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How to Defend Against Box Moth Caterpillars

AlanGardenMaster
Published on May 23rd 2020
9

by AlanGardenMaster. All rights reserved

Box tree moths on a purple flower on a plant
Box tree moth is another very unwelcome imported pest set to challenge UK gardeners. It is decimating box trees in the South East of England, but gardeners throughout the UK and Ireland should be on their guard against this voracious moth!
A close up image of a box tree moth caterpillar on a green plant leaf

Box Tree Moth

Cydalima perspectalis

What does box tree moth attack?

Box tree moth, as its name implies, attacks box trees which are mostly forms of Buxus sempervirens.
While that might not seem a big threat to our gardens, if you think how often you see box trees used as low or big hedges, topiary trained specimens or just shade-tolerant screening evergreen shrubs, it gets a bit more serious.
Make no mistake about it; this pest, along with the equally devastating Box Tree Blight (a virulent disease), has the potential to change the look of gardens both domestic and historic!
A box tree hedge damaged by box blight in a garden
A box hedge damaged by box blight

What does it look like?

For such a pest, the adult box tree moth is rather pretty!
But often the caterpillars, and it is they who cause all the damage, are hidden within the dense foliage.

What to look out for

The first sign of box tree moth is usually noticing that your plant has been defoliated (the leaves have been destroyed).
The caterpillars look similar to the Large Cabbage White caterpillar, with greenish-yellow bodies and black heads. However, unlike these, they feed inside the canopy of box trees and produce webbing.
Pupae of the caterpillar may be found hidden within the webbing.
Early signs of box tree moth would be to find the sheets of flat looking eggs glued to the underside of leaves. You might get some measure of control by squashing these.
A green box plant in a garden
Clipped box balls
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Non-chemical controls

Regular inspection of plants is essential, especially during the spring and summer months.
If your garden is not in the south-east of England, you should not be complacent. Box tree moth has already been reported in Wales, Scotland, Northern Ireland and Ireland.
If you discover the caterpillars early, you may be able to stop their spread by picking them off and destroying them. However, with larger trees, this will be impractical.
It seems that not many bird species eat box tree moth caterpillars as part of their regular diet. However, there have been reports of jackdaws and blue tits learning that these can provide a good meal.
Pheromone traps are perhaps the best means of control. These mimic the box tree moth's sex pheromones to lure them onto sticky pads.
Pheromone traps can also provide an excellent method of monitoring for the presence of this pest in your area.
Pheromone traps are becoming available in garden centres and of course online.
A knot garden of box tree hedge
Haw frost covered box hedge knot garden

Chemical controls

If necessary, forceful spraying may be required to penetrate the webbing and make direct contact with the caterpillars. You can also use contact insecticides.
Natural pyrethrins that work by contact action will require several applications. Such insecticides include Bug Clear Gun for Fruit & Veg, Ecofective Bug Killer.
Persistent contact insecticides such as the synthetic pyrethroids lambda-cyhalothrin (Westland Resolva Pest Killer), deltamethrin (Provado Ultimate Fruit & Vegetable Bug Killer) and cypermethrin (Py Bug Killer) can also be effective.
The neonicotinoid insecticide acetamiprid contained in Bug Clear Ultra might also be effective, but many gardeners prefer not to use this due to its implications in declining bee populations.
Read and adhere to the label instructions when applying pesticides.
If you do have to use sprays, they should only be applied early in the morning or late in the evening when other insects will be less inclined to be foraging.
Under no circumstances spray these materials onto plants in flower!
Knott Garden with box hedge and holly trees at Charlton House
Knott Garden with box hedge and holly trees at Charlton House

Monitoring spread

The Royal Horticultural Society (RHS) is monitoring the spread of this pest and would welcome reports if you find box tree moth in your garden or in a public place.
You can report your findings and hopefully contribute to the control of this pest on this page.

Alternatives to box trees

In areas that have a high risk of getting box tree moths and blight, it may make sense to plant alternatives.
There are many alternatives to consider. Here are just a few of my favourite:
A Japanese holly hedge in a garden
A Japanese holly (*Ilex crenata*) hedge

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