BASIC flaws in the European Union’s plant passporting system have been aired in this journal for 10 years, so it was no surprise to learn that oak processionary moth (Thaumetopoea processionea) is still arriving on oak trees exported from Europe – but the sheer scale of the problem is shocking.

News began to trickle out in June 2019 about interceptions made in Hampshire, Gloucestershire and Warwickshire (Solihull), and later Hartlepool and Cardiff, but the map released by Forest Research on 29 July was a bombshell. It showed over 60 OPM interceptions within the UK protected zone on trees recently imported from Europe with most from the Netherlands. The spread was virtually complete, from Devon to North Yorkshire, Kent to Lancashire, in Wales and as far north as Inverness in Scotland. By 8 August, Forest Research had already announced several other interceptions including two more in Scotland.

Forestry Journal: Hopefully, most nests will have been found and destroyed while still containing late instar-stage larvae (seen here) or pupae, and therefore before the adult moths have emerged, exited the nest and dispersed to lay eggs (photo courtesy of Maydencroft).Hopefully, most nests will have been found and destroyed while still containing late instar-stage larvae (seen here) or pupae, and therefore before the adult moths have emerged, exited the nest and dispersed to lay eggs (photo courtesy of Maydencroft).

The insect pest was found on a range of oak species, the majority on English oak (Quercus robur and Q. petraea) but also red oak (Q. rubra), Turkey oak (Q. cerris) and even Hungarian/Italian oak (Q. frainetto). OPM on Quercus cerris should come as no surprise. OPM and Turkey oak share the same native geographic distribution across southern Europe. When given a choice, OPM appears to prefer this native Mediterranean oak.

This outbreak occurred despite new import restrictions introduced by the UK government in 2018 applying to oak trees (Quercus L) other than Q. suber (cork oak) with a girth of 8 cm or more at a point 1.2 m above the root collar. DEFRA (Department for Environment, Food and Rural Affairs) said these larger trees represented the greatest likelihood of introducing OPM, hence the need for strengthened requirements.

These night-flying moths, presented with a range of oak tree development within plant nurseries, will tend to lay eggs on older and taller trees. Whether they actually target them as oviposition sites or whether moths are more likely to intercept these trees is unclear. Clearly, the broader branches of semi-mature oak trees present more accommodating sites for oviposition. I observed a similar pattern of oviposition by female moths of Helicoverpa armigera (cotton bollworm) in Thailand where the top-most youngest leaves on the tallest and bushiest cotton plants attracted the most insect eggs.

Forestry Journal: OPM is a potentially serious defoliating insect of oak trees, though there is no evidence as yet in the UK of this being fatal on a widespread scale (photo courtesy of Bartlett Tree Experts).OPM is a potentially serious defoliating insect of oak trees, though there is no evidence as yet in the UK of this being fatal on a widespread scale (photo courtesy of Bartlett Tree Experts).

REACTION TO THE NEWS

Dr Anna Brown, head of tree health and contingency planning at the Forestry Commission (FC), said: “Those of us involved in importing or trading plants must maintain our vigilance against exotic pests and diseases such as OPM. There is a lot we can do such as buying British, only buying stock from reputable, responsible suppliers and inspecting imported plants. Inspect, inspect and inspect again – we can’t check imported plants too often for signs of trouble. Don’t presume that because your supplier found no evidence of a pest or disease that you won’t either. You might spot something they have missed.”

What I gleaned from her statement is that European exporters, to put it mildly and politely, are ‘misinterpreting’ the requirements of EU plant passports required for oak trees exported to the UK.

Professor Nicola Spence, DEFRA’s chief plant health officer, said: “The Plant Health Service has received reports of an exceptional expansion of the OPM population in parts of Europe due to the hot weather experienced last year.”

This population explosion may well have flooded European plant nurseries with egg-laying moths, but is all the more reason for Dutch and German exporters to meticulously inspect their trees before they send them here.

Writing in the Daily Telegraph, Professor Spence said her role is “to ensure that the health and safety of our trees and plants are protected”. She goes on to emphasise how “oak trees, in particular, are the country’s most important tree for species biodiversity, supporting over 2,000 species of bird, mammal, fungi and invertebrate – from flying squirrels to moths and woodpeckers”.

DEFRA minister Lord Gardiner was reported to be “furious”. Launching the RHS Hampton Court Palace Garden Festival ‘Year of Green Action’ garden, Gardiner told Horticulture Week: “OPM can only be stopped through national measures, through legislation – that’s how we’d have to address this, by bringing in further controls. It is why we have some of the most extensive protected zones, precisely to contain OPM within London where it unfortunately arrived on an import [in 2005] and we should have dealt with it immediately then. I regret it wasn’t dealt with by the vigour we should have [deployed]. I’ll be looking to see what measures are possible to escalate the fact that no imports of oak trees should be coming in where there’s a risk of OPM.”

It was refreshing to hear a senior official, and a government minister no less, acknowledge that OPM control measures were too little and too late from the very start, and how there should be no imports of oak tree-planting material from areas where there is any risk at all from OPM. A complete ban on oak tree imports from OPM-risk countries is the obvious answer, but while the UK remains a member of the EU, that is not an option.

Forestry Journal: A truly fitting finale in the eradication of exotic insect pests. Micro-burners supplied by Kingwell Holdings seen here, late last year in Kent, burning Norway spruce infested by Ips typographus (photo courtesy of Kingwell Holdings).A truly fitting finale in the eradication of exotic insect pests. Micro-burners supplied by Kingwell Holdings seen here, late last year in Kent, burning Norway spruce infested by Ips typographus (photo courtesy of Kingwell Holdings).

DESTRUCTION BY INCINERATION

The standard procedure for dealing with these new OPM outbreaks is spraying with contact chemical insecticide to rapidly kill any susceptible insects in the nests, followed by the prompt destruction of trees and all insect debris on site.

essentialARB spoke with Justin Kingwell of Kingwell Holdings, a highly specialised forestry and arb contractor well known for its range of incineration equipment, including the company’s ‘micro-burners’ which have been deployed at some of these sites. Justin told how the equipment was hired out to one of the country’s biggest landscape contractors, tasked with clearing these sites of OPM and all infested tree material.

“These were not especially large trees,” he said. “They were recently imported from continental European nurseries as semi-mature trees and were infested with OPM before they arrived in the UK. The trees were subsequently planted at landscape sites across the country.”

Kingwell Holdings’ micro-burners have been used to deal with a number of these new OPM outbreaks, including one at Solihull in Warwickshire and another at Hartlepool in north-east England. If this kind of drastic action had been taken against the oak trees imported from the Netherlands in 2005, planted on amenity sites in the London boroughs of Richmond and Ealing and then found to be infested with OPM larvae in 2006, we almost certainly would not have a huge London-wide problem with OPM today. 

Forestry Journal: Evidence suggests that, when given the choice, OPM prefers Turkey oak (Quercus cerris), seen here re-foliating and flowering in North London in the month of May.Evidence suggests that, when given the choice, OPM prefers Turkey oak (Quercus cerris), seen here re-foliating and flowering in North London in the month of May.

HOW DID THIS HAPPEN?

When questioned by Horticulture Week about how this situation had arisen, DEFRA said: “These trees were imported by a number of different types of businesses. The infested trees were supplied from within the EU, from areas that are not designated as protected zones. Most of the affected consignments were from the Netherlands.”

Most people might reasonably assume existing legislation could have prevented the export of oak trees from areas which are not protected zones within the EU, but apparently not. The 21 August 2018 legislation relating to trees of a specified size was made with conditions as follows:

“The new legislation* prohibits the movement of oak trees, with a girth of 8 cm or more at 1.2 metres above the root collar, into UK’s OPM protected zone unless one of four specific conditions are met.

1. Trees have been grown throughout their life in places of production in countries in which OPM is not known to occur.

2. Trees have been grown throughout their life in a protected zone which is recognised as such for OPM or in an area free from OPM as established by the national plant protection organisation in accordance with ISPM No. 4 (ISPMs are International Standards for Phytosanitary Measures).

3. Trees have been produced in nurseries which, along with their vicinity, have been found free from OPM on the basis of official inspections carried out as close as practically possible to their movement and official surveys of the nurseries and their vicinity have been carried out at appropriate times since the beginning of the last complete cycle of vegetation to detect larvae and other symptoms of OPM.

4. Trees have been grown throughout their life in a site with complete physical protection (e.g. insect-proof greenhouses) against the introduction of OPM and have been inspected at appropriate times and found to be free.

*All oak trees moving into and within the UK’s OPM protected zone must still be accompanied by an EU plant passport, regardless of the size of the consignment, and all landings of oak plants in England still require pre-notification to UK plant health authorities.”

And here perhaps in condition No 3, and specifically in relation to the word “vicinity”, is one root of current problems. “Vicinity” is not a definitive word, but you might reasonably assume it to be no more than 100 m radius around a nursery. Egg-laying female OPM moths in continental Europe have been recorded to fly distances up to 5 km. It follows that if condition No 3 is claimed by the exporter to have been satisfied, then condition No 2 relating to “protected zones” becomes superfluous.

Clearly stung by these latest OPM developments, the minister promptly went some way into carrying out his threat, because on 12 July 2019, DEFRA duly announced further restrictions to the legislation. Oak trees having a girth of 8 cm or more at 1.2 m above the root collar could now only be moved into the UK’s OPM protected zone if they were:

1. From OPM-free countries

2. From designated pest-free areas including protected zones, i.e. an area of the EU declared free of OPM

3. Grown under complete physical protection for their lifetime.

Forestry Journal: The Forestry Commission will have been frantically plotting any potential adult moth spread from OPM nests by deploying pheromone traps (in late summer/early autumn) to catch the male moths. Here, a large bucket-type pheromone trap is hoisted high in an oak tree in a Hertfordshire public park.The Forestry Commission will have been frantically plotting any potential adult moth spread from OPM nests by deploying pheromone traps (in late summer/early autumn) to catch the male moths. Here, a large bucket-type pheromone trap is hoisted high in an oak tree in a Hertfordshire public park.

DEFRA clearly hopes this tightening of the legislation will provide water-tight restrictions and no further OPM introductions. Maybe so, but there still appear to be potential gaps in the system. For instance, a consignment of oak trees sent from Europe to the UK could be exported from a nursery in a protected zone, but there is no way of knowing if the exporting nursery previously purchased the trees from another location in Europe which was not a designated pest-free area.

essentialARB asked a UK nurseryman for his thoughts on such a scenario. “Documentation I receive with oak trees imported from Europe states name and location of the nursery which exported the consignment of trees but no information to indicate whether or not they were previously purchased from elsewhere,” he said.

“And there is a good chance they will have been, because movement of plants under the EU’s free-trade policy is like a game of musical chairs. If APHA (Animal and Plant Health Authority) or the FC wishes to back trace any movement of these trees prior to ownership by the exporter they would have to contact the exporter in Europe.” 

Forestry Journal: Late instar-stage OPM larvae are tough, resilient and bear stinging hairs (photo courtesy Bartlett Tree Experts).Late instar-stage OPM larvae are tough, resilient and bear stinging hairs (photo courtesy Bartlett Tree Experts).

Last but not least are the so-called direct imports over which the UK government essentially has no control. Bona fide UK nurseries have long campaigned against direct import of plants online or straight to a landscaping area. A classic example was direct import of OPM-infested oak trees from Italy by an exhibitor at the 2016 RHS Chelsea Flower Show, which subsequently contributed to RHS banning oak trees from this year’s show. Neither the FC nor DEFRA have indicated whether any of the 60+ new outbreaks have been caused by importation of oak trees from Europe by UK companies directly onto a landscaping site for planting.

Horticulture Week asked DEFRA how it could be certain that many more landscapers, garden designers and estate managers had not imported oaks from the Netherlands and of which APHA has no trace. DEFRA said: “A statutory instrument on oak was introduced in 2013, which means that all imported oak trees should be declared to the plant health service.” In other words, the so-called notification scheme introduced in January 2013.

In 2017, Forestry Journal asked APHA about the proportion of trees it was inspecting under its scheme. APHA said: “Figures will only relate to consignments of trees actually notified to APHA and given the nature of the trade we are not able to put a figure on compliance levels.”

DEFRA continually berates the public about bringing plant material back into the country from Europe, though there is no evidence of this happening on a large scale. By the same token, what’s to stop a European business loading up lorries with a variety of plants in the Netherlands or Belgium, coming in through an east-coast port and driving around the UK as itinerant salespersons selling plants to wholesalers and retailers. This practice does occur and there is apparently nothing the UK plant health authorities can do to counter it.

Forestry Journal: An OPM nest found in August 2019, scheduled for immediate removal. Hopefully, the nest still contained insects at the pupa stage, though by this time the adult moths or at least some of them could have already emerged, exited the nest and dispersed to lay eggs (photo courtesy of Maydencroft).An OPM nest found in August 2019, scheduled for immediate removal. Hopefully, the nest still contained insects at the pupa stage, though by this time the adult moths or at least some of them could have already emerged, exited the nest and dispersed to lay eggs (photo courtesy of Maydencroft).

WHAT’S THE FUTURE FOR OPM IN THE UK?

We will have to wait until April 2020 to discover whether or not OPM has become established in the UK protected zone. The question is how many more outbreaks are out there which have been missed. What’s more, the 60-plus interceptions made in the UK’s protected zone is already an underestimate because others recorded in the control zone do not appear to be included in the total. The OPM control zone was established as a buffer between the protected zone (most of the country) and the core zone, covering the west side of London and north Surrey, where OPM is considered beyond control and pest management measures are no longer required by law. The control zone covers Berkshire, Hertfordshire, Surrey and significant areas of Buckinghamshire, Bedfordshire, Essex, Kent and Sussex.

And how many of the 60-plus outbreaks were intercepted after adult moths had flown the nest? Hopefully, most were identified and destroyed while the nests still contained late instar-stage larvae or pupae, because OPM moths don’t usually start to emerge and fly until the end of July. However, if interceptions continued to be made through August and beyond, then adult moths, or at least some of them, would already have flown the nest and dispersed to lay eggs from which larvae will hatch in spring 2020. 

There is a lesson to be learned from this disaster. When confronted by dangerous pests and diseases with the capacity to degrade and radically alter the UK landscape, then much more than trust is required. It is naïve, to say the least, for the UK plant health authorities to rely solely and absolutely on the accuracy and authenticity of bits of paper (EU plant passports) completed and certified by the exporter.

Footnote:

As of 8th August 2019 affected counties and other authorities in England included Cambridgeshire, County Durham, Devon, Dorset, Essex, Gloucestershire, Greater Manchester, Hampshire, Hertfordshire, Kent, Lancashire, Leicestershire, Lincolnshire, London, Merseyside, Middlesex, Nottinghamshire, Oxfordshire, Southampton, Staffordshire, Suffolk, Surrey, Warwickshire, West Midlands, Wiltshire and Yorkshire. Three cases reported by the Welsh Government were in Glamorgan and Flintshire. The Scottish Government reported five cases in Angus, Edinburgh, Fife, Inverness and Lanarkshire (Forest Research 2019).