Box Tree Caterpillar

Recent sightings of Box Tree caterpillars in Normandy is a timely reminder to be on the lookout for these small but voracious pests that can defoliate box hedges and topiary in a matter of just a few days. Plants will eventually recover and should not be removed, but attacks are unsightly and will weaken plants.

Photograph from ebts.org

The caterpillars are the larval stages of the Asian Box-Tree Moth (Cydalima perspectalis) accidently introduced into Kent in 2007.  There can be three generations of caterpillars and adult moths in a season from March to October, so their spread from Kent has been rapid and relentless.  

Photograph from ebts.org

Various pesticides, both organic and synthetic can be effective, but repeated applications may be necessary and as caterpillars usually produce silk webbing over their feeding area, contact spraying can be made more challenging by the need to penetrate this protective web.

Biological control by spraying foliage with strains of the bacterium Bacillus thuringiensis (Bt) can be effective and there are various products available for this with Dipel and XenTari available from Amazon for example.  The bacterial spores are ingested releasing toxins into the gut of the caterpillar causing it to stop feeding and die.  As with pesticide sprays, care needs to be taken to only spray the affected plants as the bacterium will kill any caterpillar that ingests treated leaves.  

Pheromone traps can reduce the population of male moths which can reduce the numbers of eggs laid, but are generally used more for monitoring of the presence of adult months. 

Finally, it is not recommended to jet-wash plants to remove the caterpillars as this can damage the plants and increase susceptibility to Box Blight (Cylindrocladium buxicola); a fungal disease that will kill the stems of box plants.

There have been some interesting studies on developing deterrents for these troublesome pests – both using synthetic extracts of caterpillar frass (the polite name for caterpillar poo!) and by using other plant extracts; the most promising of which was Thyme extract.  Both were effective in lab-based trials, but the synthetic trials were not successful in field-use due to inconsistencies in the release of the extracts.  However, the Thyme-based extracts showed up to an 80% reduction in caterpillar numbers.  Whilst the trials are ongoing, this does offer some hope for an effective deterrent in the future.

In other tests, the RHS have been looking at whether the shape of clipped box and the tightness of leaves makes any difference to the predation rates.  Short answer is sadly, no – there was no significant difference if box plants were clipped into cubes or spheres or left unclipped.  There was however a difference between different varieties of Box.  The Common Box (Buxus sempervirens) was by far the most preferred host for the caterpillars with the highest rates of infestation and the most caterpillars.  The small leaf box, Buxus microphylla ‘Faulkner’ had the lowest rates of infestation and the fewest number of caterpillars.

This is a growing threat for gardens here in Surrey and elsewhere, and gardeners are encouraged to record all sightings and infestations on the Box Moth & Caterpillar Tracker hosted by the European Boxwood & Topiary Society.  More information is available on the ETBS site and from the RHS.