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Pine tree lappet moth (Dendrolimus pini)

Pine-tree lappet mothThe pine tree lappet moth (Dendrolimus pini) is a native of continental Europe, Russia and Asia, where it causes periodic, large-scale damage to pine plantations. This damage sometimes covers thousands of hectares, with outbreaks lasting up to eight years in some parts of its range. Outbreaks and resultant defoliation by pine tree lappet moth caterpillars can impair tree growth and tree health, making trees more susceptible to other organisms such as bark beetles and wood-boring insects, the effects of which could ultimately lead to tree deaths. This could lead to significant ecological, economic and, ultimately, social impacts.

Distribution

In Scotland, adult male moths were first caught in 2004 in a pine plantation to the west of Inverness, and then in the river Beauly catchment, but the presence of this moth was not reported to the Forestry Commission until 2008.

There have been occasional sightings of single specimens on the south coast of England and in the Channel Islands. These were probably ‘migrant’ males blown across the English Channel.

There is a report that a caterpillar found in Essex in 1999 on a pine tree imported from Italy was bred out to produce a female moth. It was first recorded in Scotland in 2004.

Its natural range is the Scots pine forests of continental Europe, where it is widespread, occurring in every European country as well as Russia and parts of Asia. It has also been reported in North Africa.

Spread

The moths move mainly by flying, although the female moths are very heavy (with eggs) and therefore do not usually fly further than a few hundred metres. Older caterpillars are able to crawl from tree to tree – but can also crawl as much as several hundred metres to reach new stands of trees. Eggs, larvae and pupae could also be spread on harvested logs being transported on lorries, or on plants or foliage.

We do not yet know how the moth got here. It could have been brought in on imported pine trees, on forestry machinery from abroad, or with wood products or wood packaging - but it is also possible that it might have been accidentally or deliberately released. We have no information that would confirm which of these possible routes could have led to the current finds, and it remains most unlikely that we will be able to trace-back unless further DNA testing finds a very close match to an overseas source population.

Impact

If conditions in Scotland become conducive to the Pine-tree lappet moth, the death and weakening of significant numbers of trees would risk serious ecological disruption to Scotland's forests. Of particular concern would be the ancient Caledonian pinewoods, where the moth is not known to be present.

Given the importance of pine to the timber trade in eastern Scotland, an outbreak could also have a very significant impact on the wider forestry sector.

While the Scottish Pine tree lappet moth population is still relatively small and localised, we have been assessing options to control or, if thought necessary, eradicate this species from Scotland in case it begins to cause serious damage and spread to other areas.

We have considered these options through an Outbreak Management Team (OMT), which includes representatives from Scottish Natural Heritage (SNH), Butterfly Conservation and Confor as well as our Forest Research agency.

Climate modelling work suggests that summer conditions in the coming decades, especially in the drier east of Scotland, may become more favourable for outbreaks of the moth. However, the potential impact on the moth population of Scotland’s variable autumn and winter climate is not yet known. 

Lifecycle

Adult moths emerge from pupae in midsummer and live for 9-10 days, during which time they mate. The females each deposit up to 250 eggs on twigs, needles and the bark of host trees. These hatch within 16-25 days, and the caterpillars feed on pine needles in the tree canopy until winter frosts begin, when they then move down the trees to over-winter between the litter and soil close to the base of trees.

Pine-tree lappet moth caterpillar eating Scots pine needle. Location: Northern Research Station, Roslin, Midlothian, Scotland.In spring the caterpillars return to the tree crowns and continue feeding until they are large enough to pupate.

The caterpillars moult through several, progressively larger stages, growing to as much as 8.0 cm long. Pupation begins in May and June and lasts for 4-5 weeks. Pupae are formed 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 6-7 months. In field conditions under our current climate a two-year cycle is believed to be the norm, but a one-year life-cycle under favourable conditions, and a three-year lif-ecycle under less favourable conditions, cannot be discounted.

In Europe natural enemies include several bacteria and fungi (particularly on the over-wintering caterpillars), parasitoids (e.g. parasitic wasps and flies) and predators such as ants and birds, which will feed on or attack the larvae. Bats will catch the adults. 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 controlling populations in Scotland.

Action 

An Outbreak Management Team was therefore set up by the Forestry Commission in late 2008 to manage the potential threat posed by the pine tree lappet moth. A contingency plan was put in place, a key part of which was introducing immediate controls on the movement of timber from the outbreak area.

We continue to take a proportionate, risk-based approach to the management of this potentially significant pest and:

  • continue to monitor Pine tree lappet moth populations throughout the Kiltarlity area

  • set up and monitor pheromone traps at those mills in receipt of timber from the Kiltarlity area

  • apply timber movement restrictions only to higher risk areas, i.e. where breeding populations have been confirmed and only in mid-May - end August when the risks of transporting egg masses and larval clusters out of the area are greatest

  • adopt practical biosecurity measures during timber harvesting operations to reduce the risk of moving Pine tree lappet eggs and larvae outside the area

  • prepare a contingency plan for aerial pesticide/biological control in the unlikely event that a mass outbreak occurs in the future and such action becomes necessary

  • subject to funding availability: continue DNA investigations; monitoring/rearing to determine growth rates and life-cycle in Scotland; and establish what natural parasitoids occur in the Kiltarlity area

Susceptible trees

The moth's preferred host is Scots pine (Pinus sylvestris), but it is also known to live and feed on other pine species and, in outbreak conditions, on other coniferous trees such as Sitka spruce, Norway spruce and Douglas fir, all of which are grown commercially in Scotland.

Movement restrictions

Initial controls restricted timber and foliage movements in the moth’s 'active period' (1 March to 30 November).  During this period only bark-free timber could be moved outside the outbreak area. These initial controls allowed time to determine whether a breeding population was present, (the capture of caterpillars in September 2009 confirmed that there was) and to investigate the life cycle, origin and likely impact of this moth.

A programme of regular monitoring shows that the population density is currently low and the moth remains confined to a small part of the Beauly catchment.

Map (PDF 2.4MB) of the contrrol area.

Based on a combination of current evidence and expert advice, the OMT considered that the level of timber movement controls should be reduced so that woodland owners could resume active management of their forests. Such active management activities will help to maintain the health and resilience of forests to the wide variety of insect pests and tree diseases found in the natural environment.

However, through the OMT we will continue to work closely with Butterfly Conservation Scotland, SNH, ConFor and woodland owners to monitor the distribution and population levels of the moth and to ensure management of this potential pest species remains balanced and risk-based.

Research

There are two possible explanations for the recent discovery of this moth in Scotland. It may be part of an overlooked remnant population or, alternatively, its presence represents a more recent introduction of the species which has since become established.

The OMT decided to explore these two alternatives by commissioning a study to compare 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 one of these groups, the members of which have a broad geographic spread from eastern France to the Mediterranean coast and 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, but this is not surprising given the very large range of this species and the impracticality of obtaining samples from every sub-population.

The pine tree lappet moth has not been caught in any of Scotland’s native Caledonian pine forests surveyed to date.

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Last updated: 12/17/2014