Have you seen this caterpillar?

The Royal Horticultural Society (RHS) and the Food and Environment Research Agency (FERA) are calling on gardeners to help the RHS map the whereabouts of box tree caterpillar (Cydalima perspectalis).

This native of East Asia eats the leaves of box plants (Buxus species) and has been reported for the first time in British private gardens. Adult moths have been recorded in Southern England since 2008, but its caterpillars had only been found at one commercial nursery in Surrey in 2009 and 2010. In May 2011 the RHS received the first specimens from private gardens in Buckinghamshire and Hertfordshire.

Box tree caterpillars were first detected in Europe in 2007 and are now widespread on the European mainland. FERA concluded that, given the pest’s rapid spread and the possibility of adult moths reaching the UK naturally, no statutory action would be taken on further occurrences in the UK.

“Box plants can be totally defoliated by this moth’s caterpillars and there is also a threat to our uncommon native box plants such as those on Box Hill in Surrey. This is yet another problem for box in the UK which is already suffering from diseases such as box blight (Cylindrocladium buxicola),” says Dr Andrew Salisbury, RHS Senior Entomologist. “It is important for us to monitor this caterpillar’s movement. Knowing how quickly it is spreading and what plants it has been found on could help us develop ways of managing this pest.”

“There is little published information on the control of Box Tree Caterpillar,” says FERA Entomologist Anastasia Korysinska. “The silk webbing around the caterpillars can make control difficult. However, insecticides available to gardeners for controlling moth caterpillars, such as pyrethrum, deltamethrin or lambda-cyhalothrin, should be effective. Physical control by cutting off infested material or picking off caterpillars could also help.”

In Germany, complete defoliation of ornamental box plants have occurred but in the Netherlands the damage has been less severe. In Europe the caterpillar has only been found feeding on species of box. However, in Asia other host plants include llex purpurea (purple-leaved holly) and Euonymus alatus (winged spindle tree).

The RHS asks gardeners finding this pest in their garden to let it know by sending samples or photographs to Entomology, RHS Garden Wisley, Woking, Surrey GU23 6QB or sending images to Advisory_Entomology@rhs.org.uk.

Box is not usually damaged by other caterpillars, so extensive defoliation and webbing on this plant is likely to be due to box tree caterpillar.