The oak processionary moth: a new pest to UK oak trees

oak processionary caterpillars in procession on an oak trunk

Oak processionary caterpillars in travelling along an oak tree trunk. Photograph by Jorg-Peter Wagner distributed under a CC BY-SA 2.5 licence.

I’ve just come back from a holiday in the English Lake District, a very beautiful area of the country where amongst other things I saw plenty of oak trees. Oak trees are widespread across the UK  but the health of the two native oak species the English oak (Quercus robur) and the sessile oak (Quercus petraea) face a new threat in the form of the oak processionary moth (Thaumetopoea processionea).

The oak processionary caterpillar feeds on the leaves of several oak species and is widespread in Central and Southern Europe (1).  The caterpillars hatch and feed between April and June and can be recognised on oak trees by their characteristic way of forming processions as they move around the tree to feed. The caterpillars have long white hairs along the body with older caterpillars also having a long orange stripe. When not feeding the caterpillars congregate in silk nests under a branch or on the trunk. The grey adult moths emerge from pupae in July with the females laying egges between July and September on twigs and small branches.

oak processionary caterpillars have long white hairs but it is the many smaller hairs that contain the toxin.

oak processionary caterpillars have long white hairs but it is the many smaller hairs that contain the toxin. Photograph by Christian Fischer distributed under a CC BY-SA 3.0 licence.

Large populations of the caterpillars can strip a tree of its leaves (1). This loss of leaves leads to a decline in health and sometimes death because the trees become more vulnerable to other pests and disease-causing microbes, and are less able to cope with environmental stress.  The caterpillars also pose a health risk to humans because of a toxin called thaumetopoein contained in tiny hairs on their bodies. Contact or inhalation of the hairs can lead to skin irritation and allergic reactions. In Southern and Central Europe their populations are contained by the activities of natural predators.  However, in recent years the moth has been spreading into areas in Northern Europe that lack these predators. In 2006 oak precessionary moth caterpilliars were first found in the UK in Kew and Richmond in South-West London. It appears that this outbreak was due to the import of semi-mature Cypress oak trees Quercus robur f. fastigiata from continental Europe on which undetected eggs were present (2,3). The London outbreak has now spread to several London Boroughs and a separate outbreak in Bromley and Croyden in South London was identified in 2012 (1).  The moth has also been present in woodland in Pangbourne, Berkshire since 2010. The original London outbreak is now so large that efforts have turned from eradication to containment but it is still hoped that the South London and Pangbourne outbreaks can be eradicated.

What actions are being taken?

All three outbreaks are likely to be due to the import of oak trees from continental Europe and as a consequence of this regulations surrounding the transport of oak trees have been tightened. All oak trees imported into the country are now required to be accompanied by a passport certifying they come from nurseries that are free of the moth and there is a limitation of movement of oak wood from affected areas (1).

To identify and monitor oak processionary moth outbreaks different methods are used depending on the time of year. In the winter months identification of old nests in affected and vulnerable areas can provide indications of where the caterpillars may emerge the following spring. During spring and summer it is possible to identify trees containing eggs, caterpillars and nests. In late summer and early autumn pheromone traps positioned in the canopies of oak trees are used to attract and trap adults (4).

To control and eradicate the outbreaks the most effective method has been to treat affected trees with insecticides in the spring soon after the caterpillars emerge (1). In urban areas ground-based insecticide treatment is favoured but in May 2013 aerial treatments of the affected woodland in Pangbourne were carried due to the difficultly of accessing the trees from the ground. In Pangbourne they are using soil-dwelling bacteria called Bacillus thuringiensis to kill the caterpillars. It is harmless to humans and other animals and only affects a small range of butterfly and moth species. It is hoped that any native species affected by the spraying would recolonize the woodland from surrounding non-sprayed areas. During the pupal stage of the moth in early summer careful removal of the nests usually using vacuum equipment can reduce the number of adult moths that emerge.

Due to the risks to human health, if you find an oak tree where the processionary oak moths are present the advice is to avoid contact with it. Please report sightings to your local council or Forest Research.  If it is your own tree call in a pest control expert to remove the infestation. Do not attempt to remove the caterpillars or nests yourself.

The arrival of oak processionary moth in the UK highlights the dangers of transporting trees and other plants across large areas. Import of ash seedlings into the UK has been implicated in the recent spread of Ash Dieback disease caused by the fungus Chalara fraxinea  from mainland Europe. Perhaps it is time to review the regulations for the international transport of plant material to help protect native plant populations.

Author’s note: I really enjoy writing “Organism of the Week” posts but from now on the these posts will be once a month to give me time to write blog articles on other interesting topics. If you have any suggestions for organisms you would like to see featured or other topics you would like me to talk about please leave a comment on my blog or contact me via twitter @SarahShailes.


  1. Website: Oak Processionary Moth (Forestry Commission) updated 05/06/2013
  2.  Townsend, M. (2008) Report on Survey for Oak Processionary Moth Thaumetopoea      processionea (Linnaeus)      (Lepidoptera: Thaumetopoeidae) (OPM) in London in 2007. Report to the Forestry Commission, March 2008    
  3. Townsend, M. (2009) Report on Survey and Control of Oak Processionary Moth Thaumetopoea processionea (Linnaeus) (Lepidoptera: Thaumetopoeidae) (OPM) in London in 2008. Report to the Forestry Commission, January 2009
  4. Williams et al (2013) Monitoring oak processionary moth Thaumetopoea processionea L. using pheromone traps: the influence of pheromone lure source, trap design and height above the ground on capture rates

One thought on “The oak processionary moth: a new pest to UK oak trees

  1. Pingback: Mare Abortions And Caterpillars

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s