Pine processionary moth (PPM) larvae (caterpillars) feed on the needles of pine trees and some other conifer tree species. In large numbers they can severely defoliate trees, weakening them and making them more susceptible to attack by other pests or diseases, or to environmental stress caused by drought or excessive moisture. PPM, like its close relative the oak processionary moth (OPM), gets its name from its distinctive habit of moving about in nose-to-tail processions.
Like OPM, PPM caterpillars represent a public health hazard because they have thousands of hairs which contain an urticating, or irritating, protein called thaumetopoein. These hairs can be blown by the wind into contact with people and animals, resulting in painful skin irritations and rashes and, in some cases, allergic reactions in some people and animals.
PPM is not currently known to be present in the UK, but it has been extending its range across Europe towards the English Channel, and we and other plant health authorities across the UK are taking steps now to consider how the UK can best prevent, or minimise the risk of, its entry to the UK. (See "Action", below)
Sightings in UK
The pest is not established in the UK, but one transient population of larvae (caterpillars) was found in a UK nursery in 1995 on Scots pine plants which had been imported from Italy in 1994. The affected trees and soil were treated, and subsequent monitoring did not detect the pest. An adult was caught in a light trap in Berkshire in 1966, but the origin of that moth was not traced.
It is native to, and until recently was only found in, the Mediterranean region, North Africa and some areas of the Middle East and southern Europe.
Possibly as a response to climate change, PPM has been expanding its range north through France since the 1990s, and is now breeding near Paris. Long-distance movement is thought to be associated with pupae carried in the growing medium or soil of infested plants. Climate change is thought to be responsible for the success of the moth's establishment once it arrives in regions north of its natural range.
The full list of countries where it has been recorded as established is: Albania, Algeria, Austria, Bulgaria, Croatia, Cyprus, France (including Corsica), Greece (including Crete), Hungary, Italy (including Sardinia and Sicily), Libya, Macedonia, Montenegro, Morocco, Portugal, Serbia, Slovenia, Spain (including the Balearic Islands), Switzerland, Syria and Tunisia.
Means of spread
a. Movement of plants for planting
There is a risk of larave, or caterpillars, being moved in association with plants for planting being brought into the UK. However, the white silken nests which the caterpillars form in trees in winter are very obvious, being sometimes as big as a football. In most cases, these nests and any associated caterpillars would be clearly visible during winter and spring when plants are usually imported, greatly lowering the risk from unintentional movement of plants into the UK, as long as shippers and recipients of plants are aware of the organism.
Spread by egg masses (pictured) being moved on plants for planting is considered to be a low risk, because pine trees are not usually imported and planted in the summer months, when adult moths lay their eggs, and the larvae will have emerged from the eggs before winter, when most pine plants are shipped and planted. Adult females are short lived and unlikely to remain with planting material being moved.
Although thought to be a low risk, pupae (right) could be brought into the UK in the soil of pine or other host plants for planting, or with any plants which have been growing in the vicinity of infested host plants before export. Inspection is unlikely to detect pupae, which can remain in the soil for up to three years before the larvae emerge. This is the pathway by which it is believed that PPM was moved to the Paris area.
There are currently no requirements for imported plants to be free of this pest. Plants for planting of many species enter the UK every year, but the volume of imports of host plants (or other plant species growing in their vicinity) with soil from areas where PPM is present is not likely to be large. The UK only imported PPM hosts (cluster pine / P. pinaster, hybrid larch / Larix x eurolepis and Scots pine/Pinus sylvestris) in containers from countries in which pine processionary moth is present on six occasions between 2003 and 2012. This is a key area of uncertainty, and additional data are being sought.
b. Natural Spread
Adults of both sexes can fly, and natural dispersal depends on the flight capacity of female moths, which is lower than that of males. Average female flying distance is 1.7km, with a maximum recorded of 10.5 km. This is consistent with the rate of spread recorded in the south of the Paris Basin, which has been reported as 5.6 km per year. Based on the moth's current known distribution, the risk of natural spread into the UK is still low compared to movement with plants for planting. However, its increasing northwards movement within the rest of Europe does increase the chance of natural spread to the UK.
Pine trees (trees of the genus Pinus) are most susceptible to attack, with the following species being particularly susceptible: Austrian pine (Pinus nigra), Aleppo pine (P. halepensis), Canary Island pine (P. canariensis), lodgepole pine (P. contorta), Maritime pine (P. pinaster), Monterey or radiata pine (P. radiata), Scots pine (P. sylvestris) and stone pine (P. pinea). Other recorded hosts include the Atlas cedar (Cedrus atlantica) and European larch (Larix decidua).
Among these species, European larch, Scots pine and lodgepole pine are widely grown in the UK, although all are present. Scots pine is our only native pine.
Defoliation of needles (right) is the main symptom of PPM attack displayed by the tree itself. Complete defoliation of needles can occur where there is a high level of infestation. Otherwise the nests formed by the caterpillars in January are the most obvious signs of the moth being present. Caterpillars feed at night during the winter, when fewer people are visiting woodlands.
The life-cycle of PPM is different from OPM's. PPM caterpillars overwinter in tent-like nests high in pine trees, and form processions on the ground in early spring before pupating in the soil until late summer, when they emerge as adult moths. This pupal stage can, however, remain dormant, extending the life cycle over two years.
The adult moths live for only about a day in the summer, during which time they mate and lay eggs in pine trees.
PPM larvae, or caterpillars, hatch in autumn from the eggs laid in the summer, and begin feeding on the trees' needles in autumn.
Caterpillars: The easiest stage of the lifecycle to recognise is the larva, or caterpillar, which is hairy and coloured orange-brown with blue bands. Like its close relative the oak processionary moth (OPM), the larvae move about in nose-to-tail processions. However:
- OPM caterpillars often form a wedge-shaped procession, with one leader and subsequent rows containing several larvae; while
- PPM caterpillars are more likely to form a single line of nose-to-tail larvae;
- OPM caterpillars have very long, white hairs which contrast markedly with other, shorter hairs; while
- PPM caterpillars do not have extremely long hairs, instead being covered with dense clumps of hairs with less variation in length;
- PPM caterpillars are most likely to be seen in winter and early spring; while
- OPM larvae are most likely to be seen in summer;
- PPM caterpillars are most likely to be seen in pine trees; while
- OPM caterpillars are most likely to be seen in oak trees.
Moths: Adult PPM moths have cream forewings with brown markings, and white hindwings, but to an untrained observer they are difficult to distinguish from other species of moth. They are flying about May to July, and individuals live for only about a day, during which time they must find a mate and lay their eggs in the foliage of a pine or other host tree.
Nests: About January the caterpillars build distinctive, tent-like nests of white, silken, webbing up to the size of a football in the branches and foliage of pine trees, and there can be several nests in a single tree. The caterpillars spend the days in these nests, and leave them at night to forage on the trees' needles.
The nests can become damaged and discoloured over time (right).
Preferred trees: PPM is most likely to be found in pine trees, while OPM is most likely to be found in oak trees.
Precautions - human health
Should the pest enter the UK and become established, people and animals should keep away from and not touch the caterpillars or nests. Old, abdandoned nests can still contain thousands of the irritating caterpillar hairs which can cause severe skin irritations. Parents, guardians, teachers and others responsible for children's welfare should teach them the dangers, and dog owners and walkers should keep dogs away from the caterpillars and nests.
People with symptoms who thought they might have been in contact with the hairs should seek medical advice, explaining that they believed they had been in contact with PPM hairs. Similarly, pet and livestock owners whose animals displayed symptoms should consult a veterinary surgeon and explain that the animal might have been in contact with PPM hairs.
Should the pest be found to have entered the UK, the first response would be to try to eradicate the outbreak to prevent establishment. Using emergency powers available to us, we would issue statutory Plant Health Notices to affected tree owners requiring them to have the infestations removed. Possible treatment options include pesticide applications during the larval stage of the lifecycle, and manual removal of nests and caterpillars by trained operators.
A Rapid Pest Risk Assessment (RPRA) has been undertaken by our colleagues in the Food & Environment Research Agency (Fera). This document provides a rapid assessment of the risks posed by the pest to the UK's pine and other host trees, and will help risk managers decide on a response to that threat. It is not a detailed Pest Risk Analysis (PRA), but includes an assessment of whether one should be developed. The recommendation, given the similarity in climate between northern France and coastal areas of southern England, is that a detailed PRA is required to assess the risk of establishment and the associated uncertainties.
A consultation is now under way and comments are invited on the RPRA by 24 December 2012.
In the meantime, statutory action will be taken on findings on a
precautionary basis, in response to the potential risk to the south of England. (See 'Outbreak Management' above). An awareness-raising programme will be undertaken to encourage vigilance by the trade and members of the public.
If you think you have spotted signs of pine processionary moth please tell us using our Tree Alert form:
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The pest is most likely to be found in pine trees. It can be confused with its close relative, the oak processionary moth, which, as its name implies, is most likely to be found in oak trees. Both species should be reported using the Tree Alert form.
Good-quality digital photographs greatly help our analysis.
- EPPO diagnostic protocol for Thaumetopoea pityocampa