Present in United Kingdom
Notifiable – see ‘Report a sighting’ below
Scientific name – Dendrolimus pini (D. pini)
The larvae, or caterpillars, of pine-tree lappet moth are a serious pest of Scots pine (Pinus sylvestris) forests in some parts of continental Europe, periodically causing large-scale damage to them. They can also attack other coniferous tree species – see ‘Susceptible species’ below.
Distribution
Pine-tree lappet moth is widespread in Scots pine forests of continental Europe, occurring in every European country as well as Russia and parts of Asia. It has also been reported in North Africa.
It was not known to be present in the UK until 2008, when a breeding population was discovered in pine forests near Kiltarlity, in the Beauly catchment west of Inverness, northern Scotland. We have so far been unable to confirm whether this is a remnant native population or a comparatively recent introduction. Further information is available under ‘Origins and background’ below.
Susceptible species
The pine-tree lappet moth’s preferred host is Scots pine. However, it is also known to live and feed on other pine species and, during population spikes when it might have consumed most of the available pine needles, on other coniferous trees. These include Sitka spruce (Picea sitchensis), Norway spruce (P. abies) and Douglas fir (Pseudotsuga menziesii), all of which are grown commercially in Scotland.
They can also attack other species of coniferous trees such as firs (Abies species), cedars (Cedrus spp.), juniper (Juniperus spp.), and larches (Larix spp.), as well as other species of pine (Pinus spp.).
The threat
Pine-tree lappet moth caterpillars feed on pine needles (pictured), causing extensive defoliation when the population gets extremely high. In some parts of its range, such as Poland and Germany, damage has covered thousands of hectares and can continue for up to eight years before the population declines. In other countries outbreaks have been less frequent and have tended to cover smaller areas.
Defoliation by the caterpillars can impair tree growth, vigour and health. This makes the trees more susceptible to other threats such as bark beetles and wood-boring insects. This can ultimately lead to tree deaths and significant ecological disruption, economic and social impacts, and an increased risk of fire as the dead tree material dries.
If climatic conditions in Scotland become more favourable to the moth its population could increase and help it to spread into new areas. Any death and weakening of significant numbers of pine trees could risk serious ecological disruption to Scotland’s forests.
Of particular concern would be damage to the ancient Caledonian pinewoods, but their structure, locations and natural diversity suggest the risk to such woodland is low. Currently only one Caledonian pinewood, Glen Strathfarrar, is known to have a breeding population present. Although the density of caterpillars and adult moths in Glen Strathfarrar was extremely low in 2019, the situation remains closely monitored.
Given the importance of pine to the timber trade in eastern Scotland, a population spike leading to large-scale damage there could also have a significant economic impact on local forestry, timber and associated industries.
Climate modelling suggests that summer conditions in the coming decades, especially in the drier east of Scotland, might become more favourable for damaging population spikes. However, the potential balancing impact of Scotland’s variable autumn and winter climate on the population is not yet known. Since its detection in the Beauly catchment there have been no known tree deaths or significant defoliation in the confirmed breeding area, reflecting the very low population density.
There is also a significant risk to forests in England and Wales should the moth establish populations in the warmer and drier southern parts of Great Britain.
Identification and symptoms
This is a large species, with adult moths having wingspans of 50-70mm (males) and 70-90mm (females). Fully grown male and female caterpillars measure up to 50mm and 80mm long respectively. Identifying features include grey-brown to brown forewings with a reddish-brown lateral band and an irregular dark-brown to black stripe along the edges. Hind wings are red brown to grey-brown. Males are usually darker than females.
Mature caterpillars have soft, grey to brown hairs, thick bands of steel blue and black hairs on the thorax, and a black mark flanked by irregular white lines on the abdominal segments.
The caterpillars pupate in highly cryptic, yellow-brown to black cocoons with steel blue hairs. Cocoons start being made in June in bark crevices and on needles and branches. They can be very difficult to see.
More images to aid identification are available in this Pest Alert. (Please note: where contact details in the Pest Alert differ from those given on this page, please use those on this page.)
Report a sighting
We encourage people who own, manage, work in or visit pine forests to be vigilant for signs of pine-tree lappet moth, especially outside the known areas of its presence, and to report suspected sightings immediately. People receiving or handling imported pine material should be especially vigilant.
- Report suspected sightings in Great Britain to us using TreeAlert.
- Report suspected sightings in Northern Ireland to the Irish plant health authorities using TreeCheck, the all-Ireland tree pest reporting tool.
Please note that TreeAlert and TreeCheck both require photographs to be uploaded. These should be clear, well-lit, close-up pictures of the moth, caterpillar or symptoms of their presence.
Alternatively, suspected sightings can be made directly to the relevant plant health authority. This is the preferred route for suspected sightings made on trade premises, such as nurseries and garden centres.
In all cases, provide precise details of the location and, if possible, clear, well lit, close-up photographs of the symptoms.
Spread
The pine-tree lappet moth disperses mainly by flying, mainly from June to August, depending on weather and location. Male moths can fly many kilometres, but female moths heavy with up to 300 eggs cannot usually fly further than a few hundred metres, limiting the rate of spread of the species by this means. They can, however, fly greater distances once they have laid their initial egg batches, but they are still believed to be much less than the distances which males can fly.
Older caterpillars can crawl from tree canopy to tree canopy, but they can also move several hundred metres on the ground to reach new stands of trees.
It is thought that very young caterpillars might get blown or ‘parachute’ off trees by means of their silken threads, although we have not yet observed this occurring in Scotland.
Eggs, larvae and pupae can also be spread on harvested logs being transported on lorries, and on plants and foliage.
Life cycle
Adult moths, both sexes of which are incapable of feeding, emerge from pupae in midsummer (typically mid-June to early July) and can live for up to two weeks, during which time they mate. They fly mostly at night, and are strongly attracted to light.
Each female deposits between about 150 and 300 eggs on twigs, needles and the bark of host trees. The caterpillars hatch within 16 to 25 days of the eggs’ being laid, and feed on pine needles in the tree canopy until winter frosts begin. They then move down the trees to over-winter in the leaf litter and soil close to the base of the trees.
In spring the caterpillars emerge from the ground and climb up to the tree crowns, where they continue feeding on the needles until they are large enough to pupate.
The caterpillars moult through several, progressively larger stages, called instars, until pupation begins in May and June, the latter lasting from four to five weeks. The pupae form inside loose, partially transparent cocoons, which can be found in tree crowns, bark crevices and under-storey vegetation.
In laboratory conditions it is possible for development from egg to adult to take place in only six or seven months. However, a two-year cycle is believed to be the norm in field conditions under our current climate, although a one-year life-cycle under favourable conditions, and a three-year life-cycle under less favourable conditions, cannot be discounted.
Control and management
Pine-tree lappet moth has natural enemies in continental Europe which help to keep its population in check most of the time. These include several bacteria and fungi (particularly on the over-wintering caterpillars), parasitoids, such as parasitic wasps and flies, and predators such as ants and birds, which will feed on or attack the larvae and pupae. A number of species of insects have been reported to prey on the moth.
Studies are under way to determine whether these natural enemies are helping to control the population in Scotland.
Given that the moth’s spread can be aided by human activity such as timber transport, it is also important that people working in pine forests practise high standards of biosecurity, or plant hygiene. This includes cleaning tools, equipment, vehicles, machinery, clothing and footwear before leaving affected or possibly affected woodlands to move to other sites.
Forestry contractors working across international boundaries must ensure their machinery and vehicles are thoroughly cleaned before moving them from one country to another. This particularly applies to machinery and equipment moving from continental Europe to the UK, although contractors moving from the UK to continental Europe should show the same respect.
Forest visitors should not remove pine material, such as foliage and branches, from affected woodland.
More-detailed biosecurity advice is available from:
Regulation and official action
An Outbreak Management Team (OMT) was set up by Forestry Commission Scotland (now Scottish Forestry*) in 2008 to manage the potential threat posed by the pine-tree lappet moth. A contingency plan was put into action, a key part of which was introducing immediate restrictions on the movement of timber from the outbreak area, and instigating the research required to inform future management decisions.
A programme of regular monitoring since 2008 has shown that the population density remains very low, and the moth’s breeding range has remained restricted to only a comparatively small part of the Beauly catchment.
Initial timber movement controls were eased following expert advice and recommendations from the OMT. The objective was to target those highest-risk areas and times of the year, thereby enabling owners to resume active management of their forests. Such management, including thinning and patch clear-felling, helps to diversify the forest and build its resilience to future tree health threats.
Scottish Forestry, with advice from the OMT’s successor, the Pine-Tree Lappet Moth Management Group, and supported by Forest Research, continues to take a proportionate, risk-based approach to the management of this potentially significant pest. This is articulated through a surveillance and containment strategy, which includes:
- monitoring by Forest Research of populations within the ‘core breeding area’, now expanded to include part of the Glen Strathfarrar Caledonian pinewood;
- maintaining wider-area vigilance, by means of pheromone traps monitored by Forest Research, to detect any new breeding areas outside the core breeding area;
- monitoring pheromone traps by Forest Research at mills receiving timber from the known breeding area;
- maintaining timber movement restrictions, including practical biosecurity measures, within the known breeding area from mid-May to the end of August, when the risks of transporting egg masses and larval clusters out of the area is greatest;
- continuing to take appropriate containment action, primarily through ‘glue banding’ trees. This to prevent expansion of the core breeding area until Scottish Forestry is confident that the pine-tree lappet moth does not present a threat to Scotland’s pinewoods. (Trees in Caledonian pinewoods are not glue-banded, so alternative approaches to containment will be required in them.);
- working with Forest Research to continue to investigate the feasibility of alternative containment methods as a ‘safety net’ in the unlikely event of a future population spike. These would include mating disruption techniques, (which would include close liaison and field work with scientists in Poland), and aerial application technologies for the application of targeted pesticides, including biological control products such as nematodes; and
- subject to funding, continuing to work with Forest Research to explore the lineage of the Scottish pine-tree lappet moth population and to understand the population dynamics of this species in Scottish conditions, including the presence and role of existing parasitoids.
With advice from the Pine-Tree Lappet Moth Management Group, Scottish Forestry continues to work closely with Forest Research, Butterfly Conservation Scotland, Scottish Natural Heritage, the Confederation of Forest Industries (Confor) and forest managers to ensure that management of this potential pest remains balanced and risk-based.
* Scottish Forestry succeeded Forestry Commission Scotland as the forestry authority in Scotland in 2019.
Origins and background
Historically, the pine-tree lappet moth’s natural range has been regarded as the Scots pine forests of continental Europe. It was unreported in the UK until 2008, apart from a small number of individuals thought to have been blown across the English Channel to the south coast of England.
Then in 2008 the Forestry Commission was notified that an adult, male pine-tree lappet moth had been caught four years earlier, in 2004, on the outskirts of Inverness. This was followed by additional captures near Kiltarlity in the Beauly catchment. Follow-up surveys confirmed in 2009 that a breeding population was established in the area, and as of 2019 it still appeared to be confined to the Beauly catchment. However, there have been signs that slow natural spread might be occurring.
There are two possible explanations for the comparatively recent discovery of this large moth in Scotland: it might be part of an overlooked remnant population, or its presence represents a more recent introduction of the species which has since become established.
Our research
The two possible explanations mentioned above for the discovery of this species in Scotland were explored in a Forest Research study. This study compared mitochondrial DNA variation in Scottish moths with that of samples from across its native range. Results to date indicate that:
- variation in mitochondrial DNA formed three groups, which showed a distinct geographic pattern. The Scottish samples belong to a group which has a geographic spread from eastern France along the Mediterranean coastal region eastwards as far as Mongolia. Membership of the other two groups consisted of samples from Spain, western France, Scandinavia and central European countries, and these areas are therefore unlikely to have been the source of founding individuals for the Scottish population; and
- so far an identical ‘match’ for the Scottish DNA sequence has not been found in other parts of Europe. However, this is not surprising, given the species’ very large range and the impracticality of obtaining samples from every sub-population.
This means that the case, based on a very few specimens, for or against the pine-tree lappet moth being either a native species which has gone through a genetic ‘bottleneck’, or a non-native introduction, cannot yet be proven conclusively either way. It is possible that it might never be fully resolved.
Further details of our research are available in the research area of this website.
Prospects
Whether this moth will ever become a serious tree pest in Scotland remains an area of debate. Given these uncertainties, Scottish Forestry is taking a proportionate and precautionary approach to its management, entailing:
- acceptance of current population levels within the core breeding area;
- containment measures designed to prevent expansion of that area; and
- wide-area monitoring to detect any unforeseen population ‘jumps’
