The oriental chestnut gall wasp, said to be the world’s worst insect pest of the Castanea genus, has spread throughout south-east England in recent years. Initially thought to be the result of long-distance spread, the cause may in fact be the import of fruit from Europe, writes Dr Terry Mabbett.
SWEET chestnut was brought to Britain by the Romans for its high-starch, low-fat nuts. However, events took a different turn and Castanea sativa became a timber tree concentrated in southern England, grown and cut as coppice usually on a 15-year rotation and remaining largely pest and disease free.
In summer 2015 oriental chestnut gall wasp – OCGW – (Dryocosmus kuriphilus), called the world’s worst insect pest of the Castanea genus, was identified by the Forestry Commission (FC) in sweet chestnut woodland (Farningham Woods) near Swanley in north Kent. It was assumed the insect pest had entered the UK on planting material through its usual mode of long-distance spread and a reasonable conclusion given that Farningham Woods is sweet chestnut coppice country.
However, subsequent events suggest OCGW may have come in on road vehicles and spread here in the same way. With hindsight, the finding of OCGW on 30-year trees lining a street in St Albans in Hertfordshire just days after the discovery in Kent should have flagged up doubts about the planting material theory.
By summer 2016, OCGW had been recorded in 42 1-km-grid squares across London and three adjoining Home Counties. The Forestry Commission said: “Now that OCGW has been found in other locations, it is recognised that eradication and containment are no longer achievable, and we should focus on developing a long-term strategy for minimising population, spread and impact.”
In just one year, OCGW had gone from notifiable quarantine insect to established pest.
Winged adult wasps are responsible for limited-distance natural spread but could not account for such an explosive situation, with infestations found up to 80 km from north Kent. What’s more, the FC had hired contractors in summer 2015 to fell trees in the affected area of Kent and destroy sweet chestnut tree material which might be carrying OCGW galls. Later pest surveys found no further infestation.
Further clues about entry of OCGW into the UK may come from Kent’s traditional position as the major fruit-growing region of England. Fruit production is still important for many farms in Kent, alongside the import of fruit from Europe, which is repacked in Kent.
A company involved in managing the initial 2015 outbreak in Kent told Forestry Journal how UK plant authorities think OCGW could have arrived on fruit-carrying vehicles from Europe and was subsequently spread by vehicles carrying repacked fruit for the wholesale and retail trades in the UK. OCGW is present in many European countries including the Netherlands, France, Hungary, Spain, Portugal, Greece and especially Italy, where the pest is widespread and endemic. At least one depot receiving, repacking and redistributing fruit is located very near the initial outbreak in Kent.
This could explain the pattern of OCGW distribution first recorded in south-east England during summer 2016. The most heavily affected areas were north Kent into south-east London and out through south-west London into the adjoining county of Surrey, along discernible linear pathways which could correspond to trunk roads into and out of London. The FC’s most recently published distribution map (1 October 2018) shows infestations recorded within one hundred 1 km grid squares across London and the Home Counties (Forest Research, 2019).
Chalara ash dieback disease (Hymenoscyphus fraxineus) was first identified in the wider UK environment in late summer/autumn 2012. Given Chalara’s capacity for rapid development and spread it quickly became apparent that common ash (Fraxinus excelsior) was essentially finished in the longer term as a hardwood timber tree. The FC promptly recommended sweet chestnut as alternative planting material. Is a rethink required, given that sweet chestnut is now faced with OCGW and Cryphonectria parasitica (chestnut blight) and with Phytophthora ramorum also in the pest and disease mix? The truth is that landowners and foresters are fast running out of unencumbered tree-planting options.
More and more UK landowners are finding OCGW for the first time. A recent find was made by Patrick Mannix, the owner of two adjoining woodlands near Guildford in Surrey. Patrick told Forestry Journal how he first noticed a very small number of galls on the leaves of a tree next to the road in 2018, but only after being alerted by me to the existence of the pest in UK. Having cut mature sweet chestnut coppice on the site after leaf fall for the previous 15 years, he thought he might well never have noticed. Recent visits to three surrounding locations within 5 km showed the same situation, symptoms difficult to find but there.
However, there is no apparent effect so far on his trees’ fruit-bearing capacity. I saw for myself in early October how 2019 had turned out to be a bumper year for sweet chestnuts in these Surrey woodlands. Patrick poses the question: Have we had a long-term benign resident, but now augmented by a more aggressive visiting relative?
References:
Forest Research (2019) Oriental chestnut gall wasp (Dryocosmus kuriphilus). https://www.forestresearch.gov.uk/tools-and-resources/pest-and-disease-resources/oriental-chestnut-gall-wasp-dryocosmus-kuriphilus/
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