The box tree moth is on the rise – here's what to look out for

young buxus sempervirens deco ball in garden nursery center ornametal box plant or boxwood in flower pots spring time close up, selective focus
RHS warning: the box tree moth is on the riseAnna Chaplygina - Getty Images

While many gardeners consider slugs and snails as one of the biggest garden pests (though the Royal Horticultural Society encourages gardeners to learn to co-exist with these creatures as they contribute positively to a garden's ecosystem), there’s one insect on the rise this year, and that is the box tree moth (Glyphodes perspectalis).

With almost five times as many reports of the insect this year (compared to the same period in 2023), the RHS says queries are growing at scale – it was the most common question asked of the RHS Advisory team at this year's Chelsea Flower Show – and it's affecting gardens across the breadth and width of the UK.

What are box plants?

Box or boxwood (Buxus sempervirens) is a hardy evergreen shrub that's typically used to create topiary shapes or as low hedging around flowerbeds and pathways. Because of its dense growth and small glossy leaves, it lends itself to many different garden designs.

What are box tree caterpillars?

Originally native to East Asia, box tree caterpillars were first found in British private gardens back in 2011. Since then, they have spread widely across England – particularly in London and the surrounding areas – as well as the rest of the UK and Ireland.

The box tree caterpillar is the larvae of the box tree moth that feeds on box plants. And gardeners may become aware of their presence when they find webbing and caterpillars on their box plants during the growing season, with the majority of damage occurring between March and October.

'There are at least two generations of box tree caterpillar in a year, though can be up to four elsewhere in Europe,' say the RHS say. However, if we are to experience a milder autumn this year, there is potential that a third generation could appear here in the UK.

caterpillar of the box tree moth cydalima perspectalis on boxwood buxus sempervirens close up of striped pest on frech leaves biggest pest for buxus sempervirens, european box, or boxwood invasive
Marina Denisenko - Getty Images

What damage do they cause to box plants?

As the caterpillars feed and grow, their damage becomes much more noticeable. In addition to the presence of webbing, you might also notice partially to entirely eaten leaves and small green pellets of insect excrement (frass) found under the plant.

‘While box is a pretty resilient plant and can recover from even complete defoliation, this does weaken the plant and where desperate, hungry caterpillars can go on to strip the bark, girdling and killing sections,’ says Dr Stephanie Bird, senior entomologist at the RHS.

box tree moth cydalima perspectalis, moth and caterpillar on common boxwood buxus sempervirens invasive species

What’s the best way to deter box tree moth?

‘Growers of box that have not yet been affected are advised to deploy pheromone traps in order to help determine when the moths are present, in order to then look for and remove the caterpillars,' explain the RHS.

And thankfully, there are several naturally occurring enemies of the box caterpillar. This includes parasitoid flies, parasitoid wasps, social wasps, spiders and ants.

If you notice that caterpillars have already infiltrated your box plants, you can try removing them by hand but it is important not to employ any pesticides to deal with them. This can have a number of negative effects and can even lead to a resurgence of the pesky caterpillars if their natural enemies are also killed by the pesticides.

beautiful green boxwood garden pruned into shapes
Havana1234 - Getty Images

Are there any box plant alternatives?

If you are looking for an alternative to box plants then there are a number of shrubs that lend themselves similarly well to being clipped. Berberis, privet, wire-netting bush (corokia), spindle tree (euonymus) and cotoneaster all have similar characteristics. The RHS has compiled a list of box alternatives and there's also an ongoing trial at RHS Garden Wisley in Surrey investigating the best compact evergreen shrubs that can be clipped into formal hedging styles.

What other pests are on the rise?

Last year the RHS Plant Health team also saw a surge in enquiries when it came to the fig leaf skeletoniser moth, who feed on fig leaves. ‘In addition, there has been an upswing in observations of an as-of-yet unidentified aphid appearing on buddleia plants,’ the RHS warn. ‘The aphid, believed to be related to the black bean aphid species, feeds on the underside of young buddleia leaves' with the leaves being crumpled and pale spots appearing on the upper side.

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