Sunday 30 July 2017

Recovery after box caterpillar

box caterpillar
Box caterpillar only eats boxes
Box caterpillar has now made several appearances in the UK and gardeners might wonder whether a completely defoliated box will ever recover.
Brenda’s son so suffered in Summer 2015 in France. My article about box caterpillar control in his garden is here.
Although Peter claims to have sprayed again after my 2015 visit I doubt it. For reasons it’s best not to go into most of the hedge had been completely neglected for two years during their temporary absence.
The box caterpillar has now completely gone. My 2017 pictures below illustrate the hedges’ recovery - or otherwise. It is a ‘natural experiment’ which might be of interest!

The small hedge at the front of the house was completely killed. Contrast the healthy euonymus hedge 
Two years after - how long can you wait?
Peter was quoted an exorbitant amount for its removal. He is going to be busy with his chainsaw! There is no read to dig out the roots
This is completely recovered and is ready for clipping
I failed to take a picture of the hedge completely smothered by this climbing weed. Not the way to recovery
The box tree is now completely unscathed and the box hedge on the left badly needs cutting
Wondering what happened
The key questions I ponder is why box caterpillar has completely gone and never returned (well, not yet) and why in different parts of the garden recovery has been everything between complete and totally absent. All the box had been completely defoliated in 2015

As to recovery - it’s taking a while. The hedge that has completely died is the small one which was completely exposed to intense Toulouse summer sunshine. In the absence of leaves and their evaporative cooling the exposed twigs would have been very warm. Perhaps the high ratio of surface area to volume in small things was significant. Or perhaps a small hedge had less stored resources than a large one.
What ever the reason it does appear that the larger the box plants the greater the recovery. I think I also discern greater recovery where plants were in a less open position. Perhaps open sites were more exposed to the moths?

I had expected that if the hedge was not re-sprayed surviving caterpillars would complete their lifecycle and there would be further damage later that season and in the following year. It does not seem to have happened. Perhaps the next generation lacking food resources on a decimated plant is programmed to fly elsewhere?

I had also expected there would be further invasion of moths in the following year. This has not happened and the living hedges continue their recovery. For some very slowly.

This post explains why I would not dig out the stumps

End days
Final Report on Peter’s box Hedge June 2018

We returned to France two years later and found our efforts had been in vain. I can only presume that a further infection of caterpillar returned - either from a further invasion or emergence of a new generation from pupae.Certainly Peter confirms he neglected to respray.
This theory is not a given. It is a possibility that the further stress of a hot Summer and continued neglect such as failure to water was what finished them off!

The smallest hedges had already been thought dead. All the larger hedges are now completely brown. A few have few green shoots if you cut all the way back to the trunk and in theory could be saved.
Some hope of that happening now and in any case recovery would take a few years. Should anyone in a similar situation try I would suggest partial removal of dead shoots to let light penetrate through. Only at first partial as complete exposure of delicate green shoots to hot sun might be in itself fatal.
The only good news from France is that Peter’s 20 foot box tree is now pinky and perky and strongly growing away. This confirms the theory that the larger and stronger the box the greater the chances of recovery from a box caterpillar infection

Although Peter must hang his shame over this sad story at least he came up with a wheeze to speedily pull out the dead hedge.

Accustomed to pulling the horse box this was a day out for the car. I do not recommend this for large shrubs!


  1. This got me thinking!
    On the 'why have they completely gone' question, could it just be the natural boom and bust of the species? On the South Downs we have years when the Chalk Hill Blue butterflies are out in their thousands. It can be difficult to walk on a path for fear of stepping on them. But other years you barely see them. It's normal for populations to vary hugely, which makes sense considering they lay their eggs in the hundreds. So maybe it was just a 'perfect storm' year and conditions had favoured them....possibly the adult box caterpillar moth in the previous season had done well?
    As to why some box lives, and some box dies, maybe it's because the smaller plants get eaten beyond just the leaves? I found some Large White caterpillars in my garden, they were on a rape seed plant which had grown up as a weed. I was just about to pull it out (ooops...pulling up weeds....tut tut Sarah!!!!) when I spotted them. I decided to leave them as I was interested to watch them. They grew rapidly and ate all the leaves. They then started eating soft stems and even stripping the soft outer layer from the woody stems. Eventually they disappeared, presumably driven by hunger they had taken the risky step of embarking on a trek across the garden. If the box was eaten to this degree, not only would it dehydrate, it doesn't have any viable bud nodes left either. On a larger plant, their maybe enough leaves to feed them and they don't do as much damage.
    It's very likely that the moths do choose to lay eggs on the plants that are in full sunshine. Some butterflies do this too. So your theory of open sites being more exposed to moths fits with this if the exposed sites are also the sunny sites?
    I'm not an expert, but I do know that two important predators of caterpillars are wasps and birds while they are feeding their young. The best thing for a box tree could be for it to have a bird box and a wasp nest in the tree! Which got me thinking about Liz's post on the other box post. Her cats use the box tree. That would deter the birds from foraging there, so it's not impossible that the cats are part of the 'perfect storm' that allowed the caterpillars have a bumper year!
    Would be interesting to know if Peter or his close neighbours remove wasps nests or keep cats. I don't for one moment think it's as simple as just those two factors, but either would work in favour of the caterpillars.

    1. Sorry that was so long! I might have written more than you did!!

    2. Your ecological ramblings are always welcome Sarah. Make them as long as you like!
      As to tut tut about weed pulling out - it's often a great control method! I am constanttly delving into my borders pulling out epilobiums.
      The weeds not to pull out are those perennials such as couch, ground elder etc that are best left intact to spray!

    3. It has struck me that my phrase 'ecological ramblings' sounds a little insulting and what I mean is 'lateral thinking'. Your insights frequently illuminate my blog!
      Your point about more of a small hedge being devoured as the caterpillars' more nutritious food disappears is a really good one. The poor caterpillars have no choice but to carry on eating.
      I wonder if small starved caterpillars give rise to small butterflies and moths.? Now its my turn to ramble.

  2. You can call them ramblings, I don't mind! Unfortunately my fascination with the garden ecosystem and identifying the native flora and fauna does rather get in the way of doing any gardening.
    I had to look up epilobiums! ah, those little willowherby things that go to seed really quickly. Do they pull up? I'm always suspicious that I'm leaving some root behind, although less so with the very tiny ones.

    1. You must have missed my post on epilobiums! (You can find it through the search box)
      Epilobiums normally come out very cleanly indeed. Unfortunately if one has made a hash and only half pulled them out they then do make a more complex root system and become more persistent.
      They don't actually flower most of the year but do so from late July through to September and are a real nuisance - indeed my worst weed.They then blow in seed from the farm field

    2. I forgot to pick up on the point about the caterpillars embarking out to find new food. I wonder if they really do - you ought to try a few experiments. I have seen nothing in the literature.
      I have told the story that in the churchyard I once flung solomon's seal sawfly caterpillars over my shoulder- rather than squashing them - and a few hours later found them returning crawling up the stems!

    3. Good plan, I just need some very small radio tracker collars! The sawfly story is hilarious, I wonder if they found their way back by smell? That's given me a new and slightly bonkers thought on box caterpillar, and I haven't even had a drink yet tonight! I wonder if well clipped hedges are less prone? I've noticed that hedge clipping often attracts wasps. Presumably the smell of sap draws them in, either because they like sap but I think it's more likely because they use the smell of sap to locate caterpillars. Perhaps topiary peacocks have a greater resistance to box caterpillar than a unkempt hedge!

    4. David J below confirms your theory about muching the wood

    5. But disproved my theory that clipping hedges might attract wasp predators!

  3. Our two very neatly clipped small box hedges have been completely destroyed in 48 hours by the box caterpillar. In five minutes I manually pulled off 60 caterpillars - the hedge is 2ft X 1ft X 1ft.

    I have read that the caterpillars also chew the green stems and damage the bark, which is what determines whether the plant recovers or not. So early action is clearly important.

    1. I hope you are soon enough
      Thanks for confirming Sarah Stu's theory David.
      I wonder if you are in the UK?

    2. I clicked your name link and found your fascinating blog about churches. I see you have been to York six times. Have you done our beautiful church in Bolton Percy and its millennium window. (try millennium window in my search box)

  4. This is fascinating. I live in north-east France and garden in a place where the previous owner planted many (long!) box hedges to create structure in the garden. Most of the box is planted in full sun. Today I finally realised that I had this dreadful caterpillar. I have already ordered Bacillus thuringiensis (today) and am going out to clip the box tomorrow. I will be monitoring results. I would be so happy if it would just 'go away' - but I doubt that will happen!!! It is a small death in the garden! Also - there are (I have read, don't KNOW) no known predators in Europe, because the moth comes from China. However today I have read stories of people seeing birds eating the caterpillars (which are reputed, by many on the internet, to be indigestible to our European predators, because they contain toxins. I also read that there's an experiment in eastern France to release a parasitic wasp (presumably coming from China). Wish I hadn't inherited/bought a lovely garden made lovelier by box. But thanks so much for your blog post - I feel I have made a little contact with 'reality' (life is definitely not predictable!)

    1. Best of luck Cathy.
      Hope you clicked the link to my main original article - its at the top of the post and is marked 'here'
      In Peter's case you will have noted it was just a penetrating spray of an insecticide that I used - although I have every confidence in Bacillus thuringensis. Let me know how you get on


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