2024 set to be bumper year for box tree moth
RHS scientists predict a bumper year for box caterpillar after reports rise nearly fivefold from last year
Since its arrival in southeast England in 2007 and subsequent discovery in private gardens in 2011, box tree moth has become widespread throughout England and is now found across the rest of the UK and Ireland, compounding the challenges already faced by gardeners from the fungal disease box blight.
Now, RHS scientists are predicting a bumper year for box tree moth, whose greenish yellow, black-striped caterpillars feed on box foliage to leave defoliation, dieback and webbing.
A bumper year for box caterpillar
The RHS Plant Health team is predicting that it could be a bumper year for box tree moth after receiving nearly five times as many reports of the insect in the first four months of 2024, compared to the same period last year, with more than 1300 reports in the past fortnight alone.
Additionally, the most commonly asked question to the RHS Advisory team at this year’s RHS Chelsea Flower Show was about managing box tree caterpillar and finding suitable alternatives to box, despite none of the show gardens featuring any box.
Guide to choosing box alternatives
List of the best box alternatives
All about the box alternatives trial at RHS Wisley
The surge in reports is most probably due to a mild winter followed by a warm spring, which meant the caterpillars’ activity started earlier than usual. If temperatures hold, the team predict that we could see an additional generation in the autumn.
What is box caterpillar?
Box tree caterpillars, native to East Asia, are the larvae of the box tree moth (Cydalima perspectalis), which feed on box (Buxus) plants. The insect, which was first found in private gardens in the UK in 2011, is now widespread across the UK.
Insect growth and development is temperature-dependent, with box tree caterpillars requiring a certain number of hours above a threshold temperature in order to ‘reawaken’. In the UK there are at least two generations of box tree caterpillar in a year, though elsewhere in Europe there can be as many as four.
The majority of damage is caused between March and October.
Signs of box tree moth – defoliation and dieback, green frass (excrement) pellets, webbing and black head capsule shed during moulting
Their presence can be identified by the small green pellets of frass (insect excrement) found under box, the partially or entirely eaten leaves, and the presence of webbing. You may also be able to see the green and black caterpillars on the plants.
What can I do about box caterpillar?
RHS Senior Entomologist Dr Stephanie Bird says: “Box growers now face this insect across the whole of the country. While box is a pretty resilient plant and can recover from even complete defoliation, this does weaken the plant, and when desperate, hungry caterpillars can go on to strip the bark, girdling and killing sections.
“However, all is not lost as there are now known to be several naturally occurring enemies, including parasitoid wasps and a fly that can provide at least a low level of control. Gardeners may need to make tough decisions, considering which plants they love and want to retain versus replacing with an alternative where management is no longer practical.”
Adult box tree moth
Caterpillar of box tree moth
If you grow box and have not yet been affected, but are concerned about possible damage, try deploying pheromone traps. These will indicate when the moths are present, in order to then look for and remove the caterpillars at an early stage.
Gardeners should focus on the benefits of having a biodiverse garden and the contribution each species makes to a healthy ecosystem. Increased biodiversity can prevent any one species becoming too prevalent and causing serious damage to plants.
Guide to choosing box alternatives
List of the best box alternatives
All about the box alternatives trial at RHS Wisley
Read our guide to box caterpillar for more information on managing box tree moth and to report if you’ve seen it in your garden.
More new arrivals getting a foothold?
As the climate changes, the RHS monitors reports and enquiries from the UK’s gardeners to provide valuable insights into emerging patterns across the country.
In 2023 the RHS Plant Health team also saw a surge of enquiries around fig leaf skeletoniser moth, which was first recorded in the UK in 2014. The larvae feed on fig leaves, grazing away the upper surfaces or folding leaves over and binding them with webbing in order to feed inside.
There has also been an increase in observations of an as-of-yet unidentified aphid appearing on buddleia plants. The aphid, believed to be related to the black bean aphid, feeds on the underside of young buddleia leaves. This renders the leaves crumped when fully expanded, with pale spots on the upper side.
The prediction of a bumper year for box tree moth follows another prediction by RHS Science, as we head into the drier months of summer, that recycling bathwater and other household greywater for watering gardens will become mainstream by 2035.
