Box Tree Moth

Everything you need to know about keeping your Buxus healthy.

Cydalima perspectalis - The Box Tree Moth

We all love buxus, many of us have box topiary in our gardens and up until not that many years ago, everything in the topiary garden was lovely. But enter Cydalima perspectalis otherwise known as the Box Tree Moth. This moth and its picky but voracious caterpillar offspring were first spotted in the UK in 2007, but it wasn’t until four years later that the caterpillars were found out and about munching on box plants in people’s gardens. They have since spread throughout the country from the south coast, through the midlands and arriving in Scotland by 2018. Cydalima perspectalis is a relatively large moth, with a wingspan measuring approximately 4 cm. The insect was probably imported into the UK on plants from South-East Asia.

One thing that I didn’t know until recently, when I found a dark grey/brown moth on my beloved box balls, is that there is a melanic variation in the colour of the moths. This is a darker colour variation caused by additional melanin. This means that there are two distinct forms of the moth. While they are mostly white with a grey/brown border around the wings, they can also be the darker colour all over. This version has one little triangular spot of white on each wing, in the same spot that there is a shape on the paler version.

The darker version of the moth

The RHS is undertaking a box tree moth survey so if you find evidence of any infestation on your buxus plants, pleased do report it via the RHS website.

There are around 70 species of buxus, the most commonly grown variety being buxus sempervirens, (sempervirens means evergreen). Unfortunately for us gardeners, all varieties are tasty to the box tree moth’s caterpillar stage. Damage is usually spotted first at the base of the plant and progresses upwards. If the problem is the other big issue that buxus has, Box Blight this will not be the case, you will find discoloured leaves at the top of the plants and leaves will fall to the ground. I do hope you don’t find either, but it’s good to know the difference and what to look out for.

The larvae of green and black caterpillars are concealed in a cocoon of white webbing, which is spun amongst the leaves. This is the tell tale sign that you have them on your plants. The caterpillars start out at between 1.5 and 2 cm long and grow to up to 4 cm, as they gorge themselves on our precious topiary. They feed voraciously on the foliage, leaving behind dead pale leaves and a covering of web. A very severe infestation can cause the death of the plant especially if the bark is eaten, which will happen when there are no more leaves available to them.

The female box tree moth lays small groups of between 5 and 20 to 30 pale yellow eggs in clutches overlapping each other. They can lay up to 700 eggs, which can be found on the underside of leaves if you look closely, however they are very difficult to spot. Unsurprising really, when you think about the size of a buxus leaf. The fine foliage is why we love them, it lends itself to neat, closely cropped shapes and makes brilliant topiary when regularly clipped.

One way to treat an infestation is to use pheromone traps to capture the male moths and stop them from breeding. I’ve just bought one from Dragonfli.co.uk. I’ve bought other controls from Dragonfli, they do all sorts of nematodes, for vine weevil, whitefly, slugs and many other beasties. I have used their parasitic wasps on a whitefly infestation in my greenhouse, which seems to have worked well.

Although many of us don’t use chemicals in our gardens, it is widely know that XenTari, (which doesn’t have a licence to be used by amateur gardeners in the UK), is available online. It is a biological control, it does not harm beneficial insects or pollinators. It’s not harmful to humans, wildlife or the environment, so if birds eat insects which have already been treated, it doesn’t kill them. When a caterpillar eats a leaf treated with XenTari it will stop feeding within one hour and will die in 1 or 2 days. If you see the moth on your plants, you can expect to have the caterpillars about two weeks later. The pupae stage is very hard to spot as the dark brown pupae are camouflaged against the internal branches of the plant.

If you decide to spray against this invasive, non native species you need to spray at least three times a year with XenTari. As box plants are generally clipped, there isn’t the issue of pollinators feeding on treated flowers. Common box is monoecious, male and female flowers are found on the plant in April and May, if they are un-clipped. Clusters of green-yellow flowers grow in the leaf axils, similar to the flowers of sarcococca. Both buxus and sarcococca are of the family buxaceae. But I can’t find any evidence of Sarcococca being damaged by the box tree caterpillar. But of course we want that for its gorgeously scented flowers so it’s no good for topiary.

A first XenTari treatment should be in March/April, which is when any overwintered caterpillars will transform into moths and the cycle will begin. Check your box from March onwards, the exact timing will depend on weather conditions. The moths fly again from mid July to September, making July a good time to give a second treatment. A third and final treatment should take place in the second week of October, to kill anything remaining before caterpillars hibernate for the winter sandwiched in a cocoon of web between two leave. Unfortunately they survive down to -30 degrees so won’t be killed off over the winter.

The stronger and healthier your buxus is, the better able it is to withstand an attack, so think about feeding yours. Either treat with a seaweed feed or a proprietary buxus feed. Westland do a great one in two separate bottle, which you combine using an ingenious measure cap that makes life very easy and Vitax do one too. Water regularly during dry spells, to stop the plants getting stressed.

Other things that you may not know about buxus, is that it can actually grow up to 5 metres (16ft) tall if not pruned. It is a long lived shrub, best trimmed in July or August, but not on a bright sunny day or you may find that the cut edges of the leaves will look scorched and brown. It is an easy plant to strike cuttings from, and I have several tiny new box balls in my back garden. Shhh, don’t tell the moths. They haven’t found them yet!

My Box Balls showing signs of damage.