NATIVE English oaks Quercus robur (pedunculate oak) and Quercus petraea (sessile oak) have been an integral part of England’s natural landscape for millennia and continue to occupy a very special place in the country’s history, culture and folklore.

The Old English phrase ‘Heart of oak’ denotes a person with a strong and courageous nature and is traditionally used to describe a brave and loyal soldier or sailor. Its historical connection is with the heartwood of the English oak tree which is unrivalled for quality, strength and durability amongst British native trees.

The revered name ‘Royal’ oak originates from an ancient hollow oak tree in which Charles Stuart (the future King Charles II) hid from Oliver Cromwell’s Parliamentarians, after Royalist forces were defeated at the Battle of Worcester in September 1651, in the final battle of the English Civil War. That English oak tree was in the grounds of Boscobel House in the county of Shropshire. 

Four centuries later, a highly invasive insect pest called oak processionary moth (Thaumetopoea processionea) is presenting serious and seemingly insurmountable problems related to the integrity of oak trees and safety of the public and pet animals. By 2018, oak processionary moth was present as an active breeding population in every single one of London’s 32 boroughs, the Home Counties (Surrey, Kent, Essex, Hertfordshire and Buckinghamshire) which border the capital city and also Berkshire. There are two particularly large and explosive OPM populations, one in north-west London, centred on Hampstead Heath, and the other in Surrey at the north end of Guildford Borough, close to the Surrey Hills area of outstanding natural beauty.

Forestry Journal: Organisers of the Chelsea Flower Show have no choice but to eradicate OPM from mature trees growing on the site, due to urticating (stinging) hairs borne by later instar larvae and posing potentially serious health risks for the public and pet animals.Organisers of the Chelsea Flower Show have no choice but to eradicate OPM from mature trees growing on the site, due to urticating (stinging) hairs borne by later instar larvae and posing potentially serious health risks for the public and pet animals.

Consequences were there for all to see earlier this year when the Royal Horticultural Society (RHS) banned oak trees from exhibitors’ displays at the world famous Chelsea Flower Show in May 2019. This was entirely due to oak processionary moth (OPM) now firmly embedded in London’s oak tree population and way beyond control. Chelsea Flower Show is a truly regal event held in the grounds of Royal Chelsea Hospital and attended by members of the British royal family, and a most appropriate event at which to show off ‘Royal English’ oak trees, but sadly not this year.

However, it is not clear why a total ban was deemed necessary. Landowners within the OPM core zone, which includes the Royal Chelsea Hospital site, are not legally bound to control OPM under government legislation. But high profile public sites like Royal Chelsea Hospital have no choice but to carry out OPM control and management measures, even though the vast majority of landowners within the OPM core zone do not. Establishment of a core zone with no legally enforced OPM control has essentially given this invasive insect pest a ‘free run’ with strong-flying, egg-laying female moths able to migrate up to 5 km.  The OPM core zone is like a ‘pressure cooker’ with moths migrating out and into the surrounding OPM control zone (a buffer zone around the core zone) to provide lots of lucrative work for specialist contractors who carry out OPM control, but real, practical and often expensive problems for everyone else.

Forestry Journal: King Offa’s Oak in Windsor Great Park (a pedunculate oak – Quercus robur). Real problems will be presented if OPM invades this priceless 1,300-year-old tree.King Offa’s Oak in Windsor Great Park (a pedunculate oak – Quercus robur). Real problems will be presented if OPM invades this priceless 1,300-year-old tree.

With regard to the ban on oak trees, were the show organisers concerned that OPM would migrate from mature oak trees growing on the site and into young oak trees on the stands? Or that infested oak trees brought in from European Union (EU) countries where OPM is endemic could infest mature oak trees growing on site? Neither scenario was at all likely because the month of May falls firmly within the OPM larval period and transfer from tree to tree is highly unlikely unless tree canopies are in contact. 

Egg-infested oak tree planting material imported from the Netherlands in 2005 and subsequently planted in west London was how OPM first arrived and established in England. Oak trees imported from Italy by an exhibitor at the 2016 Chelsea Flower Show were subsequently found to be infested by OPM. Last year, 40 OPM nests were removed from mature oak trees growing on the Royal Chelsea Hospital site prior to the show’s opening on 22 May 2018. RHS said it would carry out thorough checks and take all necessary OPM control and eradication steps before taking up residency of the site for the 2019 show. 

RHS may well have eradicated OPM from the site before the show opened on 21st May 2019, but will be faced with exactly the same problem next year, and every year thereafter, due to adult moths emerging in July/August from nests on unsprayed oak trees located elsewhere in the OPM core zone, and capable of flying up to 5 km, which oviposit eggs on oak trees at the Chelsea Royal Hospital site. Though not legally obliged to do so, RHS clearly had to rid the site of OPM before the show started due to real health risks posed by this pest.

Given that OPM is now beyond control or even containment in London, and the reason for an OPM core zone in which insect management by landowners is voluntary, a total ban on oak trees appears to be a classic case of ‘locking the stable door after the horse has bolted’. Further OPM spread could have conceivably occurred through exhibitors taking oak trees off the site (which is inside the OPM core zone) at the end of the show and from there to places within the OPM protected zone.  However, regulations and restrictions on the movement of oak tree material out of the OPM core zone and into the OPM protected zone are already governed and covered by a raft of UK government legislation, the latest passed in 2018.

Provided these regulations were adhered to, there should have been no reason for imposing a total ban on oak trees at the show. And if the show’s organisers wanted to be absolutely sure they could have ordered and indeed carried out destruction of all oak trees used by exhibitors at the end of the show. This would have safeguarded areas of the country still free from OPM while allowing continued use of England’s ‘Royal oaks’ at the Chelsea Flower Show. 

Forestry Journal: A hollow ancient oak tree is the ideal place to hide, including for a future King of England.A hollow ancient oak tree is the ideal place to hide, including for a future King of England.

Since the arrival of OPM in 2005, and its rapid rise to pest status during 2006, control measures have focused on limiting pest spread while preventing further OPM introductions on oak tree planting material imported from EU countries like the Netherlands, Germany, France and Italy where OPM is endemic. Other concerns relate to increasingly frequent findings of OPM nests close to the ground, which clearly puts people and their pets at even greater risk from the urticating (stinging) hairs borne by the later instar (L4 to L6) larvae. 

However, a major new worry is the possibility, indeed probability, of terminal damage to some of the country’s most valuable ancient oak trees. This is because OPM has been spreading westwards out of London and is now recorded in the Royal Borough of Windsor and Maidenhead in Berkshire and the Runneymede District of Surrey, which means the insect pest is now very close to if not already inside Windsor Great Park. Windsor is a royal park and home to what is almost certainly the single biggest collection of ancient native oak trees in the UK, including ‘King Offa’s Oak’ named after Offa who ruled Mercia, an ancient kingdom in Anglo-Saxon England. The tree is thought to be 1,300 years old, which means it would have germinated from an acorn during the 7th century AD when King Offa reigned over Mercia.

What on earth will the plant health authorities do when priceless ancient oak trees are infested with OPM larvae and proceed to strip the sparse foliage from these ancient and correspondingly ‘weak growth’ trees? Will they attempt to spray the trees with insecticide, because they surely cannot climb the trees to physically remove the OPM nests? Even the use of a cherry picker could cause sufficient soil compaction to kill these delicate trees.  

The worst fears for the ancient oak trees in Windsor Great Park were realised in early June 2019 following reports that OPM had been identified in parts of the 2000+ ha Windsor Estate. Reports appeared in the local press covering those parts of Berkshire, Buckinghamshire and Surrey within or close to Windsor Great Park. Windsor and Eton Express quoted from a statement on the Park’s Facebook page and telling visitors to: Please avoid contact with any caterpillars you may see. The estate is actively managing the situation.

Oak processionary moth appears to be completely classless in pest activity, being happy to chew leaves on a scruffy oak struggling in compacted ground in an east London park or foliage on a 1000-year-old ‘jewel in the crown’ at Windsor in East Berkshire.

[Footnote: There are over 400 public houses in England which carry the name ‘Royal Oak(s)’, while eight Royal Navy ships have been named HMS Royal Oak.]