The Royal Horticultural Society has warned that 2024 could be a bumper year for box tree caterpillar moth.
There have been almost five times the number of reports of the insects in the same period as last year, and the charity says that there is the possibility of more generations than usual if we have a warm autumn.
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The RHS has received nearly five times as many reports of the insect in the first four months of 2024, compared to in the same period last year, with more than 1,300 reports in the past fortnight alone.
The most common question to the RHS Advisory team at this year's RHS Chelsea Flower Show was about managing box tree caterpillar and finding alternatives to box plants.
The surge in the caterpillars is thought to be due to a mild winter followed by a warm spring, which meant the caterpillars' activity started earlier. If temperatures hold, we could see a further generation in the autumn.
Box tree caterpillars, native to East Asia, are the larvae of a moth that feed on box (Buxus) plants. First discovered in private gardens in the south east of the UK in 2011, it is now widespread across the UK.
The caterpillars need a certain number of hours above a threshold temperature in order to reawaken and start feeding. There are at least two generations of box tree caterpillar in a year, but elsewhere in Europe there can be up to four. If mild weather continues into autumn it is possible that there will be an additional generation here in the UK, meaning more damage overall to box plants.
Dr Stephanie Bird, RHS Senior Entomologist, said: '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 where 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.'
In our article on how to treat box tree moths, we list solutions many gardeners have found effective, including biological controls that contain the micro-organism Bacillus thuringiensis, sold as Topbuxus XenTari or BuxRevive. You could also try a natural solution like Plant GP's treatment. Be sure to use a knapsack sprayer if you have a lot of box to cover.
Growers of box that have not yet been affected are advised to deploy pheromone traps in order to help determine when the moths are present, in order to then look for and remove the caterpillars.
Discover more about dealing with box tree caterpillar and our recommended alternatives to box.