Wednesday, 26 August 2015

Control of box caterpillar

A defoliated box hedge in France.
Healthy last year all the box hedges are devastated. Note all the other plants are completely unaffected
News filtered home in May that Peter's garden box hedge was looking sick. My first thought was that it was the dreaded box fungus disease known as box blight. Normally thought to be the kiss of death to buxus in a garden, research on the net suggested that repeated spraying with fungicide might bring a revival. Probably not for Peter.

It was therefore good news when in June we heard the hedge was being defoliated by caterpillars! Good news in that caterpillars can be killed relatively easily by a caterpillar spray. I am not familiar with popular French pesticides but informed Brenda's son that many common insecticides are systemic and he should not use them as they are more effective against sucking insects such as aphids rather than biters. This is particularly true on woody plants. I suggested he might seek out a spray of Bacillus thuringiensis, an extremely effective biological control available in France but not in the UK.
In the event he bought a contact killer that killed the caterpillars within 24 hours. Pete needed a full fifteen litre knapsack sprayer to thoroughly drench his eighty meters of hedges. Unfortunately it was too late to reverse the damage and the hedge turned completely yellow and all the remaining uneaten leaves died.
I felt fairly certain that the culprit would be the ermine moth. It sounded to have all the classic symptoms with vast numbers of caterpillars devouring the leaves and producing copious webbing.

Peter later confessed that the more out of the way boxes had not been sprayed. Their hoped for recovery will take longer
I am now writing this in France at the end of July. We are here for a party and the hedge is yellow and brown! What a disaster! It is only the box that has died. Other hedge plants are completely unaffected even where growth intermingles.
I turned to the RHS website and found that it was not ermine moth but a pest newly reported in the UK specific to box and now dubbed the box caterpillar. It looks as if it might be a future problem at home in the UK. Here in Toulouse it is extremely hot in Summer. No surprise that hot London gardens are now starting to suffer.
The hedge is now weakly starting to sprout new green shoots.

Unfortunately this recovery is exceptional and most of the hedges are still completely brown
Optimistically I am hoping in the next few weeks it will green up again. For this to happen Peter must re-spray at the first sign of a new generation of caterpillars reappearing. I did today find half a dozen caterpillars he had missed. It was enough to just squash them.
It is extremely important that in late Spring next year he keeps his eyes peeled and at the very first sign of caterpillars emerging from within the protection of leaves webbed together he resprays. It is likely the hedge will survive being defoliated once, but twice I don't know.
I have done all that can be done at the moment. In the more obvious places I have brushed off some of the dead leaves that still clung to the plant. Not only is the hedge visually improved exposing the few delicate new shoots it might help in a very small way to speed recovery because the new leaves are more open to sunshine. I noticed some of the twigs contain chlorophyll and this will enable them to photosynthesise.

Green twigs have a small photosynthetic capacity
I have emphasised to Peter that unless the hedge improves substantially he should not clip any new green shoots this year.

Three days later and already a change of plan! The new shoots are coming quite quickly in this warm Summer weather helped no doubt by Brenda's generous irrigation. Unfortunately more caterpillars are apparent. No doubt some that Peter missed (although I now read that there can be three generations a year). I was going to spot spray but decided it would be wise to spray all the hedges again.
I checked on his spray. It was delta methrin. A very safe and effective spray. Chemically it is related to natural pyrethrum. Why it's almost organic! I was quite disappointed when I saw he had spent twenty five euros on a small 250ml bottle. I quickly changed my mind. We were in France after all. He had gone to his rural supplier - he has two horses - and bought professional product. I needed to dilute 10ml for a full fifteen litre knapsack sprayer. A cost of one euro!

I only needed 10ml of this deltamethrin to make up a full sprayer
It was great that he had a professional Berthoud sprayer that I had previously ordered, cost £130, for the spraying of weeds I do on our visits. I think in my life I must have emptied fifteen thousand full knapsacks spraying weeds but I have never sprayed a hedge! What a delight not to have to bend down. With the long lance it was very easy to spray and on the taller hedges to reach the top. Fortunately his nozzle was a cone one designed for such spraying.

The long hose and lance enabled very easy and rapid spraying
There was more box hedge than I thought with a total surface area of seven hundred square meters. Unlike Peter I emptied two knapsack sprayers. It took nearly two hours.
Peter might have to re-spray later this year, or perhaps not. He will certainly need to spray next Spring or early Summer. (It later emerged that Peter had only sprayed half the hedges – those in prominent places hence the discrepancy in the amount of spray needed. These hedges are regenerating much more slowly!)

I have mentioned before gardeners who falsely economize buying cheap amateur sprayers. The large area of mature box hedging is worth infinitely more than the cost of the sprayer. Without a decent sprayer that is safe, accurate and rapid to use I doubt if the hedges would be booked for future healthy survival.

Organic control?
What about folk who do not use pesticides?
I rarely spray insecticide or fungicide in the ornamental garden but this was force majeur. There were far too many caterpillars for hand picking. How about biological control?

A packet of biological powder
A very effective biological control is Bacillus thuringiensis. For reasons I fail to understand it is unavailable in the UK. It is a bacterial disease of caterpillars and the packeted powder in addition to millions of bacterial spores contains the disease's natural toxins that immediately kill caterpillars on contact. They recoil on contact and immediately die! The disease is completely harmless to other than caterpillars and usually maintains its infection of future caterpillar generations for the rest of the season. It is actually applied as a spray. As mentioned Peter can chose to use it in France as it is sold at the amateur store.
The RHS site lamely and unconvincingly mentions a parasitic nematode. Is this practical or just pie in the sky? Why is one biological control approved and not a better one?
I wondered if the caterpillars might be washed off with a forceful hose. If so would they climb back to return? When I picked off a caterpillar to get a picture it jerked away on what appeared to be an immediately spun thread. As I watched it, it gently threaded its way back. I doubt success but for a no-chemical gardener washing off might be worth trying.
By the time I got out my camera the caterpillar was almost returned

And a parallel story.

In my first teaching post, Lancashire Agricultural College provided a garden advisory service. For several years there were numerous phone calls every Spring when ermine moth caterpillars ravaged local hawthorn hedges. It is etched on my heart the chemical control we recommended. It is not available now.
The fascinating thing was that the phone calls in the first year were very close to Preston. Each year the radius of infection increased by ten miles. I left four years later and moved to York. Ermine moth should have arrived there by now!
I jest, nature does not work like this. Numbers of pests might explode in population for several years but eventually a balance is restored. Pests and parasites increase in numbers and maybe change their habits of feeding. Sources of nutrition might be depleted, vagaries of the weather might bring massive destruction and geographical barriers might get in the way.
I might mention that Peter's box caterpillar similarly appeared out of nowhere with no apparent infection in the previous year.
I wonder what will happen to the box caterpillar if it really establishes at home in the UK.

A box hedge in a London park. Box caterpillar has not yet arrived in Colliers Wood. I hope it never will.

The culprit and the damage it causes

A hungry box caterpillar eating a new green shoot. Note the webbing and the completely eaten portions of leaf

I suspect that if I left the caterpillar here on the ground it would find its way home

A typical skeletonized leaf. Smaller caterpillars have not completely eaten through the epidermis

We only found a single pupae. Does this mean that Peter caught most of the caterpillars in time?

There was lots of webbing
It was interesting that all the original foliage was completely killed even if not obviously eaten. The whole plant had gone yellow, then the leaves dead and brown. I wondered whether this was the plants’ defence mechanism to deprive the caterpillars of green food or perhaps in contrast it was the result of a toxin that made the leaf more palatable. Whatever the explanation it was an absolute mess


  1. OMG that looks terrible. With all the box in my garden, I hope they never get those caterpillars. Good they leaf back out though.

    1. I hope you never get it over the pond Donna!
      It looks as if there might be a panic starting over here

  2. What a sad tale! (And some 'orrible photos of the beasts too)

    1. And with great timing the BBC has reported last night it being a problem outside London - the panic is starting - 150 gardens!
      And some lousy controls are being touted such as picking the caterpillars off and someone said "kill them in the freezer"!

  3. How interesting! Both this post and the one on horse chestnut leafminer have had me wondering what the natural predators could be, and why they aren't playing their part. As you already commented, boom and bust years are normal for insect populations, but things quickly rebalance. Birds and wasps are perhaps the biggest natural enemy, but both caterpillars are well protected. Tightly clipped hedges probably deter birds though, and humans do have a tendency to get rid of wasps nests near their homes. But there is another predator that is not doing at all well, and that is bats. I wonder if the risky time for either species is when the unprotected adult moth has to fly, and if they are having a much better survival rate due to declining bat numbers?

    1. Part of the problem with alien pest invaders is that they don't bring their own parasites or predators.
      Sometimes native generalists take on the role and sometimes after just a few seasons natural selection finds an organism that just fills the niche! (Recall, Sarah, my post where near the Welsh lead mines where flora very rapidly were selected for tolerance to lead pollution)
      My piece mentioned blue tits. Sometimes native 'intelligent' mammals and birds learn new habits and their young learn to recognise tasty morsels
      And sometimes the host just caves in, witness our loss of the elms.

    2. Leaf miners appear to have already worked out their defence strategy, as no blue tit, spider or common wasp will ever find them!

      I have lived for at least ten years with a very annoying miner moth in my house plants. First thing it went for was a Queen Victoria century plant. Once it had killed that, it went for a Canary date palm. David moth and Goliath palm (it was a big one!), one day it just keeled over and died. It goes right into the fleshy part, I only find the cocoons and adult moths. It gone down in the world lately, having eaten all the expensive stuff, it is now toppling some mother in law's tongues. I've learned my lesson, and have stopped buying house plants :-(

    3. Have you tried Provado Vine weevil killer Sarah? Perhaps the plants you mention are a bit woody for it to work well.

    4. I tried it a very long time ago. It didn't seem to kill anything apart from my bank balance! The date palm was huge (boo hoo) so perhaps that just needed huge amounts. Or the moths are immune. I've tried it for red spider mite on taro and they just laugh at me and carry on breeding.

    5. I wonder which leaf miner it is Sarah. I googled it and found that leaf miner was a major pest on palm plantations
      ps perhaps we should have had this conversation on my previous post!

    6. Actually I don't think it's technically a leaf miner, so we are ok, it's still moths! I think it came in on a cactus (also dead), and it seems to go deep into the fleshy base. When they aren't destroying my property I find moths fascinating, some have such strange diets. The rare UK goat moth spends up to four years eating heartwood of trees!

  4. Yet another reason not to plant boxwood, which I don't like anyway. Though this is an test of the idea that you should be patient and let the insect population regain its own balance. Where are the limits of applying this approach?

    1. I think in this particular case the result would be a dead hedge Jason. In fact I did find a fairly hysterical reference on the net to this pest that suggested one infestation was enough to kill a hedge.
      I hope and expect this is not true

  5. Yes I think an opportunity to plant something more interesting and attractive to humans & insects.

    1. I always regard a dead plant as a new opportunity Patricia
      Box is certainly unattractive to most insects because apart from any lack of flowers it contains its own natural toxins.
      Unfortunately not to box caterpillar!
      I think you are being a bit hard on box though.

  6. I have not heard of box caterpillar in North America - another thing to look forward to. We grow less box here than in Europe.

    I think the comment about BT being a contact poison is not correct. I believe it needs to be eaten by the caterpillar and affects their gut.

    1. Thanks Robert i believe you
      Thank goodness some of you are prepared to correct me when I am wrong.
      It did seem to me that when I sprayed some caterpillars they did recoil!
      I did spray some cabbage whites last week with my French supply and they were dead within 36hours
      Interesting to hear you have BT over there

  7. Roger
    We have been attacked by the moth and have used, to good effect, BT. You don't mention moth traps. Is that because you don't think they offer a viable solution?

    1. Quite honestly I have never used pheromone traps so have no useful opinion to offer.
      I think they might work very well but for the featured hedge in France we would have needed a lot of them.
      Thanks for the thumbs up for BT

  8. Roger, I forgot to mention that I am in the Lot Valley where the infestation has been massive.
    My hedges are not as badly affected as that of your friend.

    1. Thanks for the french connection Justin

  9. Rob (ourfrenchgarden)15 April 2017 at 10:17
    Hi Roger

    I came across your blog googling about the Box Caterpillar - it's a real problem here in SW France. I've just sprayed with Decis al and seems to have arrested the problem but I can't believe that they started again so early in the season. Your blog is very informative and good to know. Keep up the good work!


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