Box tree caterpillars are a serious threat to box (Buxus) plants – they can completely defoliate them, ruining prized topiary and native box hedges.
Box tree caterpillars are a relatively new pest. Native to east Asia, they're thought to have hitched a ride on imported plants to Europe, where they were first spotted in 2007. The first moths were found in private gardens in Britain in 2011, initially in the south east, where they are now a major problem. They're also now spreading across the UK. The caterpillars cause most of their damage between March and October. A box ball can be destroyed within a week if action is not taken.
There have been reports of birds such as blue tits and jackdaws feeding on the caterpillars but it's not yet known if this will have any serious affect on numbers.
What is box tree caterpillar?
Box tree caterpillars are the larvae of a moth, Cydalima perspectalis, which lays its eggs on the undersides of box leaves. The resulting caterpillars create cobweb-like webbing over their feeding area, and devour the box leaves. After around a month, the caterpillar forms a chrysalis which emerges as a box tree moth, which then mates, perpetuating the cycle. Box tree caterpillars can be a problem from spring to autumn, producing multiple generations. The caterpillars overwinter among box foliage, resuming feeding the following spring.
Box tree caterpillar symptoms
The signs of box tree caterpillar can be confused with box blight. You are likely to become aware of box tree caterpillar when you see the tell-tale webbing, dieback and droppings on your box plants. As the caterpillars conceal themselves deep within the plants, this often becomes apparent when you have trimmed or shaped your plants.
Parts of the plant are covered in dense, white webbing, under which the caterpillars feed.
Your plant may also have small patches of dieback – the leaves turn pale and papery. This is particularly noticeable on newly trimmed plants. This can look a little similar to box blight, so be sure to check the symptoms for this too.
You may also see caterpillars, often concealed deep among the foliage or at the base of the plant. The young caterpillars are green/yellow, with black heads, and are around 1cm long. Older caterpillars reach 4cm long, and are also green/yellow, and have black and white stripes on their bodies.
You will also see copious amounts of droppings, known as frass – pale yellow flakes.
You may also notice the pupae, in a cocoon of white webbing among the foliage.
You may see the eggs of the caterpillars on the undersides of the leaves – these are small, pale yellow and flat and overlap each other.
If you shake the plants, you may also see the moths flying off – they have white wings with a brown border, or completely brown wings, around 4cm across.
The moths can also strip the bark of box.
Controlling box tree caterpillar
You can try to remove the caterpillars by hand if the infestation is small or you only have a few plants – you will need to do this every day once signs of their presence have been spotted, and check deep within the plant. You can also prune out the stems covered in webbing, using secateurs.
If the caterpillars are really taking hold, a biological control that contains the micro-organism Bacillus thuringiensis is said to be effective. Treatment needs to be repeated several times across the season, when the temperature is at least 15°C. Spray thoroughly, coating both sides of the leaves so it penetrats deep into the plants.
Insecticides can be used on box tree caterpillar but they are not thought to be as good as the biological control and will also kill other insects. Multiple applications are needed. Do not spray insecticides near plants that are in flower to avoid harming beneficial pollinators.
Preventing box tree caterpillar
Pheromone traps feature a synthetic pheromone that mimics the one produced by the female box tree moth. The male moths are then attracted to the pheromone and become stuck inside the trap, disrupting the breeding cycle. The traps need to be replaced frequently and are unlikely to catch all of the male moths in your garden. However they are a useful indicator of the presence of box tree moth so you can take prompt action.
Box tree caterpillars can be hard to spot as they conceal themselves well. Therefore check your plants regularly, looking deep inside the plant and around its base.
Alternatives to box
If you have lost plants to box tree caterpillar (or box blight) or are unwilling to plant more due to potential problems, there are plenty of alternatives to box.
Some, such as box leafed holly (Ilex crenata) or Lonicera nitida look similar to box and can be grown and clipped in the same way, and yew is a great alternative for hedging and topiary. While other plants are not direct replacements, some have the added advantage of flowers that are attractive to pollinators (such as lavender, Mexican orange blossom and podocarpus), while others have attractive variegated foliage (Euonymus fortunei or Euonymus japonicus). Some, such as berberis, have fiery autumn foliage.
Read more about our recommended alternatives to box.