Box hedges, balls and topiary are a fixture of British gardens, and the box tree caterpillar has become one of their most damaging problems. Unlike a household pest, it is a horticultural one, and the choices made by owners, gardeners and contractors are mostly about monitoring and timing. This article summarises the Royal Horticultural Society (RHS) profile of the pest. The RHS describes the situation across the whole UK, and this article does not cover pesticide approvals, which are not addressed in the profile. It is general information.
What it is and how widespread it has become
The box tree caterpillar is the larva of Cydalima perspectalis, a moth native to East Asia. The RHS says young caterpillars are greenish-yellow with black heads, and mature ones reach about 4cm, with a greenish-yellow body marked with thick black and thin white stripes. The RHS reports that the pest was first detected in Britain in 2007 to 2008 and appeared in private gardens in 2011. By the end of 2014 it had become established in parts of London and the surrounding area. It now describes the moth as widely distributed across England, especially the south-east, and present throughout Wales, Scotland, Ireland and Northern Ireland as a common resident.
The yearly cycle
According to the RHS, the moth completes at least two generations a year. It overwinters as small caterpillars sheltered between webbed leaves, and in warm spells feeding can begin as early as February. By June caterpillars pupate and adults emerge; a second generation appears in late July or early August, and life stages overlap through the summer. By late September or October, most of the population consists of early-stage caterpillars. Because overwintering caterpillars are small, damage can appear suddenly when they grow, which is why the RHS stresses regular checks.
Recognising damage
The caterpillars defoliate box by eating the leaves and produce webbing over the areas where they feed. Affected plants may show patches of dieback, especially on trimmed plants. The RHS adds that caterpillars can also strip bark, which may girdle sections of a plant. Because the feeding is often inside a clipped surface, the first sign can be webbing, brown patches or droppings rather than visible caterpillars.
What the RHS recommends
Monitor early and often
The RHS advises frequent checks from early spring, and particularly from May onwards, so that action can be taken before a damaging population develops.
Hand removal
Where practical, caterpillars can be removed by hand. This is realistic for a few plants but slow for a long hedge.
Pheromone traps
The RHS says pheromone traps can show whether moths are present in the area and how active they are. They are a monitoring tool, and the profile does not present them as a way of clearing an infestation, so they suit gardeners who grow box and have not yet been affected.
Biological control
The RHS says the mixed nematode product sold as Fruit and Vegetable Protection has some effect on the caterpillars, but adds that nematodes could infect non-target animals and should be used with care.
The RHS’s separate page on biological control adds that nematodes are living organisms supplied mainly by mail order, mixed with water and watered onto affected plants, and to be used as soon as possible after they are received. It says products containing Steinernema feltiae and S. carpocapsae, including one sold for box tree caterpillar, are available, and that nematodes should be used only when there is a specific problem to treat.
Alternatives to box
The RHS suggests considering other hedging or topiary plants in place of box, and links to guidance on alternative shrubs. For households with repeated losses this may be more sustainable than yearly treatment.
Buying help from a contractor
A garden contractor or pest control company offering treatment should be able to say what they will apply, how it fits the RHS steps above and when they will return to check. The site’s advice on quote comparison and follow-up visits applies here as well: a single visit is unlikely to suit a pest with overlapping generations. Note that legal conditions on products, including what an amateur may use, are covered in the site’s explanation of how pest products are regulated in Great Britain; they are outside the scope of the RHS profile, so no product is recommended here.
Common questions
Is it a notifiable pest that must be reported?
The RHS profile does not describe any reporting requirement. Pests that are reportable, such as those covered in the site’s TreeAlert guide, are a different category.
When should box be checked?
The RHS says from early spring, with attention from May onwards, since feeding can begin as early as February in warm spells.
Will pheromone traps solve an infestation?
The RHS describes them as a way of detecting moths and gauging activity, not as a control on their own.
Does it occur outside England?
Yes. The RHS reports it in Wales, Scotland, Ireland and Northern Ireland.
The bottom line
The box tree caterpillar is now a common resident across the UK, with at least two generations a year and small overwintering caterpillars that can go unnoticed until damage appears. The RHS emphasises early and frequent checking, hand removal where practical, pheromone traps for monitoring, cautious use of nematodes and considering alternative plants. Any paid treatment should be judged against that timing rather than a single visit.
Sources
- Royal Horticultural Society, “Box tree caterpillar” (rhs.org.uk)
- Royal Horticultural Society, “Biological control in the garden” (rhs.org.uk)