Ips cembrae: implications and consequences for UK forestry

IT’S bad enough to have one exotic bark beetle putting exports of timber to pest-free countries like Ireland in jeopardy, but following the European spruce bark beetle Ips typographus settling in for the duration across southern England, Ips cembrae (large larch beetles) were found at an Irish port on logs imported from Scotland. This led to an indefinite ban being placed on the movement of conifer wood from parts of Scotland’s west coast. Unlike Great Britain, the island of Ireland (Republic of Ireland and Northern Ireland) is free of I. cembrae.

That said, a significant swathe of western Scotland from the Mull of Kintyre, Mull through to Fort William and over the sea to Skye is called the PFA, which stands for ‘Pest Free Area’ – but apparently no longer. Indeed, I. cembrae was first identified in the PFA back in 2022. What’s more, in February 2024 similar ‘preventative action’ was taken after Scottish Forestry chiefs admitted they could no longer guarantee that Dendroctonus micans (the great spruce bark beetle) was not within the PFA. All this comes hard on the heels of I. typographus being found in Fife woodland in Scotland and then on Sitka spruce in the south of England. It all goes to show the wider international implications and consequences of exotic quarantine pests establishing on our trees and timber. 

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Ireland is understandably proud of its pest-free status for so many forestry pests that appear to be running and flying around here at will.

“Ireland is free of many of the bark beetles that are found in Europe and in Great Britain,” boasted Senator Pippa Hackett, Ireland’s agriculture minister. “It is very important for the future of the forestry sector in Ireland that we do everything we can to keep the island free of these beetles. My department has a clear plant health and biosecurity strategy and has built up a strong multi-disciplinary plant health team. These findings demonstrate the importance and effectiveness of our surveillance work at ports.” 

So where is I. cembrae found? According to EPPO (European Plant Protection Organisation) Ips cembrae is restricted to Europe, albeit a large part of the continent. Ips cembrae is native to Europe and occurs there throughout the natural range of Larix decidua (European larch) and most of the planted range of Larix decidua and Larix kaempferi (Japanese larch). It is present only in Europe (including non-EU regions of Central and Northern Russia, Ukraine, Switzerland and mainland United Kingdom); reports of its presence in Asia are considered to relate to the closely related species Ips subelongatus. I. cembrae is absent in some areas, such as Greece, Ireland (Republic of) and parts of the United Kingdom (Northern Ireland, Isle of Man), which have protected zone status.

Ongoing infections by P. ramorum mean a lot of larch logs across the landscape for Ips cembrae to feed on (Image: DARD)

And what is the economic impact of this pest? I. cembrae is generally regarded as a secondary pest in Larix (larch) plantations where it breeds in recently felled logs, wind-thrown stems, and dying trees. That said, an abundance of potential breeding resource such as fresh thinning residues has the capacity to trigger major population growth and thus facilitate and furnish a subsequent invasion of live standing trees. Drought conditions on drier sites will weaken trees and leave them susceptible to attack.

Adult beetles carry out so-called maturation feeding on young twigs, which means large populations can cause additional damage to live trees when this results in shoot pruning and defoliation. Initial attacks upon live host trees may be concentrated within a relatively limited area on the stem, as governed by pheromone aggregation, resulting in crown dieback. That said, damage tends to be more diffuse along the stem on wind-thrown or dying trees.

Like other conifer bark beetles (including I. typographus) I. cembrae is closely associated with a wide diversity of fungi, most notably ophiostomatoid species, including ‘blue stain’ fungi. The most significant of these is Ceratocystis laricicola which kills the living bark and cambium (meristematic tissue) in the host tree, stains the sapwood and most likely contributes to the dieback of and killing of larch during I. cembrae outbreaks. 

One area requiring further investigation is the relationship between I. cembrae and Phytophthora ramorum, the invasive Oomycete pathogen, which initially caused large-scale dieback and death of larch plantations across the United Kingdom and was subsequently detected across western and central Europe. Research into whether there is any interaction between this lethal fungus-like pathogen and I. cembrae, and particularly whether the pathogen has an influence on population growth and timber damage caused by the larch beetle is clearly needed.

So what are the implications for Great Britain (mainland UK)? The Forestry Commission considers it a much less damaging pest than the closely related spruce bark beetle Ips typographus, with the risk presented to quarantine areas being much lower. But that did not stop the Irish authorities acting like greased lightning when beetles were discovered on timber imported from the west of Scotland. As far as the UK is concerned it is the pre-eminent status of Sitka spruce and the fading importance of larch within UK commercial forestry that will clearly colour the official view on I. cembrae, even if specific damage done to larch as a species is severe. 

Potential damage from Ips cembrae needs to be weighed up against that caused by P. ramorum (Image: FC)

Any damage done to larch plantations in the UK clearly has to be weighed up against the considerable damage already inflicted on larch by Phytophthora ramorum and as such making a judgement on whether I. cembrae is worth worrying about – but of course it is. Despite Forestry Commission statistics and disease reports tending not to differentiate between the three larch species grown commercially in the UK, the bulk of the damage caused by Phytophthora ramorum has been to Japanese larch (L. kaempferi), whereas the main host of I. cembrae is European larch (L. decidua). 

That said, I. cembrae will also attack Japanese larch and Hybrid larch (L. x eurolepsis). Though not affected anywhere near to the same degree as Japanese larch by Phytophthora ramorum, all three larches have suffered collateral damage by being tarred by the same brush. 

Should the industry want to stage a comeback for larch on the back of European larch then the last thing we want is I. cembrae crawling all over the landscape. And with Phytophthora ramorum showing no real signs of abating until it has exhausted the supply of larch, there will clearly be a lot of larch logs laying on the landscape for the beetle to feed on.