Box Tree Caterpillar (Cydalima perspectalis)

It seems that the UK is becoming a much sought-after residence for a number of plant insect pests and diseases.  One such pest is in the process of devouring many of our manicured Box (Buxus) topiary features and trimmed or informal box hedges; The Box Tree Caterpillar. The moth is also present of course, but in itself is relatively harmless other than the fact that it needs a host for its clutch of minute eggs.

The moth has been visible for a number of years, but the effects of its offspring the caterpillars only since year 2011.  It would have been present since first reports of moth sightings of course, but the increasing damage to Buxus shrubs makes it more evident now. The moth is capable of three hatchings generations a year, so the spread of this pest is a foregone conclusion.

You may well have other plants being chewed by similar caterpillars, but they will probably be the caterpillars of the cabbage White butterfly. As the name suggests, the Box caterpillar feeds exclusively on box trees, hedges, shrubs and topiary specimen.  

Photograph by Böhringer Friedrich

The Box tree caterpillars start life as light yellow stringy crawlers with a pronounced black head. Later developing into caterpillars of 2.5 3cms long (1 inch) and having bands of yellow and black along body again with the black head at the business end.

They eat voraciously, and craftily hide themselves under a webbing mesh that will cover the defoliated skeletons that remain once they have taken hold. The damage will be seen in the growing season being April through until late October. As you would expect being from Asian it is happiest in the warmer areas of the UK Greater London and the Southern counties. It will most certainly spread unless some effective control measures are put in place.

Control of Box Caterpillar

Firstly be diligent in your inspection of any Box (Buxus) plants you have wherever you live in the UK! It is far better for you to find it at an early stage, rather than waiting for it showing you where it has been. It will strip a shrub of foliage in a matter of days.

Physical control

Non chemical control is basically confined to picking off the caterpillars by hand when you first see them. As there is little known about the toxicity or irritation aspects of the caterpillar, you would be advised to wear gloves. At least, wash your hands as soon as possible. This will probably be necessary in any case depending upon how you ‘dispatch’ your catch!

Do not discount looking for the eggs on undersides of leaves. They are easily recognized; being pale yellow and neatly arranged in clusters. Rub these off in the first instance. As most Box plants have a multitude of small leaves, this can be an impossible task for larger shrubs.

If any trace of eggs are found, then follow up physical action with preventative spraying.

Chemical Control with Pesticide Spray

Most popular pesticide sprays will kill all caterpillars some will also kill helpful insects, so do be careful in your choice and spraying times. Avoid flowering time, as pollinator insects will be visiting. Early evening or morning is always favoured to avoid killing all manner of other insects.

A light spray will not ensure success. You will need to drench spray, in particular where the protective web mesh is encountered.