardening for biodiversity. She explained that slugs and snails help our compost heaps work better.
If you have birds and ladybirds, they’ll eat most of your aphids. So, as long as you’re prepared to tolerate some nibbled leaves, then you can put down the chemical sprays and let nature get on with it.
However, no-one suggests you should take that approach for box moth caterpillars. Because they arrived recently, they don’t currently have natural native predators in the UK. There have been some early reports of sparrows, starlings and blue tits beginning to feed on them in the UK, but not in sufficient numbers to control the spread.
In Asia, Asian hornets eat box moth caterpillars, but these would cause other problems if we imported them here.
How to get rid of box moth caterpillars
If you want to treat box moth caterpillars without harming other insects in your garden, you’ll need a biological control. You can get nematodes (Nemasys Fruit & Veg is a popular brand for this purpose). Note that links to Amazon are affiliate, see disclosure.
You’ll need to read the instructions on nematodes carefully and follow them precisely. You can’t often store them for long, for example, and you may have to keep them in the fridge.
There is also a product called Top Buxus Xentari, which several professional gardening friends around the world have used successfully. It’s safe for bees and birds.
It’s a bacterial insecticide with an active ingredient based on Bacillus thoringiensis subs Azawai. Another product with Bacillus thoringiensis as the main active ingredient is now available in the UK as PlantPro BuxRevive. That is what I am currently using to control my box moth caterpillar.
You can find a professional treatment by searching online ‘box tree moth professional treatment’. Check that they are using products that minimise damage to other insects or wildlife.
Any insecticide that works against several kinds of pest will probably also affect the natural balance of pests and predators in your garden. So avoid the all-purpose sprays if you can. If you do use one, make sure you use it on a still, dry day so the spray doesn’t drift.
You’ll need to repeat any treatment several times throughout the summer.
Note that links to Amazon are affiliate, see disclosure. Other links are not affiliate.
Prune and feed the box plants
Several professional gardening friends have pointed out that clipping the box means that you’ll be cutting some of the caterpillars off (or cutting them in half). This isn’t a cure, but it’s a useful way of reducing the numbers.
So we’ll clip all our box topiary. Always bag up the clippings and dispose of them – don’t leave them lying around or put them in the compost heap.
Box moths can fly around 5m/10k so they can work their way down the street and out into the next county.
I’ve also been advised to feed the plants. Once again, this isn’t a cure, but it’ll help the plants recover faster and should minimise the damage.
Healthy, well established plants always withstand pests and diseases better than plants which are struggling or new, recently planted plants.
Do box moth caterpillars eat other plants?
No, they don’t. If you see caterpillars on your other plants, it’s not the box tree moth caterpillar.
The moths that eat your clothes or carpets are different too. There are hundreds of variety of moth. Only two or three types will harm your clothes or furnishings.
The best way to stop box moth caterpillar from destroying topiary…
Sadly, the very best way of avoiding both box moth caterpillars and box blight is to STOP planting box (boxwood plants)!
There may be a blight-resistant box variety in future. And treatments against box moth caterpillar – such as TopBuxus Xentari – will also probably be licensed more widely.
It’s a pity because box is such a brilliant plant, especially for topiary. There’s been much discussion about the best alternatives to box in the gardening world. These include euonymus, yew and privet.
But everyone has been disappointed by the performance of Ilex crenata, the plant that looks most like box. It doesn’t thrive in many gardens and often fails to clip into a tight shape.
You can clip conifers into shape, and some naturally have sharp enough shapes to look like topiary. See how conifers can transform your year-round garden.
I’ve also used privet, holm oak and holly in my own garden. I bought them as young plants and had them topiarised which was cheaper than buying them ready-made. See how to buy sensational topiary on a middle-sized budget.
If you’re in the UK and have just spotted box moth caterpillars in your garden, let the RHS know by completing their survey on this page.
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