How do I deal with box tree caterpillar?
Telling caterpillar damage from box blight, the monitoring and treatment cycle that keeps box alive, and the point at which replacing the plant is the sensible answer.
- Difficulty
- intermediate
- Time
- 1 hr
- Read
- 3 min
Short answer
Look for webbing with green-and-black caterpillars inside it, and stripped stems with the bark grazed off. Pick them off and cut out webbed shoots, then treat with a bacterial Bt spray in the evening, repeating with each generation from April to October.
Box tree caterpillar arrived in the UK in 2011 and is now widespread in England, particularly in the south east. It can strip a mature box hedge in a couple of weeks, and it produces two or three generations a year, so a single treatment is never enough.
What you'll need
- Pheromone trap
- Bacillus thuringiensis spray (optional)
- Secateurs
- Bags for disposal
Step by step
- Confirm it is caterpillar and not blight.Caterpillar damage shows webbing, droppings, stripped stems with bark grazed away, and visible green caterpillars with black heads and stripes. Blight shows dark leaf spots, black streaks on stems and brown patches without webbing.
- Hang a pheromone trap in spring.It catches male moths and, more importantly, tells you when a generation is flying so you can time treatment. It does not control the infestation on its own.
- Search the plant thoroughly.Caterpillars start deep inside the bush where you cannot see them. Part the foliage and look into the centre, not just at the surface.
- Pick off what you can reach.On a small plant or a topiary specimen, hand picking and cutting out webbed shoots is genuinely effective. Bag and bin the material.
- Treat with a bacterial spray.Bacillus thuringiensis is a biological control specific to caterpillars. Spray thoroughly into the plant, not just over the surface, in the evening, and repeat as each generation hatches.
- Time treatment to young caterpillars.Bt works on small caterpillars and poorly on large ones. Treating within a week or two of a trap catch is far more effective than treating visible damage.
- Repeat with every generation.Two or three generations from April to October in most years. Treating once in May and stopping is why many people lose the plant in August.
- Cut out and bag webbed material.Do not compost it in an open heap. Bag it and bin it or take it to green waste.
- Feed and water a stripped plant.Box regrows from bare wood if the root system is sound. A stripped hedge given a spring feed, a mulch and water through summer usually refoliates.
- Consider replacing with an alternative.Where box is failing repeatedly, Japanese holly, Euonymus 'Jean Hugues', yew or Lonicera nitida all clip to similar shapes without this pest or blight.
Tips
- Webbing means caterpillar, no webbing means blight. That is the fastest test.
- Bt only works on small caterpillars, so timing matters far more than dose.
- Box regrows from bare wood — a stripped plant is often not a dead plant.
Common mistakes
- Treating once and assuming it is dealt with — There are two or three generations a year. The plant survives May and is stripped in August by a generation nobody was watching for.
- Spraying only the outside of the bush — The caterpillars work from the inside out. A surface spray misses almost all of them. The spray has to be worked into the centre of the plant.
If it doesn't work
Webbing and green caterpillars inside the bush
Cause: Box tree caterpillar — Fix: Pick off, cut out webbed shoots, and spray Bt thoroughly, repeating per generation.
Brown patches, no webbing, black stem streaks
Cause: Box blight, a fungal disease — Fix: Improve airflow, avoid overhead watering, clip in dry weather and disinfect tools. Cut out affected material and consider replacing severely affected plants.
Stripped to bare stems
Cause: Heavy caterpillar infestation — Fix: Continue treatment, then feed, mulch and water. Box usually reshoots from old wood.
Returns every year
Cause: Reinfestation from surrounding gardens — Fix: This is now normal in affected areas. Monitor with a trap and treat each generation, or change to a different clipped evergreen.
Questions people ask
How do I tell box caterpillar from box blight?
Webbing and droppings mean caterpillar. Dark leaf spots, black streaks on the stems and leaf drop without webbing mean blight. Both can be present at once.
Will my box recover if it has been stripped?
Usually, if the roots are healthy. Box reshoots from old wood. Feed, mulch, water through summer and keep treating the caterpillars.
What can I plant instead of box?
Japanese holly, Euonymus 'Jean Hugues', yew for larger shapes, or Lonicera nitida for fast hedging. All clip well and none are affected by this pest.