Dr Terry Mabbett reports on a troubling outbreak of beetle-ravaged trees in an Essex woodland – and efforts to identify the culprit.

LIKE most people I was essentially locked up from 23 March to 23 June 2020, so when the opportunity came last August to visit an ailing conifer plantation on the Essex border with Hertfordshire I jumped at the chance.

While the entire country was in the grip of a coronavirus pandemic, this stand of Norway spruce just a stone’s throw from Stansted Airport was suffering from a plague of bark-boring beetles. This was much to the chagrin of the landowner, and my forestry contractor colleague was hired to clearfell the stand, realise some revenue from what was left of the standing timber and carry out re-planting of the site. Beetle infestation was extremely high, with widespread resin bleeds and bark sloughing on large numbers of trees. By the time of my visit, a significant proportion of the trees were already dead.

It was four decades since I had last been confronted with a bark beetle infestation of such plague proportions and that was on a rubber plantation in West Africa. A dry-season fire had ripped through hundreds of hectares of young pre-tapping rubber trees, scorching the bark and laying the trees open to attack by Ambrosia beetles which, as I recall, were Xyleborus perforans.

The Norway spruce monoculture was around five hectares in size, significant for the species in this neck of the woods, especially planted at such a close spacing of 1 m x 1 m. The substantial property had changed hands and functions several times in the past half-century and there were no available records as to the actual date of planting. With no thinning having apparently taken place, radial growth of the trees was way below what you would expect for Norway spruce planted decades ago.

The site manager recalled how some 25 years ago they had cut the tops of these same Norway spruce for personal use as Christmas trees. This suggested the trees were planted about 40 years ago, before Nigel Lawson’s infamous 1988 budget. Tax changes in this budget delivered an almighty blow to the future of commercial, timber-led forestry in the UK. Effects were psychological as well as financial and are still being felt today in England’s poor tree-planting figures.

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I was intrigued as to why anyone would want to plant at such a close spacing and then compound the problem by failing to thin. David Gwillam, owner of Prees Heath Forest Nurseries in Shropshire, whose business was already well established at this time, said the close spacing was common to continental Europe, but always followed by a much stricter regime of rigorous thinning than carried out in UK.

“The tax changes in Lawson’s 1988 budget were not only a disincentive to commercial conifer planting but also a disincentive for post-planting care and management,” he said. He surmised the landowner had planted the trees in the early 1980s and subsequently walked away. Other possibilities are that the motivation for planting was privacy with the resulting dense stand of evergreen conifers providing a living screen around the property, or that the trees were originally planted for harvesting and use as Christmas trees, but that this was never realised.

Be that as it may, the forestry contractor was now faced with clearfelling a stand of an inherently low timber value, falling all the time due to the damage caused by bark beetles which spread like wildfire through the trees in just three months. However, the contractor’s greatest initial concern was that the culprit could be one or both of a pair of spruce bark beetle pests with the capacity to act as primary pest invaders, listed as ‘notifiable’ on the respective Forestry Commission websites.

FOREST RESEARCH TO THE RESCUE

By far the biggest fear was for Ips typographus (the larger eight-tooth spruce bark beetle), identified for the first time as a breeding population in December 2018 in Kent on the other side of the Thames Estuary. It was found in two woodlands in the Ashford area of Kent, specifically on Norway spruce which is its primary plant host. Ips typographus preferentially attacks weakened trees as a secondary pest invader, but as the population rises it will attack and damage healthy trees as a primary pest invader.  Another possible outbreak was flagged up on Norway spruce by a forestry contractor working in the Surrey Hills, but without divulging the exact site location. Apparently, this report was not acknowledged or investigated by the UK plant health authorities. Of additional but much less concern would be Dendroctonus micans (great spruce bark beetle), first identified in the UK in Shropshire during 1980 with many outbreaks reported since then.

Forestry Journal: The Norway spruce was already self-thinning at a rapid rate, thus providing an ample supply of weakened and dead trees for bark beetles to invade as secondary insect pests.The Norway spruce was already self-thinning at a rapid rate, thus providing an ample supply of weakened and dead trees for bark beetles to invade as secondary insect pests.

No live beetles, adults or larvae, were in evidence on the day of my visit, but insect signatures in the form of gallery patterns – where adult beetles and larval stages had eaten their way through the layer of phloem immediately under the bark – were widespread. 

Dedicated beetle traps loaded with pheromone lures specific for the suspected beetle pest species would ideally be deployed to secure and identify the potential culprits. The expertise required to carry this out is at Forest Research, but was difficult to access at the time due to COVID-19 and the resulting lockdown. Forestry Commission restrictions on the distance of travel allowed in one day by their officers required staged travel plans that would take several days at least for the round trip, making a visit logistically difficult and too costly. However, pictures of the site including damaged trees and beetle gallery patterns were sent to Forest Research, which provided some assurance as to the cause of this unusual pest outbreak.  

There was great relief all round when Forest Research was able to eliminate
I. typographus on the basis of there being no evidence (from the pictures) of the specific gallery systems produced by this spruce bark beetle pest. Larval galleries radiate outward from wider and deeper linear galleries where adult females lays the eggs. Larval galleries become wider as the larvae grow when they burrow along. This pattern is sufficiently unique to be used as the sole identification tool for I. typographus.

Indications, from my point of view, against it being D. micans were found on the Forest Research website, which said that since first discovery in the county of Shropshire in 1980, D. micans had become an established pest in Wales and western England before expanding into southern Scotland, but with no mention of outbreaks in the east of England (although it later transpired this was not the full story).

Forest Research said D. micans is currently establishing all over England, Wales and southern Scotland and how they would not be surprised if it was present in or near to this site on the Essex border with Hertfordshire, although they saw no evidence from gallery patterns in the pictures that would indicate the presence of D. micans. They also clarified the situation around whether D. micans still had ‘notifiable pest status’ (as indicated on the Forest Research website) given it now appeared to be well established across most of the British Isles. “D. micans is not notifiable, although we do ask to be informed of findings in order that we can treat any affected woodland with Rhizophagus grandis, a predatory beetle which specifically preys on the larvae of D. micans,” they said.

Forestry Journal: Adult beetle exit holes (pin-holes) on the bark of Norway spruce at the site in Essex.Adult beetle exit holes (pin-holes) on the bark of Norway spruce at the site in Essex.

SECONDARY INVADERS AS THE MAIN CULPRITS

Forest Research had essentially eliminated the two big, bark beetle fears, but did have tentative answers which lay in one or more secondary bark beetle pest invaders encouraged by the advanced state of self-thinning of the stand prior to invasion by the insects. Forest Research picked out Polygraphus poligraphus (small spruce bark beetle), which they say is becoming increasingly common in the UK. “We are getting many enquiries related to this bark beetle but always associated with weakened, dead and dying trees, in which it breeds,” they said. 

Drought, which the UK has been on the periphery of experiencing over the last few years, is impacting disproportionately on tree species such as Norway spruce, said our contact at Forest Research. This, coupled with increased attention on unmanaged and unhealthy woodland, which this clearly is, brings associated problems like secondary pest attack more into focus. Our contact at Forest Research said they had never encountered Polygraphus poligraphus attacking healthy trees.

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Having seen a full range of pictures, they summarised the situation as follows: “The stand is self-thinning and thus generating a large amount of dead and dying trees susceptible to attack by many species of bark beetle and which is the start of the rot-down process. Polygraphus poligraphus is likely to be the main culprit, but a full survey of the woodland would almost certainly reveal many species of bark beetles and longhorn beetles contributing to the damage, although none will be the specific cause and culprit of the damage seen.”

Forestry Journal: Norway spruce sawlogs loaded up and ready for the mill at the Cowdray Estate in West Sussex, March 2015.Norway spruce sawlogs loaded up and ready for the mill at the Cowdray Estate in West Sussex, March 2015.

POLYGRAPHUS POLIGRAPHUS (SMALL SPRUCE BARK BEETLE)

There appear to be no reports of this beetle within an insect pest context or capacity within UK. Past interest in this beetle, such that it is, appears to be within a biodiversity context and arena with 23 scattered reports from south-east England, East Anglia, the English Midlands and Wales, from 1938 to 2006.

However, there are reports of this insect causing significant pest damage in European countries such as Sweden, where P. poligraphus, like I. typographus, prefers weakened trees but does attack healthy trees. Polygraphus poligraphus is a European native species, but nothing could be found to indicate whether P. poligraphus is regarded as native to the UK; the first documented report of this particular spruce bark beetle in the UK appeared in 1938.

LONGHORN BEETLES – NATIVE AND EXOTIC

There are 69 longhorn beetles in the UK considered to be native or naturalised, with many more occasional alien longhorn beetle species arriving all the time on imported timber. Of these, one is of particular interest. Tetropium fuscum (brown spruce longhorn beetle), which attacks spruce and pine, was considered to be of sufficient importance for the Forestry Commission to carry out a Rapid Pest Risk Analysis (PRA) in 2017. There had been a few previous records of this insect in the UK. Surveillance during that time revealed breeding populations in Scotland. The Forestry Commission said the likelihood was the pest is more widely established in the UK than known at that time (2017). Apparently it prefers to attack already stressed-out trees, although in both Canada and to a lesser extent in Europe, attack by this bark beetle has led to the death of trees that would otherwise have recovered, and therefore by implication had been healthy.

Forestry Journal: By late August 2020 many of the trees were already dead.By late August 2020 many of the trees were already dead.

CONCLUSIONS AND CONSIDERATIONS

Despite Ips typographus having previously been found in Kent (December 2018) on Norway spruce (the primary plant host), insect gallery patterns carved out of the bark in Essex did not match the unique pattern left by I. typographus. The pattern is sufficiently unique to be used as the sole identification tool for this specific beetle and would appear to rule out I. typographus.

There are few worthwhile pictures of galleries carved out by Dendroctonus micans which can be used for comparative purposes. From those which are available, there appears to be a linear dimension to the pattern, with the width and depth of galleries comparable to those of Ips typographus. The Forest Research website shows a picture of resin bleeds and resin tubes caused by entry of the female beetles and characteristic of this beetle attack on Norway spruce. Resin bleeds were photographed on beetle-infested Norway spruce trees at the site in July 2020. Forest Research did not rule out the possibility of D. micans attacking trees in this stand of Norway spruce and others in this region of eastern England.

Forestry Journal: Bark sloughing on dead trees was widespread.Bark sloughing on dead trees was widespread.

Standard comparative pictures of gallery patterns written by Polygraphus poligraphus lack the linear dimensions seen with those carved out by I. typographus and D. micans. Gallery writings by P. poligraphus are more of a scribble and it is difficult to discern any distinct or organised pattern. Furthermore, in both width and depth the individual galleries appear much less substantial than those carved out by the aforementioned bark beetle insect pests. At 1.8 – 3.5 mm in length, the P. poligraphus adult beetle is considerably smaller than either I. typographus (4.0 – 5.5 mm) or D. micans (6.0 – 8.0 mm). 

Standard pictures of gallery writings of P. poligraphus are similar to those observed on site. They lack a distinct pattern, with a relatively small width and depth of the galleries carved out by insects, and thus logically associated with a much smaller beetle like P. poligraphus. Polygraphus, which is derived from the Ancient Greek words ‘poly’, meaning many, and ‘graph’, meaning to ‘scratch, scape or graze’, aptly describes the hieroglyphic-like scribble of Polygraphus poligraphus. Given the number of different longhorn beetles in the UK environment, it would be surprising if none were involved here.

BEETLE MANIA BACK ON THE AGENDA

Beetle mania is currently well and truly back on the agenda following the latest findings of I. typographus as breeding populations in two woodlands in Kent, confirmed by the FC on 26 June and 1 July 2021.

Ever since this notorious, notifiable bark beetle pest was found on Norway spruce in two separate woodlands near Ashford in Kent during December 2018, a demarcated area covering virtually the whole of Kent and a slice of neighbouring East Sussex has been enforced under order of the Forestry Commission. Movement restrictions have been in place on susceptible conifer material capable of spreading this bark beetle pest and originating within the demarcated area.

These new findings of I. typographus must be concerning for the Forestry Commission, because two and a half years have elapsed since the first finding of this notifiable insect pest in December 2018 and the subsequent instalment of the plant health notice.

Nicola Spence, the UK chief plant health officer, said: “Two outbreaks of the eight-toothed spruce bark beetle in areas of woodland in Kent have been confirmed. This beetle can have a serious impact on spruce tree species and the forestry industry.”

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You bet it can. Ips typographus is the most serious and destructive insect pest of spruce species within its native range, which includes Europe and Asia. Norway spruce is the primary host of I. typographus although all spruce species, including Sitka spruce, are susceptible. Ips typographus is generally a secondary pest found attacking weakened or felled trees. However, its main economic impacts are through population build-up and mass attack of living trees, which can result in extensive tree mortality.

Forestry Journal: Further west across southern England, Norway spruce produces good commercial timber for milling. Here, P60 Norway spruce undergoes felling by the Forestry Commission in the Surrey Hills in 2011.Further west across southern England, Norway spruce produces good commercial timber for milling. Here, P60 Norway spruce undergoes felling by the Forestry Commission in the Surrey Hills in 2011.

AUTHOR’S NOTE ON NORWAY SPRUCE

There is a paucity of Norway spruce in this part of eastern England including Essex, Suffolk and Norfolk. Forester Matthew Allen, who works the East Anglian area of England, puts the scarcity of Norway spruce grown purely for timber in his area down to water stress experienced as the trees gain height. “In its native European distribution, the tall canopies are frequently shrouded by mists and fogs, but this is clearly a facility which does not exist in East Anglia”, he told Forestry Journal.

“Where found, Norway spruce has either been grown for Christmas trees or as a nurse crop for oak.” He commented that the best Norway spruce observed in the region is invariably found in mixed woodland with deciduous broadleaves. Given the 1 m x 1 m spacing, Matthew suggested it could have been a Christmas tree planting that was never realised. It would be interesting to learn of any other stands of Norway spruce within the region and whether bark beetle infestation and damage of this nature has been observed.

I always have a chuckle when the UK plant health authorities announce the finding of a ‘new’ insect pest. It appears quite all right that the beastie won’t bite or sting you or your mutt, although it could well wipe out one or more commercial tree species.

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