The recent discovery of pine processionary moth on a number of trees in England and Wales has prompted the banning of pine and cedar imports from several countries. So what threat does the pest truly pose? 

THE last 20 years have seen a succession of exotic pests arrive in the United Kingdom, culminating with Ips typographus, the world’s worst insect pest of spruce trees, and a deadly, fungus-like pathogen called Phytophthora pluvialis with Douglas fir and western hemlock at the top of its host hit-list. So it was not unreasonable to hope for some respite in the short term at least, but no such luck. 

Easter 2022 had barely passed when up popped DEFRA with the news that pine processionary moth or PPM (Thaumetopoea pityocampa) had been intercepted on unspecified pine species imported from France. Following identification at an unspecified number of nurseries in England and Wales, the notorious lepidopteran insect pest – predominantly on pine but also other conifers – is now subject to long-overdue preventative action. 

READ MORE: Pine Processionary Moth: Ban on imports of pine and cedar after pest discovered in UK

PPM possesses potential for extremely high levels of damage to pine and some other conifers. Affected trees may defoliate, thereby reducing growth rate and timber yield, while tree mortality can be high in young conifer plantations. From the third instar (L3) stage onwards, PPM larvae bear irritating, stinging (urticating) hairs which pose potentially serious risks to human and animal health in the same way as oak processionary moth – OPM (Thaumetopoea processionea) – a closely related species.


A main plank of the government’s PPM programme is an emergency regulation which DEFRA says will protect treescapes and strengthen biosecurity against this ambidextrous insect pest with dual tree-damage and public-health qualifications. DEFRA says the new regulation, effective from 29 April, will strengthen requirements for the importation of pine (Pinus) and cedar (Cedrus) trees* into Great Britain**. Imports of these trees, which are plant hosts of PPM, will only be permitted from:

• Countries officially confirmed by the National Plant Protection Organisation to be free of pine processionary moth
• Areas officially designated as free of pine processionary moth
• Plant nurseries where Pinus and Cedrus trees have been grown under complete physical protection for their entire lifetime.

*European larch (Larix decidua) and Douglas fir (Pseudotsuga menziesii) are also recorded hosts of PPM. 
**Apparently not applicable to Northern Ireland.

DEFRA’s new controls for PPM apply to all businesses importing living pine and cedar plants and their constituent parts, including live plant foliage, into Great Britain.

Restrictions do not apply to processed plant products such as timber, wood chips and packaging materials.

The restrictions appear to be sound and secure enough until you investigate the small print and relate the wording to similar situations with other exotic insect pests over the last 20 years. The exclusion of imports from countries not confirmed as PPM-free by the NPPO would appear to exclude imports on the basis of designated pest-free areas and trees grown under physical protection if the sites concerned are in countries where PPM is present. 

Forestry Journal:  Pine processionary moth nest in a pine tree. Pine processionary moth nest in a pine tree.

The potential dangers of fiddling at the edges can be seen from the situation 10 years ago with oak processionary moth, which does to Quercus what PPM does to Pinus, both insects delivering a nasty sting in the tail to anyone unlucky enough to come into contact with the hairs borne by the older larvae.

Similar arguments raged around the security of importing oak tree-planting material from continental European countries where OPM was already widespread and endemic.

This included the Netherlands, from which large volumes of Quercus robur and Quercus petraea continued to arrive into the UK, including some carrying OPM infestations subsequently responsible for further outbreaks around London. 

At the time, the UK was a member of the European Union and subject to its rules and regulations effected through EU plant passportation. At the time, EU plant passports accompanying oak trees arriving into the UK from the Netherlands (or another EU member state) were required to confirm the trees were nursery grown and sourced from an OPM-free area (similar language to that used 10 years later for PPM and pine trees). 

At that time, OPM was already widespread throughout the Netherlands, where ‘immediate vicinity’ was a largely subjective measure that could mean distances down to 50 m. Reasonable enough you might say, except that female and male OPM adults (moths) can fly up to 5 km and 20 km, respectively. PPM adult moths have similar flight capabilities.


Announcing the new legislation, Professor Nicola Spence, UK chief plant health officer, alluded to the increasing role played by the globalised plant trade – in conjunction with climate change – to accelerate new pest and disease risks for the UK. 

In the case of PPM, which is native to lands surrounding the Mediterranean Sea, progressive movement from its native distribution to threaten more northern reaches of the continent is what currently preoccupies minds. Expanding plant trade is clearly helping to propel the pest out of its native distribution, while a warming climate increases the self-sustainability of new, more northerly located infestations.
PPM is established in northern France, which means it is now less than 300 km (as the crow flies) from Brighton Beach, approximately the same distance separating London and Manchester. But all this is nothing new. When the last pest risk assessment (PRA) for PPM was published by DEFRA in 2015, it was already well established in the Paris region, in Brittany, Austria and Switzerland. 

DEFRA said the implementation of such controls is all about risk, but also relevant are any lapses – confirmed or implied – in biosecurity measures put in place by the importing country. Professor Nicola Spence said: “Strengthening our rigorous standards of biosecurity – already among the highest in Europe – will both minimise the net potential losses to our existing treescapes and serve to realise our long-term vision for the nation’s trees and woodlands.” 

Sadly, experience over the last two decades does not bear this out. 20 years ago, Forest Research scientists identified the first outbreak of horse chestnut leaf miner in the UK.

There followed a succession of new exotic pests and pathogens into the UK which in most instances have established and spread. In date order they include: horse chestnut leaf miner (2002), Phytophthora ramorum (2002), Phytophthora kernoviae (2003), oak processionary moth (2006), citrus longhorn beetle (2010), Phytophthora lateralis (2010), Phytophthora austrocedri (2011), sweet chestnut blight (2011), Asian longhorn beetle (2012), Chalara ash dieback (2012), Oriental chestnut gall wasp (2015), Ips typographus (2018), Phytophthora pluvialis (2021), and pine processionary moth (2022).
Pre-2000, the UK had a tight and tidy system of biosecurity, but this is no longer the case. And if our standards are already among the highest in Europe, I dare not think how sieve-like the others are.

Forestry Journal:

Reading DEFRA’s 2015 PRA, you get a flavour of the sense of urgency existing at that time. “During the last 25 years, PPM has been increasing its range, spreading northwards in France, and is now found in the area around Paris, as well as parts of Brittany,” said the PRA. “This northwards expansion in range has meant that this moth is of increasing concern to the UK.”

So why did UK plant health authorities not play safe and implement today’s restrictions in 2015 instead of waiting seven years until the blighter arrived, leaving an already under-staffed and over-stretched FC to deal with yet another alien, industry-destroying insect invader? According to all recent accounts, including those of the National Audit Office and the EFRA parliamentary committee, there are not enough FC personnel to direct and deliver the UK government’s tree-planting plans for England. 

It’s bad enough that new tree planting in England is stalling through lack of new boots on the ground, but it could be catastrophic if existing boots, who should be getting new tree roots into the ground, are spending their time scouting new exotic pest introductions and felling otherwise sound trees which have already succumbed.


So what could we be dealing with should PPM escape into the wider UK environment and establish a foothold on native Scots pine and other conifers? The UK would be confronted with an insect of high potential pest status and a biology and ecology even more complex than its close cousin OPM. The added complexity is likely to complicate the selection of preventative and control measures. 

There are several differences between the life cycles of PPM and OPM. The OPM life cycle is annual and occurs entirely above ground, while the PPM life cycle can extend over two years when the insect is breeding at high altitudes and in cooler northern latitudes, of which UK would be an example. 

What’s more, PPM has two phases – an aerial (above ground) phase involving the adult moth, egg and larval stages, and a hypogeal (underground) phase featuring the pupa stage. Under the most favourable conditions for PPM, such as those prevailing within its natural Mediterranean distribution, the insect may complete its development cycle in six months, although the fourth and fifth larval instar stages may be prolonged during the winter months. The pupal stage may be extended considerably by the onset of diapause, which adjusts at a given location and within certain limits, to ensure constant and consistent adult emergence from the pupa stage from one year to the next. 

You might think that environmental requirements of a Mediterranean insect, especially conditions related to temperature, would be difficult to satisfy in northern Europe.

However, the clear ability of this insect to establish in countries like Austria, Switzerland and Hungary shows low winter temperature is not the only environmental constraint on PPM development. 

In fact, daily average sunshine has a significant part to play in defining the northern European limit of PPM distribution. Over 60 years ago, scientists proposed 2,000 hours of sunshine per year determined the northern limit of PPM, and they were bang on. Paris, the most northerly limit for PPM, recorded 1,984 hours of sunshine in 2018, whereas Nice, on the south coast of France, where PPM has the potential to run riot, had more than 2,550 hours of sunshine in the same year. 

As a comparison, England as a whole really struggles to reach 2,000 hours, with the nearest contenders being areas along the south coast and the Devon/Cornwall peninsula, which come in close to 1,900 hours. Dates of adult moth emergence are generally earlier in more northerly latitudes and at altitude. The adult moth emergence period is generally less than four weeks for vigorous PPM populations which you would expect to find in the Mediterranean area, and six weeks for weakened populations, like those trying to gain a foothold in northern European countries, such as the UK, and in less-than-favourable conditions.

As a general rule, adult moths emerge from the pupa stage and fly during July. Moths will have mated within hours, with the female moth usually moving just a short distance to oviposit eggs on the nearest pine tree. However, these female moths can fly several km, with the capacity to extend subsequent pest outbreaks over large areas. 

Eggs are laid in cylindrical masses and in a helicoid pattern around a pair of pine needles. A large proportion of egg masses will be laid on the pine tree’s peripheral shoots in the crown of the tree and typically comprise 70–300 individual eggs, depending on the quality of the feeding conditions for emerging larvae.

READ MORE: Phytophthora Pluvialis: All the British areas where new tree disease has been found

After 30–45 days larvae bore a hole in the chorion (outer shell of the insect egg) to emerge from the egg stage as the first instar larval stage. These neonate larvae aggregate in colonies and spin silken nests, which enlarge until the fourth instar, when the definitive winter nest is built. In general, this is situated at the branch tips in the upper part of the conifer crown. Larvae change colour with each ecdysis (moult) and during the third stage they develop urticating hair patches on the body. If the autumn is warm and sunny larvae can reach the fifth stage in early winter.

Pupation ‘processions’ in late winter and early spring present an amazing sight and expression of social behaviour by the larvae. 

The processions occur at temperatures of 10–22°C. At lower temperatures, the colonies will regroup, and at higher temperatures larvae will bury themselves wherever soil texture allows. As a result, the cooler the soil, the more extensive the spread of pupation sites at the forest margins. At higher temperatures, the procession moves towards trunk bases in the shade of trees and may even bury itself close to the base of the original tree. During one such journey, a PPM larval colony observed in a cold, high-elevation site in Spain travelled 37 m over a two-day period, with the first 35 m accounted for by the first 24 hours of travel. 

There are more than 100 species of Pinus, though relatively few have been called out as especially susceptible to attack by the pine processionary moth. These are not, as you might expect, all closely associated with the pest’s natural distribution within areas around the Mediterranean Sea. 

Pinus species documented as more prone to attack include Aleppo or Syrian pine (Pinus halepensis), Monterey or radiata pine (Pinus radiata), Scots pine (Pinus sylvestris) and lodgepole pine (Pinus contorta). Of the cedars, Atlas cedar (Cedrus altlantica) is recorded as most susceptible to attack by pine processionary moth


It is clear multiple finds at different nurseries were intercepted, but less clear if all came from the same French nursery. However, DEFRA does refer to “affected nurseries” in France. 

Forestry Journal:

Infestations, presumably at the egg stage, were only found in pine trees. Some cedars had also been imported from the same nurseries, though they had apparently been grown at different sites in France. DEFRA said the affected pines found at UK nurseries were contained and destroyed, though the same treatment was not dealt to the imported cedars. They were treated with insecticide, barrier glue and contained as a precautionary measure, though given what is at stake you might think they should have been destroyed along with the pines.

Equally strange is the way these interceptions appear to have been kept hush-hush, perhaps for some weeks. DEFRA said the infested pine trees were imported from France in February 2022, but does not say when the interceptions were made.

On the day DEFRA released a statement for general consumption, I spoke with the owner of one of the bigger forest nurseries in England who had not been informed (until I informed him) that PPM was rattling around the UK nursery sector. He speculated that the nurseries involved were most likely dealing with trees destined for landscape, amenity and garden planting. 

Be that as it may, there is an increasing amount of buying and selling among UK nurseries, especially with ongoing shortages of some species due to a variety of factors including the pressure on planting to meet government targets. The forest nursery owner told Forestry Journal that a lot of current between-nursery trading within the UK is from England to Scotland to meet the country’s highly successful tree-planting programme.

PPM most probably entered the UK in the egg stage on container-grown pine trees.

However, the hypogeal (underground) pupa stage of PPM offers extra opportunities for pest transfer between countries in soil that originated metres away from the infested pine tree and adhering to or around the roots of any tree growing in the vicinity of PPM pupation underground.

DEFRA said: “We encourage those who work in the live plant trade to continue to be vigilant for signs which may indicate the presence of PPM on imported pine and other conifer plants, and if symptoms are identified, to report them immediately to the plant health authorities.”

It might help if all those concerned and especially other nurseries are promptly informed about the situation.