After receiving a spike in recorded sightings of box tree caterpillars, the Royal Horticultural Society (RHS) is warning the nation it could be facing a bumper year for the insects as a result of changing climate.

Nearly five times more reports of the insect have been brought to the RHS’s attention in the first four months of 2024, compared to this time last year, with over 1300 reports in the past fortnight alone.

The most common question asked of the RHS Advisory team at this year’s RHS Chelsea Flower Show was about managing box tree caterpillars and finding suitable alternatives to box plants – none of the gardens at this year’s show featured any box.

The RHS speculates the surge in reports is “very likely” due to a mild winter, followed by a warm spring and if temperatures continue at their current rate more sightings could be reported throughout autumn.

A buxus bush destroyed by the Box tree moth caterpillar.

Box tree caterpillars, native to East Asia, are the larvae of a moth that feed on box (Buxus) plants. The insect was originally found in private gardens in the UK in 2011, it is now widespread across the UK.

The insects’ growth and development is temperature dependent, with box tree caterpillars requiring a certain number of hours above a threshold temperature in order to ‘reawaken’.

There are at least two generations of box tree caterpillar in a year, though there can be up to four elsewhere in Europe, with the majority of damage caused between March and October.

The RHS warns that if the mild weather were to continue into autumn, it is possible there will be an additional generation.

Their presence can be spotted via the small green pellets of frass (insect excrement) found under box; the partially, to entirely, eaten leaves; and the presence of webbing.

An ongoing trial at RHS Garden Wisley, Surrey, is investigating alternatives to box trees with a range of compact evergreen shrubs that can be clipped into formal hedging styles.

For more information on managing box tree moth and to report sightings, visit: www.rhs.org.uk/biodiversity/box-tree-caterpillar