Bug of the Month June

Box Tree Caterpillar

Though relatively new to the UK the spread of box tree caterpillar moth (Cydalima perspectalis) appears to be very rapid and is becoming a serious pest in many parts of the UK, this combined with Box blight is giving the poor Buxus plant a hard time.

We have not yet seen this particular pest in the garden here at Hidcote but, as a precaution, we are using pheromone traps placed within our hedging to monitor if the moth makes an appearance. However, I did come across some infestations while in Holland and having seen the damage it can do felt I should cover it this month.

The Box tree moth is Native to East Asia, it was first seen in Britain in 2008 although the caterpillars were not found in private gardens until 2011. By the end of 2014 the moth had become established in parts of London and surrounding areas and has since become widely distributed across England being present in wales, Scotland, Northern Ireland and Ireland.

In the UK there can be 3-4 life cycles per year, most active from March to October. You may first be aware of the box tree caterpillar when you find webbing and caterpillars on the plants, this protects them and can make treating with sprays more challenging. The adult moth usually has white wings with a brown border and lays its eggs on the underside of the leaves.

They have no natural predators here and can develop into large numbers, feeding on the leaves can cause defoliation of the plant, but the box can survive if the larvae don’t eat the bark of the stems.

The damage done by a Box Tree Caterpillar
The damage done by a Box Tree Caterpillar
The damage done by a Box Tree Caterpillar

 The caterpillars can also survive over winter, hidden between the box leaves that have been spun together in late autumn and will then complete development in the spring. The caterpillars become a pupa in a chrysalis before emerging as a white semi-transparent moth. The female moths can fly around 10km from where they emerge.

So, what can we do?

Initially pheromone traps can help monitor adult moth activity, but this is not a control method on its own and once they are seen it is likely the box tree caterpillar will follow a few weeks later. Where practical caterpillars can be removed by hand.

This may not be possible with a large infestation, so a combined approach is necessary involving trapping of moths, destruction of eggs and killing of the caterpillars using nematodes or biological insecticides. Research is still being carried out on timings, application techniques and potential new natural predators or parasites of this damaging and invasive pest.

We should all be keeping an eye out for signs of the box tree caterpillar where we have box in our gardens to enable early treatment. If you do see the box tree moth you can record it at the RHS website which monitors the spread of pests and diseases. More detailed information and advice can be found on the European Box and Topiary Society’s (EBTS) website.