Guides · Dover Hedges

Box tree caterpillar in east Kent: the 2024 lesson

Box (Buxus sempervirens) has taken two hits in a decade in the south-east. Box blight came first. Box tree caterpillar (Cydalima perspectalis) came second, and by now it is the more visible day-to-day problem for Dover gardeners. The RHS classifies it as "widespread in London and the south-east of England", common resident, two or more generations a year. If you have box in Dover, you will meet it sooner or later.

What it looks like

The adult moth is small (about 4 cm wingspan), white with a dark brown border. You will rarely see the adults in daylight. What you see is the damage:

• Fine webbing between the twigs, most visible on a dewy morning.
• Skeletonised leaves where the caterpillar has stripped everything but the veins.
• Green-and-black-striped caterpillars up to 4 cm long, often hidden inside the webbing.
• Rapid bare patches that can strip a mature hedge in a fortnight during peak feeding.

The lifecycle

Adults overwinter as pupae inside the box canopy. First-generation caterpillars start feeding in April to May. Adults emerge in June. Second-generation caterpillars run July to September. In a warm summer we can see a partial third generation stretching feeding into October. That is the difference between one shot at spraying and needing to monitor all season.

What actually works on a hedge scale

For a single topiary specimen, hand-picking caterpillars off the plant every three or four days through the feeding window is realistic and works. For a hedge of any length it is not; we need a spray programme, and even that has caveats.

Bacillus thuringiensis subsp. kurstaki (Btk)

The biological pesticide of choice. Btk is a bacterial spore that produces a protein toxic specifically to lepidopteran (butterfly and moth) larvae, and is broken down within about 48 hours by UV. It is safe for bees, mammals and other invertebrates in normal use. Two important limitations:

• It has to be sprayed onto the caterpillar (or ingested by it) — that means good coverage of the whole hedge, both sides, both faces.
• It works only on actively feeding caterpillars. Miss the window and you are wasting product.

Typical Dover programme: two treatments per generation. First when caterpillars are visible (mid-April, early July), second 7 to 10 days later. Repeated for the second generation. That is four applications a season minimum in a wet summer.

Pheromone traps

Useful as monitors to time your Btk applications, marginal as controls in themselves. Hang them in the hedge from late May; when moth numbers start rising, you have about 10 to 14 days before caterpillars will be feeding.

What does not work

• Contact insecticides that are still available to amateurs mostly hit the caterpillar poorly because it is protected inside webbing. And they hit bees.
• Cutting the hedge hard in mid-feed does not remove the population, it just displaces them onto whatever is left.

The strategic question

Between box blight and box tree caterpillar, a mature box hedge in Dover is now a demanding, ongoing management project. If you love the hedge and are willing to run a Btk programme through summer plus watch for blight in the wet months, you can keep it. If you are inheriting a badly damaged hedge and want a low-maintenance solution, replacement with Japanese holly (Ilex crenata) is usually the right answer — the visual match is close and neither pathogen affects it.

We are honest with clients about which category they fall into. Where a hedge is still worth saving, we will run a programme. Where it is not, we will say so and quote replacement instead.

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