Tackling the box caterpillar invasion
by Sarah Wilson
In May this year I began to hear from gardeners in Kingswood, Ashtead and Banstead about the devastation caused to their box hedges and topiary by the box caterpillar. One friend reported that her hedge was alive with caterpillars and she was picking them off in the hundreds – visiting the Epsom tip she saw people lobbing box balls , topiary and small shrubs into the garden waste because they believed that the box shrubs were irreversibly destroyed by the box caterpillar .
I looked at my box shrubs and they seemed to be fine. We went away in June and on return two of my box shrubs looked decidedly unhappy with a lot of defoliation, typical cobwebs, and frass (caterpillar excrement) under the shrubs.
I looked on line for advice and there were three suggestions:
- Apply a general insecticide such as “Bug clear “
- Remove all box and replace with an alternative evergreen shrub
- Call in a registered pesticide sprayer to spray your box hedges
None of these options appealed to me.
- General insecticides can kill off useful insects, invertebrates and birds, so I do try to avoid them.
- I do love my box and would like to preserve it
- I’m not sure why the recommended treatment can only be sprayed by a registered sprayer
In mid-June I was at the Plant Societies Fair at Wisley and one of the exhibitors and lecturers was the European Boxwood and Topiary Society (www.ebts.org or firstname.lastname@example.org)
I learnt a lot from them – look at their website for more useful information – they recommend the following:
- Affected shrubs should be cleaned up by removing exfoliated stems, cobwebs and the “frass “under the shrub. Then water well and apply a box specific fertilizer which you can get from a garden centre or on line from the EBTS.
- Get some biological insecticide – Xen Tari, Dipel, or Bruco – available to order on line – all contain a bacterium called Bacillus thuringiensis, which will kill the caterpillars as they eat the box leaves by destroying their intestine. Buy a 5-litre sprayer and make up the bacterial solution and spray all your box. If possible, spray on a dry day and repeat every two weeks. Heavy rain will wash off the bacteria.
- Buy some Box moth pheromone traps – these will attract male box moths to the specific pheromone – you can assess the presence of the moths and reduced the incidence. After 4 weeks all my traps are full of box moths – they have white wings with black edges,
- Apply the bacterial spray every three weeks from March to October
- Put out the pheromone traps from May to October
I seemed to have reduced the impact of the box moth and seen no more evidence of caterpillar damage since late June.
Visit the EBTS website and report any sighting of the caterpillars and moths caught in your pheromone trap.
Good luck with your box moth extermination programme!
Dr Sarah Wilson, a long time resident in The Warren, regularly opens her garden in the summer months with the National Garden Scheme.
To contact Sarah please click here