Oak processionary

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Oak processionary
Thaumetopoea processionea, closeup.JPG
Oak processionary caterpillars
Scientific classification
T. processionea
Binomial name
Thaumetopoea processionea

The oak processionary (Thaumetopoea processionea) is a moth whose caterpillars can be found in oak forests, where they feed on oak leaves, causing significant damage. They travel in nose-to-tail processions (hence their name), often arrow-headed, with a leader followed by rows of several caterpillars abreast.[1] They are a human irritant because of their venomous setae (hairs), which can cause skin irritation and asthma.


The moths are widely distributed in central and southern Europe, and are occasionally found as far north as Sweden. In the southern countries of Europe the populations are controlled by natural predators, but these predators are not present in northern Europe. Their range is expanding northward, possibly or partly as a result of global warming. The moth now has an established population in the UK. The eggs arrived on oak imported to the Richmond and Ealing areas of London in 2006[2] and the range of the species in the UK has been steadily expanding despite efforts to eradicate it.[3]


The imago (adult stage)

The wingspan of adult stage moths is between 25 and 35 millimetres (0.98 and 1.38 in). Their pattern of tan, brown and white make the adults difficult to see against oak bark. Adults fly during July and August. The larvae construct communal nests of white silk from which they crawl at night in single file, head to tail in large processions to feed on foliage in the crowns of trees, returning in the same manner.

Oak is its preferred food source, but the moth also eats the leaves of hazel, hornbeam, sweet chestnut, birch and beech.[4]


The caterpillars live and feed almost exclusively on oak trees. They may march in procession across the ground between oak trees, and cluster together as they feed on oak leaves. In early summer they build silk nests on the trunks and branches, but not in the leaves, of oak trees, and leave silk trails on the trunks and branches. The nests and trails are originally white and visible, but soon become discoloured and hard to see.[1]

The nests may be hemispherical, teardrop shaped, bag-like, and blanket-like (surrounding part of a trunk or branch), and may be at any height on the tree. The diameter may range from about 25mm (one inch) to stretching several meters up the trunk. The caterpillars stay in these nests during the day between feeding periods, and later in the summer they remain in the nests to pupate into adult moths.[1]

The caterpillars are mostly found in oak trees or on the ground under them in late spring and early summer, and do not live on fences, walls, etc. as other caterpillars do. They have very long, white hairs contrasting markedly with shorter hairs.[1] Several other caterpillars may be mistaken for OPM.[5]

Public health problem[edit]


The moths pose an increasing nuisance to humans as their range is extended. The backs of older caterpillars (3rd to 6th instars) are covered with up to 63,000 pointed defensive bristles,[citation needed] which contain an urticating toxin (e.g., the protein thaumetopoein).[6] The setae break off readily, become airborne and can cause epidemic caterpillar dermatitis (lepidopterism), manifested as a papular rash, pruritus, conjunctivitis and, if inhaled, pharyngitis and respiratory distress, including asthma or even anaphylaxis; however, as of this date,[when?] there have been no known deaths related to or caused by such exposures to this toxin.[citation needed]

It has been found that the skin irritation and itching caused by contact with these hairs can be largely eliminated by the use of cetirizine-based antihistamine tablets.[citation needed]

Transmission of the hairs can be airborne, by ground contact via plants or grass or even by water contact in stillwater e.g. garden ponds. The toxicity of the hairs remains active beyond the normal life cycle of the moth and in some cases can remain a problem for several seasons.[citation needed] Mowing a lawn can bring a person into contact with these hairs. One alternative is to adopt a grass mulching technique to reduce possible contact, and to speed up the biological breakdown of the irritant hairs.[citation needed]

Damage to trees[edit]

Large populations can strip trees bare, leaving them weakened and vulnerable to other threats.[1]



Nests can be removed, or the caterpillars sprayed with pesticides soon after they hatch. However, neither approach is 100% effective.[7] Male moths can be trapped in pheromone traps; this does not significantly reduce the population, but provides an indication of moth distribution.[1]


The caterpillars were accidentally introduced to the UK in 2005, almost certainly as eggs on live oak plants imported from continental Europe. Later distribution of the pest probably arose from several similar introductions, in addition to spread from the original point of introduction.[1] By 2019 they had spread to all 33 London boroughs, and the Government had spent £37 million trying to control them.[7] The general public have been asked to look out for these caterpillars and to report them, rather than deal with them themselves. The London Boroughs of Brent, Ealing, Hounslow and Richmond upon Thames set up task forces to deal with outbreaks. Sightings of these caterpillars in other areas should be reported to the Forestry Commission,[8] whose research agency issued guidance on the way to contain outbreaks and deal with infestations, so as not to increase the risk to the public.[9]

On 31 March 2008 an emergency amendment added the moth to the list of pests in The Plant Health (Forestry) Order 2005, and has required all oak trees coming into the UK from the rest of Europe to have Plant Passports.[10]

In 2013 the Forestry Commission announced helicopters would be deployed to "blanket spray woodland" where the caterpillars posed a health threat.[3]

In April 2018 an outbreak of the caterpillars was reported across Greater London and surrounding areas.[11] In 2015 fifteen OPM nests were found in Hampstead Heath, Highgate Wood and Queen's Park; in 2018 over 2,000 were found at those sites.[7]


The moth is reported as being fairly common in Belgium, notably in the Campine but also elsewhere, the population fluctuating from year to year.[12] In 2007 infestations in the province of Limburg were so acute that soldiers were deployed to burn them.


In the Netherlands, the caterpillars are dealt with by biological pest control. Local authorities use, like in Germany, fluids containing bacteria that are toxic only to the caterpillars. These fluids are sprayed onto the infected trees. Furthermore, an experiment with bird houses for the great tit has been carried out since 2016. It has been observed that the great tit likes to eat the young, not yet hairy caterpillars in the month of April. In serious cases of contamination, the use of relatively mild chemical pest killers has been allowed by local authorities.[citation needed]

Because chemicals may be harmful to other insects, an alternative is the use of vacuuming equipment to remove the caterpillars, followed by incinerating the caught specimens.

See also[edit]


  1. ^ a b c d e f g "Oak Processionary Moth - Tree pests and diseases". Forestry Commission (UK). 11 September 2018. Retrieved 31 January 2019.
  2. ^ forestry.gov.uk
  3. ^ a b Helen Dixon (7 May 2013). "Helicopters to spray woodland to halt march of toxic caterpillars". Daily Telegraph. Retrieved 7 May 2013.
  4. ^ Tree News, Autumn/Winter 2007, page 4 internal Sylva supplement
  5. ^ "Native species that may be mistaken for oak processionary moth". Forest Research. 2019. Retrieved 31 January 2019.
  6. ^ Lamy, M; Pastureaud, M. H; Novak, F; Ducombs, G; Vincendeau, P; Maleville, J; Texier, L (1986). "Thaumetopoein: An urticating protein from the hairs and integument of the pine processionary caterpillar (Thaumetopoea pityocampa Schiff., Lepidoptera, Thaumetopoeidae)". Toxicon. 24 (4): 347–56. PMID 3087028.
  7. ^ a b c Sophia Sleigh (31 January 2019). "Toxic caterpillar invasion spiralling out of control following 'phenomenal' population explosion". Evening Standard. Retrieved 31 January 2019.
  8. ^ The Forestry Commission. Oak processionary moth. Accessed 2008-05-31
  9. ^ Forest Research Survey and intervention in relation to different phases of the oak processionary moth life cycle. Accessed 2008-05-31
  10. ^ Office of Public Sector Information. Explanatory memorandum to the Plant Health (Forestry) (Amendment) Order 2008 No. 644.[1] Accessed 2008-05-31
  11. ^ "Health warnings after toxic caterpillar outbreak in London". BBC News. 28 April 2018. Retrieved 28 April 2018.
  12. ^ >> "<<Thaumetopoeia processionea>>". April 30, 2018.
  • Maier, H.; Spiegel, W.; Kinaciyan, T.; Krehan, H.; Cabaj, A.; Schopf, A.; Honigsmann, H. (24 November 2003). "The oak processionary caterpillar as the cause of an epidemic airborne disease: survey and analysis". British Journal of Dermatology. 149 (5): 990–7. doi:10.1111/j.1365-2133.2003.05673.x. PMID 14632804.
  • Rahlenbeck, S.; Utikal, J. (1 August 2015). "The oak processionary moth: a new health hazard?". British Journal of General Practice. 65 (637): 435–6. doi:10.3399/bjgp.15X686341 (inactive 2018-05-06). PMID 26212843.

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