Developing a strategy for Box Tree Caterpillar moth control

The spread of Box Tree Caterpillar moth (Cydalima perspectalis) in the UK appears to be very rapid with new reports of its further spread coming in all the time. This combined with Box blight is giving poor Buxus plants a hard time! With calls for it even to be replaced in favour of other plants. For this reason; a strategy to protect Box plants from Box Tree caterpillar attack is urgently required. The Caterpillar can appear in high numbers and is capable of totally defoliating box plants and in severe cases killing plants all together. The caterpillars are greenish-yellow when they first emerge and will spin webs to pupate in.

This year Dragonfli has been investigating potential biological control programmes to keep this invasive pest in check. Work in France carried out by Koppert Biological Systems, where the pest is widespread already, suggests that treatments of nematodes, especially the species; Steinernema carpocapsae can provide control of the caterpillar stage of the life cycle. The key to success is timing and coverage with plenty of water. Nematodes are UV sensitive, so need to make contact quickly with the caterpillars and should not be applied on bright days. Here in the UK Dragonfli has been working with the National Trust at Ham House to evaluate other biological controls. The first step being to monitor and catch male Box tree moths with pheromone traps, which has been largely successful but is not a control method on its own. Large numbers of moths can be caught, and this provides an early warning of the caterpillar infestations that will follow. Biological control can then be timed for maximum effect on the caterpillars. At Ham House we introduced some tiny parasitic wasps that are egg parasites to try and take out a different part of the Box tree moth life cycle. Lacewing larvae were also introduced to eat the eggs. Both appeared to have some effect, but the trial needs to be repeated and on a larger scale.

The most effective treatment available to professional gardeners and growers in the UK is the biological insecticide; Bacillus thuringiensis. This acts as a stomach poison to the caterpillar but is safe to other insects, animals and humans but can be difficult to apply to already heavily infested Box plants and hedges, as the caterpillar needs to ingest it directly from the leaf it is feeding on. In the UK the product is available as DIPEL DF.

A combined approach involving trapping of adults, destruction of eggs and killing of the caterpillars with nematodes and biological insecticides will be required to halt infestations of Box Caterpillar moths but more work is required on timings, application techniques and potential new natural predators or parasites of this very damaging invasive pest. If you have any queries about biological control for Box tree Caterpillar moth , please contact . Dragonfli is also working on biological solutions to prevent Box Blight.

[Prepared by Julian Ives for PlantNetwork, November 2018]