In the last two decades, Great Britain has battled a series of invasions from alien tree pests – with mixed results. Aggravating the situation is the talent these invaders seem to have for landing in the most ideal spots for their own establishment and spread.

FOLLOWING on from the dawn of the new millennium, the United Kingdom began to suffer invasion by a succession of alien-invasive, economically damaging insect tree pests, with more arriving over the last 20 years than did in the preceding 200. Stranger still is their apparent uncanny ability (almost an insect ‘sixth sense’) to land and establish in or nearby some of the highest-profile sites in the country, turning up in places offering an abundance of potential host trees and conditions conducive to growth, development, reproduction and spread.

HORSE CHESTNUT LEAF MINER

Forestry Journal: Close-up on the larval mines of the horse chestnut leaf miner moth.Close-up on the larval mines of the horse chestnut leaf miner moth.

The first to appear on the radar is a classic case in point. Entering the United Kingdom as the pupal stage inside dead horse chestnut leaves attached to motor vehicles arriving from continental Europe, horse horse chestnut leaf miner (Cameraria ohridella) was subsequently dropped off on Wimbledon Common in south-west London. Not only did it take up residence in the south of England, where the more benign climate allows the insect pest to complete more generations per spring/summer than is possible further north, but it started life in the home of the world-famous Wombles. 

The first report of horse chestnut leaf miner damage was made in 2002 by a lady living on Wimbledon Common, left dismayed when her treasured white-flowering horse chestnut tree had an early autumn in late summer. Forest Research duly arrived, confirming the foliar browning and dry leaf necrosis (resembling the tree’s autumn attire) was caused by mining of the leaves by the larvae of a lepidopteran insect (a tiny moth only 4–5 mm in length) commonly called the horse chestnut leaf miner.

Ironically, it was the case of one alien attacking another. According to purist parlance, white-flowering horse chestnut is an alien exotic species brought to these shores in the mid-16th century from Turkey. The natural distribution of white-flowering horse chestnut stretches from the Balkans, through Eurasia and right into the Himalayan region of the Indian subcontinent. Non-native it may be, but white-flowering horse chestnut is domiciled and naturalised in the UK. However, I have only seen the odd mature tree in woodland. The tree’s white, soft wood was never of interest to the forester and generally failed the test as sawlogs. It proved ideal for milkmaid’s buckets and provided the slats for shelves in Kentish apple-and-pear stores, but this limited use by carvers was about all it was ever put to.

However, white-flowering horse chestnut found fame and fortune in the landscape and amenity sector and was soon prized for a well-shaped, imposing canopy and beautiful candelabra-like blossoms. White-flowering horse chestnut soon became first choice for avenues, groves and single trees on estates, in parks and on common land.

South-west London, where Wimbledon is located, has a high concentration of parks and other open spaces, studded with white-flowering horse chestnut trees. Putney Heath, Richmond Park, Kew Gardens, Barnes Common, Bushey Park, Syon Park and Hampton Court are among the most high profile, and all with large numbers of white-flowering horse chestnut trees of all shapes, sizes and ages.

Forestry Journal:  Loading up debris for incineration at Paddock Wood in 2012. Loading up debris for incineration at Paddock Wood in 2012.

The only brake on pest spread northwards would be much shorter and cooler summer seasons, restricting the number of insect pest generations required for the insect to become invasive as it essentially has in southern England. The latest map provided by Forest Research is for 2014, showing horse chestnut leaf miner to have saturated most of England except the Devon/Cornwall peninsula, struggling to colonise Wales and virtually absent north of a line from north Lancashire to north Yorkshire. However, that was eight years ago and the pest may well have colonised more of the country since then. 

As far as southern England is concerned, white-flowering horse chestnut trees appear to be measurably less affected than they were some 10 to 15 years ago. Indications are that local authorities and other stake-holders have learned a valuable lesson – that you can lessen (though not eradicate) horse chestnut leaf miner by the simple act of sweeping up and destroying the leaves in autumn.

OAK PROCESSIONARY MOTH

Forestry Journal: Adult oak processionary moth larvae, which strip oak trees and cause very unpleasant symptoms for anyone who comes into contact with their stinging hairs.Adult oak processionary moth larvae, which strip oak trees and cause very unpleasant symptoms for anyone who comes into contact with their stinging hairs.

Next to arrive was oak processionary moth (Thaumetopoea processionea), almost certainly as the insect’s egg stage on oak trees imported from Europe and planted during autumn/winter 2005. Unlike the entry of horse chestnut leaf miner, which was essentially unavoidable unless all vehicular traffic from Europe had been banned, the entry of OPM was entirely preventable by proper biosecurity involving inspection, quarantine and import bans from high-risk areas and countries as and where appropriate. 

The pest was discovered in 2006 on cypress oaks (Quercus robur fastigiata Koster), planted the previous year at amenity sites in the London Boroughs of Richmond and Ealing after importation from mainland Europe. The columnar canopy of cypress oaks is clearly an advantage for urban areas, where aerial space is at a premium and where other oaks would inevitably outgrow their welcome. 

The OPM pests planted along with their trees clearly fell on their feet, because the western half of London provides a rich tapestry of open spaces supporting thousands of ancient oak trees, including some of England’s most high-profile sites. The Royal Botanic Gardens at Kew was one of the first sites to register OPM infestation, so soon after the first finding that some people still believe (albeit erroneously) that the original infested oak trees were planted there. 

However, Kew was not a million miles away from the landscape site where the trees took root and the strong-flying OPM adults soon found their way onto its oak trees and other sites in the London Boroughs of Richmond and Hounslow. 

With hindsight, measures taken to eradicate OPM were entirely inadequate. 2008 appears to be the year it got away, and by 2011 the Forestry Commission had abandoned its initial aim of eradication and settled for one of containment. Spread continued on all sides, with more and more oak trees in London’s prime parks succumbing to OPM infestation.

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OPM eventually disrupted the Chelsea Flower Show, held annually in the grounds of the Royal Chelsea Hospital, causing oak trees to be banned from the 2019 Show. Despite warnings from the authorities, OPM-infested oak trees imported from Europe were planted at the Olympic Park. 

Sixteen years on from the first finding of OPM in 2006, breeding populations are now well established in all 32 London boroughs and within all the Home Counties (at least) – Surrey, Berkshire, Buckinghamshire, Hertfordshire, Essex and Kent. 

Despite the obvious success of this invasive pest in colonising virtually the entire land area of south-east England, it appears to have happened relatively slowly – especially when you compare it with how quickly the entire land area of the Netherlands was invaded and colonised. 

Perhaps we should count ourselves lucky, because the establishment of OPM within an urban area almost certainly allowed for more prompt identification of the pest. It also gave the FC a better chance of containing the pest. Imagine if OPM had first put down roots in a very rural area. Not only would it not have been spotted for a much longer time, but the network of woodland and hedgerow oaks would have allowed for a much greater and quicker spread. By now, the whole of England and Wales could have been living with OPM.

ASIAN LONGHORN BEETLE

Following a welcome six-year pause, Asian longhorn beetle (Anoplophora glabripennis), indisputably one of the world’s worst insect pests of trees, turned up in Kent at a place with the unlikely name of Paddock Wood. The Asian longhorn beetle had arrived in wooden crates assembled in China, used to ship ornamental stone slabs to the UK. 

Forestry Journal: The ultimate fate of white-flowering horse chestnut affected by the horse chestnut leaf miner – a dry necrosis with leaves rolled up to look like brandy snaps.The ultimate fate of white-flowering horse chestnut affected by the horse chestnut leaf miner – a dry necrosis with leaves rolled up to look like brandy snaps.

However, this pest would have had the potential to cause huge damage to standing trees and timber wherever it turned up in the UK. This is firstly because Asian longhorn beetle has such a huge recorded tree host range and secondly because its natural Asian distribution in China indicates ability for activity across a wide climatic spectrum. 

Scientists in North America, where Asian longhorn beetle appeared as an alien and potentially invasive insect pest five times between 1996 and 2008, calculated susceptible trees in North American climatic zones from southern Mexico to southern Canada were at risk. 

Forestry Journal: Samples from trees infested with Asian longhorn beetle are bagged up for later identification.Samples from trees infested with Asian longhorn beetle are bagged up for later identification.

Primary hosts within its native Asian range include poplars, willows, elms, maples, limes, birches and horse chestnuts. Outside of its Asian range (i.e. USA, Canada and Europe) you need to add London plane, mountain ash, hornbeam, beech and plum (Prunus) to the recorded host range of Asian longhorn beetle. For most insect pests it is usual to list the tree species which are attacked, whereas for Asian longhorn beetle it is far easier and more convenient to list those which are not.

The Paddock Wood outbreak of Asian longhorn beetle was detected in March 2012, with UK plant health authorities conducting a ruthless campaign aimed at pest eradication extending through summer 2012. During this time, contractors, backed up by the police where necessary, removed and incinerated all susceptible standing host trees, together with fences and garden sheds made of susceptible wood. Ironically, householders who had planted conifers and other exotics like eucalyptus (often to the annoyance of native-tree nutters) were spared the pain of seeing their gardens reduced to something resembling a moonscape. 

Even nesting birds were not spared on this occasion, with everything coming down and going into the incinerator. However, it was all deemed worthwhile as Asian longhorn beetle was eventually eradicated, though it took almost 10 years for the authorities to be sufficiently sure in announcing the all-clear. 

ORIENTAL CHESTNUT GALL WASP

Forestry Journal: A delicate-looking adult insect it may be, but the oriental chestnut gall wasp is described as the world’s worst insect pest of chestnut species. In the UK, it was first found in Kent, the heart of sweet chestnut country (picture courtesy of Gyorgy Csoka). A delicate-looking adult insect it may be, but the oriental chestnut gall wasp is described as the world’s worst insect pest of chestnut species. In the UK, it was first found in Kent, the heart of sweet chestnut country (picture courtesy of Gyorgy Csoka).

Three years on and another ‘beast form the East’ was found in Kent – the ideal place for oriental chestnut gall wasp (Dryocosmus kuriphilus), since around 90 per cent of the Castanea (sweet chestnut) growing as coppice and/or standards in the UK is located in Kent and East Sussex. Originally distributed in China, the insect pest has subsequently spread out into neighbouring Asian countries, including Japan and South Korea, and much farther afield into Europe and the United States. CABI now describes oriental chestnut gall wasp as the ‘worst insect pest of Castanea worldwide’.

The Kent infestation almost certainly arrived from Europe, though why it should choose 2015 to emigrate, given it has been embedded as an exotic pest of Castanea sativa (sweet chestnut) for some years – in Italy since 2002 – remains a mystery.

The full force of the UK’s pest-eradication capability descended on Farningham Wood near Swanley in north Kent, where the insect pest was first found. Whether the eradication work was successful is unclear, although in the event it would have made no difference, because by 2016 the pest was being reported all over the London area and at too many points for it to have spread so quickly from north Kent. 

UK plant-health authorities have never said exactly how the insect pest got into the UK, but given the location and the pattern of subsequent outbreaks in the years following 2015, it would appear the import of fruits from Europe, and their subsequent re-distribution in UK, is as likely an explanation as any.

Kent was once described as the ‘Garden of England’ because it boasted the biggest concentration of fruit orchards in the country. Though still an important fruit-growing area, Kent is also a hub for fruit imports from Europe and hosts many fruit-packing plants from where imported fruit consignments are re-distributed by road across the UK.

Indeed, there is at least one of these plants near to Farningham Wood in north Kent, while a map illustrating the subsequent flush and rush of outbreaks reported in 2016 and the years thereafter showed a distinct concentration of outbreaks along several trunk roads going into and out of London.

IPS TYPOGRAPHUS

Forestry Journal:

Ips typographus popping up in 2018 in Kent, with further outbreaks in Kent and East Sussex last year, has proved to be an exception to the rule – and fortunately so. Its primary host is Norway spruce, but by and large the only trees it can seek out in this neck of the woods are small pockets left over as nurse trees for oak plantings or redundant Christmas tree plantings. 

READ MORE: Ips typographus: Further finds of spruce bark beetle in England

DEFRA and the FC are increasingly adamant these 2021 outbreaks are the result of adult beetles flying in from continental Europe, though why an insect pest native to Europe which has attacked Norway spruce (also native to continental Europe) with gusto from time immemorial should wait until 2021 to fly to the UK raises some questions. Be that as it may, if Forest Research scientists can prove this theory then potential fame and fortune awaits, because after centuries of research across Europe the established view on the movement of Ips typographus is as follows: Adult Ips typographus beetles can fly up to 4 km in search of suitable host material.

They can also be transported by abiotic factors such as wind. Dispersal over longer distances depends on transportation under the bark of logs. It is one of the most commonly detected pests travelling on solid wood packaging.