A defoliated box hedge in France.
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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.
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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.
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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?
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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
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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 |
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We only found a single pupae. Does this mean that Peter caught most of the caterpillars in time? |
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