Box tree caterpillar in Thanet: what it does and what works
Cydalima perspectalis has been in Britain since 2007 and is now a common resident in the south-east. Here is what it does, how to spot it before you lose the plant, and what actually works to protect the box you have left.
How the box tree moth reached Britain
Cydalima perspectalis, the box tree moth, is native to East Asia and became established in Europe in 2007. The first adult moth arrived in Britain in 2007, caterpillars were not found in private gardens until 2011, and by the end of 2014 it was established in parts of London and the surrounding area. The RHS now describes it as a common resident, widely distributed across England and particularly the south-east, and a problem that is likely to recur repeatedly through the growing season and in successive years.
The larval stage (the caterpillar) is what does the damage. Greenish-yellow caterpillars with black heads, up to 4cm long when mature, with thick black and thin white stripes along the body. They feed inside the box canopy, spinning webs between the leaves, and they eat the foliage from the inside out. A homeowner often sees no external damage until they push their hand into the canopy and find a hollow shell of webbing and frass.
Generations through the year
The RHS records at least two generations a year, with most of the feeding damage between March and October. The moth overwinters as small caterpillars sheltering in webbed leaves, which become active again as the weather warms in spring and have been seen feeding as early as February. Adults emerge in June and again in late July to early August, and generations overlap through the summer.
Repeated heavy defoliation over successive seasons can kill an established box hedge.
How to identify infestation early
- Push into the canopy. Do not judge by the outer skin. The caterpillars work from inside out. If you feel silk webbing when you part the foliage, look for green caterpillars.
- Frass on the ground. Small green-black pellets under the hedge. That is caterpillar droppings. If you are sweeping it up every few days in June, you have a heavy infestation.
- Adult moths. Usually white with a brown border on the wings (some are all brown), roughly 4cm wingspan, attracted to light. If you see them around the hedge, eggs are probably being laid.
What actually works
Pheromone traps
Pheromone traps tell you whether the moth is in the area and how active it is, which helps you time checks and treatment. They will not eradicate a population on their own.
Bacillus thuringiensis kurstaki (Btk)
A biological treatment that targets caterpillars and is far less harmful to bees and other beneficial insects than broad-spectrum sprays. It is sprayed on the foliage and works when the caterpillars eat it. It is timing-critical: the aim is to hit young caterpillars in each generation, so check the hedge from early spring and again through the summer, and follow the product label.
Hand-picking
The RHS advises removing caterpillars by hand where practical. Wear gloves: the caterpillars do not sting, but the webbing carries frass and is unpleasant. Drop them into soapy water. Boring, but it works on small hedges.
Nematodes
A mixed nematode biological control has some effect on the caterpillars, according to the RHS, but nematodes can infect non-target animals so should be used with care. Realistic on a low knot hedge, impractical on a large screening hedge.
What does not work
Broad-spectrum insecticides are a poor choice. Contact sprays tend to hit the outer canopy but do not reach the caterpillars deep in the webbing, and they harm bees and beneficial insects on the way. Btk is the better spray choice.
When to give up on the box and replace
Heavy defoliation year after year with no visible recovery flush is the practical trigger. If the plant is skeletal by September and there is no re-flush in the following May, the roots are giving up and the plant will not recover. Better to remove and replant than to spend another two seasons managing a dying line.
Replacement candidates for Ramsgate: Ilex crenata for the closest visual match, yew for anything over 60cm, Euonymus japonicus for shade or salt exposure. Full detail in the box blight replacement guide.
The blight overlap
Box tree caterpillar and box blight often turn up in the same hedge, and both are common in south-east gardens. A weakened plant is more vulnerable to the pathogen; a defoliated plant scars up and gives the fungus access. If you are dealing with one, check for the other.
Want this looked at?
Send a couple of photos and your postcode to hello@ramsgatehedges.co.uk or 07763 100 477.
Sources: Royal Horticultural Society box tree caterpillar and box blight pages (rhs.org.uk/biodiversity/box-tree-caterpillar, rhs.org.uk/disease/box-blight).