A Whole Lot of Processioning Going On

Oak processionary moth caterpillars (Thaumeetopoea processionea) (Photo by Joanne Jordan)

Dear Readers, the poor old woods have taken a right battering over this past few months. Firstly, I would say that footfall is up by about 300% as people go for a walk before, after and probably during work. Then, it’s been extremely dry. Add to that the flooding in the spring, and the amount of litter that some people think it’s ok to leave after a picnic on the fields, and it’s all been very trying. I imagine the oak trees have seen it all before, so I can only imagine how delighted they were when a bunch of oak processionary moth caterpillars emerged and started, well, processioning.

They look cute and fluffy, but the hairs can be very irritating, both when in direct contact with the skin, and if they are inhaled.  Furthermore, a heavy infestation can seriously defoliate a tree – the caterpillars prefer pedunculate and turkey oaks, but on the Continent, where they originated,  they will also munch their way through beech (although they can’t complete their life-cycle without oak). Unfortunately, the ‘treatment’, which involves a heavy dose of insecticide, can be as almost as bad as the insects themselves. Normally this treatment is most effective when young caterpillars are sprayed – later in the year it can be just as efficient to remove the nests, which will be full of pupating caterpillars and will hence help to prevent the adults from flying off and affecting other trees.

OPM (as I will call the insect from now on to save my typing fingers) was ‘accidentally’ introduced to the UK on some imported oaks in 2005, and is largely London-based at the moment, so the plan is to try to prevent it from spreading to the rest of the UK. To give you an idea of how quickly they are spreading, there were fifteen nests on Hampstead Heath, Highgate Wood and Queen’s Wood in 2005: by 2018 there were over two thousand nests. These are prolific little devils, to be sure. From 2019 we have implemented much stronger restrictions on the import of oak trees, but the words ‘stable’, ‘horse’ and ‘door’ come to mind, though not necessarily in that order.


The caterpillars live in a nest of silk, usually high up on the trunk of a tree.

OPM nest (Photo by Joanne Jordan)

But in the evening they head out in search of tasty leaves. Each one follows a silken strand left by the one in front. They are actually rather attractive caterpillars in my view, with their wizard-like flowing white hair, but it’s the little short hairs underneath that cause all the problems – they float down from the trees and irritate people’s skin and eyes, and can also be problematic for dogs and cats.

Photo by Joanne Jordan

Where one OPM goes, they all seem to go – they feed together, they pupate together in the original larval nest, and when the adult moths lay their eggs they do that in a solid, single-layer of eggs called a plaque. The adults only live for four or five days, so they have to get down to business at great speed.

Photo One byBy Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org - This image is Image Number 5371232 at Insect Images, a source for entomological images operated by The Bugwood Network at the University of Georgia and the USDA Forest Service.(cropped), CC BY 3.0 us, https://commons.wikimedia.org/w/index.php?curid=7021924

Adult Oak Processionary Moth (Photo One)

In the Netherlands, an alternative control method that’s being tried is to set up lots of nestboxes in affected woods for great tits to use: these birds apparently greatly enjoy the younger caterpillars, before they get those irritating hairs. You can also, apparently, use a special vacuum to hoover them up, which must surely be more environmentally friendly (unless you’re an OPM of course).

And so, the UK’s history of lax biosecurity continues. Dutch Elm disease (in timber), ash dieback, chestnut blight and box tree moth, xyllela and bleeding canker have largely been introduced with imported plants. Climate change is making it easier for these various organisms to flourish, and is causing stress to plants more accustomed to a colder, wetter world. I suspect that our landscape is going to look very different in a few decades’ time. Who can say with any certainty who the winners and losers will be?

Photo Credits

Photo One By Gyorgy Csoka, Hungary Forest Research Institute, Bugwood.org – This image is Image Number 5371232 at Insect Images, a source for entomological images operated by The Bugwood Network at the University of Georgia and the USDA Forest Service.(cropped), CC BY 3.0 us, https://commons.wikimedia.org/w/index.php?curid=7021924



6 thoughts on “A Whole Lot of Processioning Going On

  1. Anne

    There is almost a ghoulish fascination here: both an admiration of these little horrors as well as the denouncement of the effect they have. It is interesting the way countries (wilfully or accidentally) import plants or insects and then have to find (hopefully biological) controls to stem the tide as the said plants or insects thrive with no natural enemies in their new abode. We have several examples here too.

  2. Alittlebitoutoffocus

    The one at the front in your first picture looks a little larger than the rest. So is that simply because it gets to the food first, or is it truly the leader of the pack for some reason do you think? And is that one also first to return, or perhaps the last?

  3. Bug Woman

    All very good questions, Mike, and I have no idea, but let me see what I can dig out on the internets. I too wonder if it’s always the same caterpillar who takes the lead, or if it’s just a larger one who barges the others out of the way.

  4. Bug Woman

    Found on New Scientist – the author actually reared some pine processionary moths, which are a very close relation to the oak processionary.

    ‘There doesn’t seem to be a natural leader – whichever caterpillar finds itself at the head of the column determines where they will go. If you join the head of the column to the tail, the caterpillars will quite happily march around in circles.

    Read more: https://www.newscientist.com/lastword/mg20827885-700-on-the-march/#ixzz6P2cXZg4k

  5. Sarah

    I reported a sighting of OPM last weekend, and have had a reply from the Forestry Commission saying they will conduct further surveys around the area this winter and “should follow up with spraying treatment to help deal with caterpillars next spring”. Now I’m feeling sad and wondering if I did the right thing. I wish they’d said they would follow up with great tit next boxes!

    1. Bug Woman

      🙁 I know what you mean, but yes, you did the right thing – the caterpillars really do cause problems, not just with people but also with any animals that come into contact with them. It would be better if the Great Tits could do the job, for sure, but I think the forestry commission is trying to make sure that they don’t spread any further. It would make more sense to vacuum up the pupae at this point, but I guess they don’t have the equipment or the man power.


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