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Why do Oak Processionary caterpillars form processions?

Why do Oak Processionary caterpillars form processions?


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Firstly ,I don't live in Europe. I live in South India, and my garden has a Night Flowering Jasmine tree.

And these trees are quite popular, but notorious for their caterpillars. In our region ,there are no specific names or identification these caterpillars and the thick mat they form on the tree trunks.

And one day, I was looking through images of caterpillars online ,when one seemed similar. It was called Oak Processionary.I am not sure if the caterpillars found in my area are the same species or something else entirely, but their behavior is similar . As per as Wikipedia and some other websites, they are usually found head to tail moving together.

My question is

  • Do they have some chemical signaling agent like pheromones ,or like ants when they follow each other?
  • Why do they form such processions in the first place? Wouldn't having such a large ,conspicuous gathering make it easier for predators to spot them from afar?

Toxic processionary caterpillar plague spreads across Europe

The mild winter and warm spring this year boosted caterpillar numbers.

In Louvain, Belgium, firefighters had to destroy nests of the invasive species before a rock concert.

The caterpillars turn into pupae, then moths in late July, and the threat diminishes.

Germany's western Ruhr region is densely populated and among the worst affected by the caterpillars.

Some schools and parks have been closed to allow specialists to attack the nests in oak trees.

The caterpillars - measuring 2-3cm (about one inch) - march in long processions to the treetops at night, and can wreak havoc in oak trees, as they feast on the young leaves.

One mature caterpillar has up to 700,000 hairs, which can be spread by the wind. The oak processionary moth pest is known as OPM for short.

The Fredenbaumpark in Dortmund was closed for three weeks, as nearly 500 trees were found to be infested there, broadcaster Deutschlandfunk reported.

"The oak processionary infestation this year is very intensive - much more than last year," said the park's manager Frank Dartsch.

Special teams there and elsewhere have donned protective gear and used firefighters' lifts to reach the treetops, where they have attacked OPM nests with blowtorches or big vacuum cleaners.

In the Netherlands, OPM infestations have also increased compared with 2018, with the oak-rich provinces of Noord-Brabant, Drenthe and Overijssel especially affected.

A video of an elderly woman attacking the caterpillars with a heat gun in the city of Enschede has gone viral, the nltimes.nl website reports.

Vasthi Alonso-Chavez, a specialist at Rothamsted Research in the UK, told the BBC that two factors were probably responsible for OPM's spread northwards: the trade in oak plants and a warming climate.

Broadcaster RTL says the caterpillars have spread all over Luxembourg, a heavily forested country. The Luxembourg City authorities have issued a health warning, as the caterpillars are in the city too.

The caterpillars are also infesting the densely wooded French island of Corsica.

They were a problem in parts of England last year.

Ms Alonso-Chavez says there are established OPM populations mainly in London and surrounding areas. OPM was first identified in London in 2006, she says, and probably arrived as eggs on imported oak plants.


How to spot an OPM caterpillar

They move in nose-to-tail processions (hence the name)

They sometime form arrow-headed processions or in clumps

Most likely to be found in oak trees, or on the ground underneath

Seen normally during spring and summer

They have long, white hairs as well as shorter, more furry ones

They rarely live on fences, walls or places other than oak trees

If you are unfortunate enough to see any OPM caterpillars do not touch them or let children or animals approach them.

Don’t try removing the infestation yourself, and instead call the Forestry Commission on 0300 067 4442.

The commission will be treating trees until about mid-June, but it’s still important to be wary.

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If you do come into contact with any larvae hairs and begin feeling unwell call 111.

Symptoms in humans include itching skin rashes and, less commonly, sore throats, breathing difficulties and eye problems. In pets they include hyper-salivation, tongue swelling, conjunctivitis, gagging, vomiting, respiratory distress and mouth inflammation.


BALI Issue Advisory Alert for the ‘Oak Processionary Moth’

The ‘Oak Processionary Moth’ (Thaumetopoea processionea) caterpillars are a fairly recent introduction into the UK having been imported from the Netherlands (most likely in egg form on young trees) have been sighted on trees in Hampshire. BALI have issued a notice for all members to remain vigilant and report all sightings. The OPM is an established pest in many areas of London, whilst the rest of the country is a designated Protection Zone (PZ).

Why are they are risk?

Whilst in caterpillar life stage, the OPM poses a risk not only to our native oak trees but also humans and some animals.

The moth gets its name from its unusual processing behaviour of the caterpillars because they tend to move in nose-to-tail lines – consuming foliage as they move. The caterpillars consume the oak leaves which can then lead to severe loss of foliage, weakening the trees and making them more vulnerable to other diseases, drought or flooding.

The second risk is one to human health. The OMP caterpillar’s tiny hairs contain a toxin which can lead to itching skin lesions, rashes, and less commonly sore throats, breathing difficulties or eye problems. The hairs from the caterpillars are commonly found in their nests so it is advisable to avoid contact with both nest and caterpillar.

What do they look like?

Mature caterpillars are characterised by long, white hairs, on darker bodies. They are likely to form processions (hence the name), and found primarily on Oak trees. They are most likely to be seen in summer. Nests are silky in appearance, and almost always on the trunk or large limbs of host trees.


What does pine processionary moth damage look like?

  • Presence of the large hairy caterpillars.
  • The caterpillars' nests in the branches and foliage of pine trees. They are white, silky webs that can reach the size of a football.
  • Discoloured needles (yellow and then brown) or completely eaten needles which leads to defoliation of the tree.
  • The extent of damage increases with severity of infestation and these signs and symptoms only become noticeable when populations are large and infestations severe.
  • If there are enough caterpillars they can defoliate the whole tree.

Credit: Dave Watts / naturepl.com


More information

Processionary caterpillars are a real pest. Not only are they are a threat to humans and animals, but they’re also a threat to the pine trees themselves. If you find evidence of the processionary caterpillar in your area, contact the authorities or a removal expert immediately. Furthermore, if there’s someone new to your community, alert them as to the risk – especially if they’re a dog owner.

And remember that if you have any questions or you would like more information about Spain, its legal system or the procedures for buying a house here, you can call us or fill out this form and we will get back to you as soon as possible.


The caterpillars' thousands of tiny hairs which contain an urticating, or irritating, substance called thaumetopoein. Contact with the hairs can cause itching skin rashes and, less commonly, sore throats, breathing difficulties and eye problems. This can happen if people or animals touch the caterpillars or their nests, or if the hairs are blown into contact by the wind. The caterpillars can also shed the hairs as a defence mechanism, and lots of hairs are left in the nests, which is why nests should not be touched without protective clothing.

OPM caterpillars can threaten the health of several species of oak trees (Quercus species) because they feed on the leaves. Large populations can defoliate, or strip bare, large parts of oak trees, leaving them vulnerable to attack by other pests and diseases, and less able to withstand stresses such as drought and flood. They will only feed on other trees if they run short of oak leaves to eat, and have been seen on hornbeam, hazel, beech, sweet chestnut and birch trees.

Simple precautions to minimise the health risks to you and your pets

DO NOT:
&bull touch or approach nests or caterpillars
&bull let children touch or approach nests or caterpillars
&bull let animals touch or approach nests or caterpillars or
&bull try removing nests or caterpillars yourself.

DO:
&bull teach children not to touch or approach the nests or caterpillars
&bull train or restrain pets from touching or approaching them
&bull keep horses and livestock a safe distance from infested oak trees. Covering or stabling livestock can help
&bull see a pharmacist for relief from skin or eye irritations after suspected OPM contact
&bull call NHS111 or see a doctor if you think you or someone in your care has had a serious allergic reaction
&bull consult a vet if you think your pet or livestock has been seriously affected
&bull call in a pest control expert to remove infestations in your own trees

OPM caterpillars are most easily recognised by their distinctive habit of moving about in late spring and early summer in nose-to-tail processions, from which they derive their name, and the fact that they live and feed almost exclusively on oak trees. They can sometimes be seen processing across the ground between oak trees, and clustering together as they feed on oak leaves.

In early summer they build distinctive white, silken webbing nests on the trunks and branches of oak trees (almost never among the leaves), and leave white, silken trails on the trunks and branches. These nests and trails become discoloured after a short time, and more difficult to see as a result.

The nests can occur in a range of shapes, including hemispherical (half a ball), tear-drop shaped, bag-like, and like a blanket stretched around part of a trunk or branch. Sizes range from as small as the width of a 50p coin to stretching several feet up the oak tree trunk in some cases. They can occur anywhere from ground level to high in the oak tree, and can fall out of oak trees and be found on the ground.

The caterpillars rest up in these nests during the day between feeding periods, and later in the summer they retreat into the nests to pupate into adult moths.

move about in nose-to-tail processions

often form arrow-headed processions, with one leader and subsequent rows containing several caterpillars abreast

are most likely to be found in oak trees, and sometimes on the ground under oak trees

are most likely to be seen in late Spring and early Summer

have very long, white hairs which contrast markedly with other, shorter hairs and

do not live on fences, walls and similar structures, as some caterpillar species do.


Oak Processionary Moths [OPM]

Oak processionary moths (OPM) are a species whose caterpillars cause significant damage to oak trees by consuming their foliage, and can also pose a public health risk to both people and animals.

Be alert for Oak Processionary Moth caterpillars when you are out and about in Surrey Heath. ​They are mainly present on Oaks but can also colonise other tree species. If you spot any do not touch.

Take a photo if possible, make a note of the location and report at [email protected] - more information below.

The Forestry Commission has produced both a short leaflet and an in depth "OPM Manual".

As the name suggests, the caterpillars move in nose-to-tail processions along the main stems and branches of oak trees or on the ground and often cluster together. They build white, silken webbing trails and nests – usually dome or teardrop-shaped – on tree trunks and branches rather than amongst the leaves.

In the UK, OPM (scientific name Thaumetopoea processioneais) is now known to be present throughout the whole of London [Richmond Park currently spends £250k annually to contain their infestation] and many neighbouring boroughs and counties. A protein in the caterpillars' barbed hairs can cause severe skin and eye irritations, sore throats and breathing difficulties in people and animals who come into contact with them. The caterpillars pupate into adult moths in their nests on the trunks and branches of Oak trees from early July to early September and has resulted in a government-led programme of survey and management in these areas to minimise populations, spread and impacts on both trees and humans.

To trees: OPM caterpillars can threaten the health of several species of oak trees (Quercus species) because they feed on the leaves. Large populations can defoliate, or strip bare, large parts of oak trees, leaving them vulnerable to attack by other pests and diseases, and less able to withstand stresses such as drought and flood. They will only feed on other trees if they run short of oak leaves to eat, and have been seen on Hornbeam, Hazel, Beech, Sweet Chestnut and Birch.

To people and animals: The caterpillars' thousands of barbed hairs contain an urticating, or irritating, substance called thaumetopoein. Contact with the hairs can cause itching skin rashes and, less commonly, sore throats, breathing difficulties and eye problems. This can happen if people or animals touch the caterpillars or their nests, or if the hairs are blown into contact by the wind. The caterpillars can also shed the hairs as a defence mechanism, and lots of hairs are left in the nests, which is why nests should not be touched without protective clothing.

Forestry Commission and the NHS advise people in affected areas to take the following precautions to minimise the health risks to themselves, their pets and livestock:

  • touch or approach nests or caterpillars
  • let children touch or approach nests or caterpillars
  • let animals touch or approach nests or caterpillars or
  • try removing nests or caterpillars yourself.
  • teach children not to touch or approach the nests or caterpillars
  • train or restrain pets from touching or approaching them
  • keep horses and livestock a safe distance from infested oak trees. Covering or stabling livestock can help
  • see a pharmacist for relief from skin or eye irritations after suspected OPM contact
  • call NHS 111 or see a doctor if you think you or someone in your care has had a serious allergic reaction
  • consult a vet if you think your pet or livestock has been seriously affected
  • call in a pest control expert to remove infestations in your own trees and
  • report any sightings to Council Tree Officer on 01276 707100 or [email protected] or [email protected] or the Forestry Commission using the Tree Alert on-line reporting form or e-mail [email protected], or telephone 0300 067 4442.

Following your report of nests to Forest England you can engage specialist contractors to remove the nest and caterpillars at your own expense from your trees, these contractors can be found on the Arboricultural Association website.

Tree surgeons, forestry and ground-care workers, and others working on or close to oak trees in the affected areas, should wear full protective clothing, and familiarise themselves with the signs of OPM presence and the regulations applying to handling and moving oak material.


Dispersion of the bio-aerosol produced by the oak processionary moth

The oak processionary moth (Thaumetopoea processionea L.) is found in oak forests in most European countries. The caterpillars bear urticating hairs (setae) as a chemical defence. These hairs break off and are small enough to become airborne and be transported by the wind. Upon contact with humans the toxin can cause an allergic reaction that ranges from a skin rash to respiratory distress. In order to measure the terminal settling velocity of this bioaerosol, we used a small elutriator and tested its functionality with particles of known aerodynamic diameter. We determined that the mean settling velocity of the setae is about 1 cm/s, corresponding to an aerodynamic diameter of 19 μm for setae with a diameter of 6 μm and a length of 190 μm. The dispersion of the hairs in the atmosphere for a typical summer day was calculated by means of an Eulerian model. The results of this calculation revealed that the maximum concentrations in the atmosphere on a typical summer day reach 20–30% of the concentration found directly at the source. Those maximum concentrations are reached at a distance from the source that varies between 174 and 562 m, depending on the atmospheric stability and the settling velocity.

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Why are OPM caterpillars bad for oak trees?

OPM caterpillars can damage our precious oak trees by feeding on the leaves. Large numbers can strip whole trees almost bare of leaves, weakening the trees and making them vulnerable to other threats such as drought and disease.

Are OPM caterpillars harmful to people and animals?

The caterpillars emerge about April every year and develop thousands of tiny hairs which contain an irritating substance called thaumetopoein. This can cause itching skin rashes, eye irritations and sore throats in people and animals who come into contact with them. In rare cases they can cause breathing difficulties and allergic reactions.

It is important to avoid contact with the hairs, to teach your children to avoid them, and to protect your pets from them. Curious pets might need to be restrained from approaching nests and caterpillars.

However, if you are affected, the symptoms, although unpleasant, are not usually medically serious and will pass in a few days. You can ask a pharmacist for something to relieve the symptoms.

If you do have a serious allergic reaction, call NHS111 or see a doctor. Similarly, consult a vet for badly affected animals

What do OPM caterpillars look like?

These caterpillars (pictured above) have very long, white hairs and a distinctive habit of moving around oak trees in nose-to-tail processions, which gives them their name.

They make silken webbing nests, which are white when new, and often have silken trails leading to them. They quickly become discoloured and harder to see against the dark colour of oak tree bark.

For more details of how to identify an OPM caterpillar please see the Forestry Commission website.

What to do if you see OPM nests or caterpillars

If you see any OPM nests or caterpillars, do not touch or approach them.

Please report them immediately to the Forestry Commission, which is leading efforts to control its population, spread and impacts.

How to report OPM

The preferred way to report sightings is with the Forestry Commission’s Tree Alert on-line pest reporting form, which you can access via the Forestry Commission website page about OPM. You will have to add a photograph to your report, but do not risk contact to get a photograph.

If you cannot use Tree Alert or get a photograph, you may report them by:

Control programme

The Forestry Commission are working with councils and major oak tree owners on a programme funded by the Department for Environment, Food & Rural Affairs (Defra) to survey for and treat affected oak trees and remove nests to minimise the population, spread and impacts of the pest. You can help us by reporting sightings as above. Details of the programme are available on the Forestry Commission website.

If you have an oak tree on your property you can find out more in the Forestry Commission OPM manual.


Watch the video: Time lapse captures processionary caterpillars crossing footpath (July 2022).


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