The box tree caterpillar has been announced as the top critter in the 24th RHS annual pest and disease ranking for the third year running. The organisation offers the following guide to this nuisance pest.

Where does it come from? It’s native to East Asia but became established in Europe in 2007, although it wasn’t reported in private gardens until 2011. The caterpillar is the larvae of a box tree moth, which feeds on box plants.

l What does it look like? Its pale yellow eggs can be spotted on the underside of box leaves. Pupae are hidden in a cocoon of white webbing spun among twigs and leaves. Newly hatched caterpillars have black heads and a greenish-brown body and can reach up to 4cm at maturity, when their colour will have turned to greenish yellow with thick black and thin white stripes along the body. Adult moths have white wings with a brown border and a wingspan of around 4cm.

l What are the signs that you have it? You may simply see the caterpillars or the webbing that they produce which covers their feeding area. Box plants can also show dieback areas, particularly visible when plants have been trimmed.

l How much damage can it do? Quite a lot - severe defoliation in many cases. Where it has become established it is likely to pose a recurring problem throughout the growing season and in subsequent years.

l What is its life cycle? It is most active from April to October. It overwinters as small caterpillars, hidden between box leaves that have been spun together with silk in late autumn. According to the European Boxwood and Topiary Society (EBTS), adult moths can fly up to 10km during their life and start laying eggs two to three days after they start flying. Each female can lay up to 700 eggs.

l How can you control it? If you’re going down a non-chemical route, remove the caterpillars by hand. To monitor adult activity, you can buy pheromone traps. Alternatively, use a nematode which will provide some biological control.

EBTS advises gardeners to carefully comb through the plants with a small hand claw/rake to remove the cobwebbing and clear away the debris under the plants. It may be easier to put a cloth under the plants to catch the debris.