We’re fighting a losing battle when it comes to the box bush caterpillar.
Have you seen the devastation a certain caterpillar is causing in our gardens? It’s an issue that’s affecting many gardeners in The Five Valleys, but it’s not just happening here in Gloucestershire. The insect, known as the box tree moth (Cydalima perspectalis), is running riot up and down the UK. The box tree caterpillars are the larvae of a moth that feeds on box tree. So why is the box tree caterpillar causing so much damage, and can anything be done to stop it?
I was first made aware of the box tree caterpillar when I was walking along the towpath near my home in Ebley. Two elderly ladies had stopped on the path in front of me discussing the state of my poor neighbour’s hedge! I’d seen my neighbour Mark lovingly plant 50 Buxus hedge plants 3 years ago, to form a border between his garden and the towpath.
Initially, the plants grew really well, but last Summer Mark noticed them looking rather unwell. On closer inspection, he noticed what looked like dense spider webs forming within the hedge containing tiny ‘eggs’.
“There was nothing on the market that would kill the little critters, and it was only a matter of time before our hedge would be completely taken over and killed”, Mark said. Mark ended up removing and binning the whole of his hedge, and replacing with Llex Cranata. So far it’s cost him around £300 to replant.
Initially, I thought Mark’s hedge was perhaps a one-off, until another neighbour posted on our community Facebook group that his box bush had also succumbed to the box tree caterpillar. Everywhere I walked from thereon after, I began noticing the calling card of the box tree caterpillar!
Time to call in our expert. Graeme Davis is a conservationist and entomologist from Stroud, specialising in moths and butterflies. I went for a walk and talk with Graeme around my estate, and it wasn’t long before we tracked down the culprit!
Here’s all you need to know about the box tree moth and caterpillar. We asked Graeme to answer the following questions:
Where has the box bush caterpillar come from?
The box tree caterpillar arrived in the UK in Kent in 2007. By 2011, it was recorded as being a bit of a pest in gardens, and soon became a widespread issue. Numbers started to grow in London and the Southern counties, but it has now been recorded as far as Scotland. The insect is not native to this country. It originates from South East Asia and has been completely brought in by the trade in plants. Graeme believes the demand from gardeners to get hold of cheap plants, is what has fueled the rise of box tree caterpillar numbers in this country. Commercial nurseries have imported the insect on the box trees themselves. When it comes to wildlife, our border controls have been lapsed. Box trees have gone through border control carrying eggs, without being detected. Before 2007 it wasn’t a problem, but it could have easily been prevented, especially if we’d had better bio-controls at our borders, but also if we grew our own plants in this country.
What does the box bush caterpillar look like?
The caterpillar is quite striking. They’re lime green in colour, with a streak of yellow down the body and black dots. They’re easy to spot when you have your eye in and you can get hundreds on an individual tree. They create webbing as they’re feeding, which looks like spider webs. They leave tiny little brown balls everywhere, which is actually their poo not eggs! The eggs and pupae are harder to find.
What does the box bush moth look like?
Naturally, the box tree caterpillar turns into a moth and it’s actually rather lovely! It’s large and striking. It’s about 14mm long and bright white, with a browny/black border. There is a melanistic version, which is black with white spots, which make up 20% of the population. The moth is more likely to be taken by predators than the caterpillar, which may be the key to controlling numbers.
How is the box tree caterpillar killing the plant?
The caterpillar will munch away at the leaves and strip it completely. Once it’s had its fill of the leaves and if it hasn’t pupated at this stage, it will then move onto eating the bark. It then strips aways the cambian of the tree, which will kill the bush completely. If you can stop the caterpillar from eating the bark, you can save the tree,
Can we control the population?
The moth has been introduced to the UK where there are no known predators, which would explain why numbers have been allowed to grow. There’s no way of really controlling box tree caterpillars at this current stage. The only way we could control numbers, is if we stop buying imported box bush. Eventually the caterpillar would run out of food, but at the moment, people are just replacing infected box bush and giving the caterpillar more food. There are multiple examples of where humans have acted too late, like the introduction of grey squirrels or pheasants even, who’ve been linked to dwindling adder and butterfly numbers.
Do pesticides work?
A definite no, no! There are no known pesticides that make a difference on the caterpillar itself, but what it does tend to do, is kill everything that is good – for example, any predatory insects that might have eaten them, you’ll kill them and not the caterpillar. There are biological methods you can try. Nematode worms you can spray directly onto the caterpillars, or picking the caterpillars off the box tree each morning and dispatching them in some way. I’ve heard of some people collecting all the caterpillars off a bush, and encouraging birds to eat them off a bird table. The issue is, British birds don’t know what a box bush caterpillar is, because it’s not on its menu. It’s almost like we need to train British birds that this is food!
What can you do if your box bush is infected?
If the box tree caterpillar has eaten the bark, then that’s the end of your box bush. If they’re just stripping the leaves, you can keep on removing the caterpillars and staying on top of the population. It should recover, but you need to check it every day. Don’t replace it with more box bush, but replace your box with an alternative. The RHS does a list of alternatives insert website here: https://www.rhs.org.uk/plants/types/shrubs/box-alternatives
What is box blight?
Box blight is a fungal infection, but it can be slightly more controlled.
Is there a threat to other insects or plants?
Not at the moment, but the threat might be if the box tree caterpillar evolves
to eat British plants, and then we will really have a problem. Eventually numbers will come down when predators realize they’re food, but they can take a long time. As long as we continue buying Asian box tree, they’re here to stay.
Are all moths bad?
There are 2,500 species of moth in the UK. Of them 0.5% of those are pests, and a large proportion of those are ones we’ve introduced. There are very few British moths that are a problem. You think of carpet and clothes moths, but they all come in with the cotton trade from China. Most moths are beneficial to us and our environment. Moths are our nighttime pollinators, so they’re very important to our food crops. They’re doing the night time pollinating, which is 50% of the pollinating, but also there’s a lot of daytime moths so they’re working day and night, of the total numbers of pollinators, but also there’s a lot of daytime and they work day and night. People think that bees are very important for pollination, but they’re only doing the day job!
