Every now and then in life you gain sudden clarity of vision about an issue; perhaps triggered by listening to someone erudite, reading something written with super clarity, or seeing it with your own eyes.  In my case it is the latter and I’m worried – super worried in fact. I am not prone to exaggeration.

Deer at rest in Richmond Park. Notice the horizontal browse line of the trees.

I recently visited Richmond Park, the largest of London’s Royal Parks. It is a vibrant place full of ancient trees, home to several herds of Red Deer, and it teems with joggers, dog walkers and families. Around it on all sides is the highrise metropolis of London including Wimbledon, Putney, Richmond and Roehampton. In a few weeks it will be host to the Olympic cycling road race. Most visitors scarcely notice the metal fencing and red tape around the occasional oak tree; presuming probably that the tree may have a dangerous branch or needs extra protection from the browsing deer. But they would be mistaken.

OPM leaf damage on a young oak tree. Other species of caterpillar can cause similar defoliation but signs of silk among the stripped leaf veins should start to raise alarm bells and a search for nests on the bark should be undertaken in July.

I saw with my own eyes the oak trees of Richmond Park infested with the caterpillars of the oak processionary moth (OPM) Thaumetopoea processionea. The caterpillars can cause serious damage to trees by defoliating entire canopies but this is a minor problem in comparison to their effect on human health. The caterpillars of the third to sixth instars have poisonous hairs or setae that carry a toxin that can cause serious irritation to the skin, eyes, nose and throat of humans and other animals.  If inhaled they can cause respiratory distress, asthma and even anaphylactic shock.

An oak tree under treatment for OPM attracts public interest in Richmond Park. The groundsman directs his colleague on a telescopic platform – the only safe way for a worker to gain access to the upper canopy where the caterpillars build their nests in July.

The human health danger is scary in its own right but it is the consequences of the increasing spread of OPM that is my real concern. Three years ago the authorities at Richmond Park spent £50,000 in one season trying to control the pest but costs have been doubling every year, year on year! A decision was taken last year by all those concerned with OPM in London that eradication was not an option; instead aiming for containment. Meanwhile the moth continues to be found in an ever-increasing range within Greater London. An outbreak in Sheffield was successfully controlled, while a population in Pangbourne near Reading evaded extinction in 2011 and is currently being retreated. How long before the various authorities decide that it is no longer worth spending a six figure sum trying in vain to control OPM when all around them trees in private gardens are going uninspected by the unaware general public? Sadly it may take a child blinded or even a death being reported in the red tops, like the Daily Mail, before notice is taken. This is not a forestry problem – it is a human health problem!

While huge amounts of public money are being spent on attempts to control OPM with little effect, the pest is not yet at large among private gardens and larger forests.  When, and it is surely ‘when’ and not ‘if’, OPM becomes widespread on private land then it will be the landowners that will be required by law (under the Plant Health (England) Order 2005) to report their presence and pay for their control. I can’t imagine any owner of an oak tree infested with OPM being pleased when required to spend up to £1,000 per tree. Forests in Holland and Germany have been so badly infected that they become out-of-bounds; the timber unusable for several years as the toxic hairs remain dangerously potent even on the forest floor.

I fear that OPM is a huge elephant in the room that everyone is wishing would go away. But, it is still early days in England, and if a German or Dutch arboriculturist were to see the extent of OPM in Britain they would surely urge us to try harder: to go for eradication across all of London rather than to give in now. Yes it will be expensive. Yes it will need co-operation between lots of different authorities. Yes the public will have to be involved too and be encouraged to report OPM in their gardens. Ignoring OPM is not an option – its spread is a disaster already happening – soon impacting our gardens, parks and forests across England unless action is taken urgently.

Further information on the Oak Processionary Moth (OPM)

Oak Processionary Moth – management implications

A looming threat to man and tree – a post I wrote in 2011

More information on OPM from Forest Research


  1. I was surprised to learn that Defra have now decided to be a bit more proactive in controlling OPM in the core London area and seem to be throwing £1.5 million at it in a pilot programme :


    Although I’m a bit worried that the information on their map is too light (surely Richmond Park had tons more incidences than that in 2012) and I was disappointed that the decision to do prophylactic spraying was promulgated too late to do much in 2013. For instance the oak wood I am familiar with had at least one nest identified in 2012 and is on the “front line” – but no spraying occurred in May/June.

  2. The amount of plant diseases that have been brought over by imported flora makes me pessimistic about our chances of getting rid of this. If we eradicate it then it’s only going to be a matter of time until someone imports some logs with the caterpillars on or some get blown in from France.

    Plant collectors brought most of the diseases to this country. Don’t get me wrong – I love gardening with many non-native species, but there needs to be some sort of quarantine period. Maybe in addition to spraying, plant importers should be required to pay for housing imported specimens in isolation in a glasshouse for a year where the plant can be tested?

    It seems everything is going for our oak trees. Around here it’s hard to spot any young ones without mildew.

    I just looked up the species – this highlights my case:

    “The moth now has an established population in the U.K. ***The eggs arrived on oak imported to the Richmond area of London*** in 2006, the range of the species in the U.K has been steadily expanding despite efforts to eradicate it.”

    Typical. We should be spraying imported timber and plants at the very least.

    Since the species is from Southern Europe it may be worth studying its predators there and seeing if any exist here or can be brought her without any further detrimental effects. Maybe introduce predator species of one gender that can’t hybridise with anything here? One gender so that the predators themselves can be controlled.

    1. The pine processionary caterpillar has already made its UK appearance at Sherwood Center Parc. We saw them in 2009 and friends did in 2010. The Forestry Commission are now aware and are treating the problem as urgent.

  3. This year has been particularly bad for butterflies with the wet spring summer period devastating the colonies of indigenous species. I also know that bees have been having a terrible time of things with minimal nectar and pollen available. I know because I have taken up bee keeping with a colony of British Black Bees. They don’t mind the rain but if there is nothing out there to collect then they struggle to survive.
    If we introduce any species to chase the other one we run the very serious risk of destroying what is already a very weak and diminished butterfly population. A bit like sending one cheese down the hill after the first one. Also once these harmful caterpillars and also very likely the butterfly larvae have been disposed of what will this or any other beetle or predator turn to for sustenance? We have already messed up the ecosystems for a great many species. Maybe BugLife have an answer to this dilemma?

  4. I fear that after two seasons where the policy was NOT eradication in the core area, it is now too late to hope to try.

    It appeared to me that the decision was made on quite narrow plant health grounds rather than considering the full public health impact.

    I bumped into a couple of surveyors on the Olympic road race route outside Richmond Park and they indicated how much worse the incidence was in 2012 compared with 2011. There are interesting papers from the Dutch on the population dynamics of OPM with possibilities of a seven-ish year cycle building up to plague levels at peak. The Forestry Commission pages also have a very worrying report from the guys who went on a study tour to East Germany.

    Regarding natural predators, have you come across Calosoma sycophanta ?

    If I had any money. I’d put it into commercially breeding them ! (allegedly they were native to UK in the last century).

    1. Author

      Thank you Tim. No I was not aware of Calosoma sycophanta and that’s a great video you linked to, thank you. I will find out more although it seems that it devours caterpillars of many species indiscriminately, so it’s reintroduction may be an issue for our native species of butterflies and moths. As you say, perhaps there is someone out there who knows more about it and may be interested in investing!

  5. Can’t we employ more Cuckoos to come to our woodlands; sadly, they are going the way of much of this earth’s wildlife. My reason for mentioning the Cuckoo is that they have a liking for hairy caterpillars.

    Let us hope we (man) can do the right thing and eradicate this pest. I believe we all agree, out of the 2000 plus non-native biodiversity killers we currently have, this needs man’s intervention immediately.

    Best Wishes

    Tony Powell

  6. Well done. May I make one or two corrections.

    ‘The caterpillars can cause serious damage to trees by defoliating entire canopies but this is a minor problem ‘…… myth. OPM KILLS trees – viz tree losses in Germany. So OPM is not only a health problem but also a tree problem.

    ‘A decision was taken last year by all those concerned with OPM in London that eradication was not an option; instead aiming for containment.’ It was not. That was the decision of the Forestry Commission. Many of us understand that with adequate resources and recognition of the public health problem, OPM could be eradicated. Only look at the minute size of the infested zone on the scale of the whole of the UK.

    ‘While huge amounts of public money are being spent on attempts to control OPM with little effect…..’ . The amounts of money are very small compared with – you name it , plenty of un- or less important issues and the amounts are simply not adequate – the problem here being that treatment is not centrally funded but has to be met by tree owners.

    ‘……. the pest is not yet at large among private gardens and larger forests.’. This is guesswork. Only presence can be proven, not absence. These areas supposedly free of the pest have simply not been looked at

  7. If I get this right we are facing a serious threat that will devastate the woodlands completely and prevent us mere mortals from indulging in our walks under the cooling canopies. It makes our efforts to prevent the sale of the forests seem totally pointless if we will inevitably be banned from them on health and safety concerns relating to a mere caterpillar.
    So in my humble opinion it would seem that it is imperative that this damnable caterpillar is removed totally from the UK. No matter what it costs to do so. Otherwise the government will have achieved a backdoor victory over the people, hollow though it be. The only consolation for us then would be the fact that the wood would not be sale-able for a long time due to the pines of this caterpillar.
    Oh and the Daily Mail is not a red-top. The Mirror and the Sun are. But I take your point. Sensationalist press one and all. 🙂

    1. That should read ‘due to the spines of the caterpillar’. My apologies.

      1. Author

        I rather liked ‘pines’ of caterpillars! On a serious note though I agree that this could/is become/ing a very real issue. Yes in our oak forests then this pest could be devastating. I can’t imagine the true horror of this reaching the New Forest or Sherwood, Forest of Dean etc.

        1. I imagine there is no natural predator for this caterpillar hence its toxic spines? So the alternative to manual collection and destruction (with the necessary protective gear) would be what precisely? Sprays? Nematodes? Surely there is something which can be applied which has an immediate effect upon this caterpillar?
          How about some form of internal toxin for the trees? I recently read that some plants, when attacked by insects send out a signal to the surrounding plants of its type which triggers a sudden development of toxins within these other plants which repels the insects. This is particularly noticeable in vegetables when there is a slug invasion like we are witnessing this year. Its worth investigating whether there is some method of stimulating this effect in trees or alternatively maybe a toxic serum to kill the caterpillars that could be fed to the trees which due to the evapotranspiration effect would release this through the leaves where the insects may be munching. Its an interesting idea however I bet I am not the first to think of this?

  8. Here in the south of France the same desease has been plaguing many oak and pine wood forests since a few decades!!..(in the Mont Ventoux large mixed plantations of Alep pine trees and different species of oak groves are being infested too not to speak of the famous gorges of Ardêche exposed to the pest since a long time ..depending of the weather variations during winter and spring time -I noticed a mild and dry winter season inducing an “explosion “of OPM nests everywhere on the following summer time !( such was the case two years ago )…now and again we happened to climb on the trees to remove the nests in our private gardens !
    Many thanks and congratulations for your forestry and artistic projects in England

    1. Author

      Thanks Frédérique. It was interesting to read of your experiences in France with this pest. Of course on your pines it is a different moth species – the pine processionary moth – that causes the same problems as OPM. It is my understanding that OPM is not yet present in Normandy and other areas of northern France?

      1. In fact OPM has already been spotted in Normandy for the first time in 2007 in Nojeon-en-Vexin (27), but the plague area was confined to a few isolated oak trees. Than the desease has been spreading out to the edge of forests,till it gained the very core of oak groove wild expanses! ( for instance around Etrépagny). Unlike previous years, it has been presently recorded that some oaks could’nt recover from the renewed moth attacks which cause damageable defoliation – may be due to global warming and rainfall deficit !-idem in other northern regions of France..

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