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Tree Wardens

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Tree Wardens

Tree health news and biosecurity action from across the sector

The Tree Council

February 26, 2020

oak processionary moth

An update for Tree Wardens from Jon Stokes, Director of Trees, Science & Research

UK tree health continues to be a rapidly evolving space. On 25 October, the House of Commons Environmental Audit Committee called for a ‘citizens’ army’ to tackle the growing threat from invasive species, estimated to cost Britain’s economy £1.8 billion a year.The Committee called for trained volunteers to help identify and respond to biosecurity outbreaks, modelled on a system developed in New Zealand.  The report called on the Government to:

  • Train a ‘biosecurity citizens’ army’ of 1.3 million volunteers to identify and respond to outbreaks of invasive species
  • Establish a dedicated border force by 2020 to improve biosecurity at UK borders
  • Set up a rapid response emergency fund to enable agencies to tackle a threat before it becomes out of control

The Tree Council will be talking to Defra about the central role Tree Wardens could have in this ‘citizens army’ over the coming months. 

Ash dieback

The ash dieback challenge continues to evolve, and to help tree owners manage the disease, the Forestry Commission has put together some updated advice for owners of ash trees in woodlands in its leaflet, Managing ash dieback in England. The Royal Forestry Society and the Forestry Commission have also published case studies of experiences dealing with ash dieback from ten sites. Case studies range from woodlands within historic landscapes, to owners coming together to look at roadside trees. 

The Tree Council is producing new guidance for managing ash dieback in non-woodland places like gardens parks and roadsides, which will now be published after Christmas.

Resistance to ash dieback

A paper published in the journal Nature Ecology and Evolution shows that a small number of trees are showing some natural resistance to ash dieback. Professor Richard Buggs from Kew said “we’ve discovered about 3,000 locations in the DNA of these ash trees that are contributing to the resistance and we hope to bring together all of the genetic differences that are contributing to resistance into a single population of ash trees that will have higher resistance than any of the ash trees that we currently have.”

Unfortunately he added that “this will not save the trees that are currently dying but it could mean they could eventually be replaced and ash could live on in the countryside”.

Emerald ash borer

Another significant non-native pest threat is the emerald ash borer (EAB), which has been confirmed to have spread from Russia to Ukraine on its journey westwards through Europe. In September, official surveys were conducted in a forest area near the village of Markivka (Luhansk region of Ukraine) where insect larvae and damaged ash trees had previously been reported by scientists. During the survey, 50 ash trees were found to be infested. To prevent the possible spread of the pest outside the quarantine zone, all ash trees within a radius of 100 metres around each infested tree were destroyed.

In another paper published in November, a study found that the frequency with which larvae of the EAB developed to later stages in European ash was much lower than in the highly-susceptible black ash. It was found that European ash had similar resistance to that of Manchurian ash which co-exists with the beetle in East Asia.

However one of the authors of the study, Professor James Brown of the John Innes Centre, said, “if the beetle were to arrive in UK it would encounter an ash population weakened by exposure to ash dieback. The combined effect may prove highly destructive initially to woodland and urban plantings.”

He also said, “the implication of our study is that the emerald ash borer must be kept out of the UK for as long as possible particularly by restricting imports of ash wood, both timber and firewood, from areas affected by EAB or neighbouring areas.” 

An oak processionary moth outbreak

The Forestry Commission’s November Tree Health update outlines how Defra launched a coordinated response to the Oak Processionary Moth (OPM) outbreaks this summer. The OPM caterpillar was found by a Government plant health inspector while carrying out routine targeted inspections of oak trees. They discovered recently planted trees imported from the Netherlands were infested with oak processionary moth (OPM) caterpillars.

As a result of this finding the Plant Health Service looked at oak tree imports into the country and have since traced over 2,000 consignments of oak from the Netherlands, Germany and Belgium.

“It soon became apparent that there had been exceptional expansion of the OPM population in Europe, due to the hot weather experienced last year. Thanks to a swift and coordinated response, oak processionary moth was intercepted at over 70 planting sites in the UK Protected Zone. All infested trees and material were rapidly destroyed to eradicate the pest”.

Dr Anna Brown, Head of Tree Health & Contingency Planning, Forestry Commission

If any Tree Wardens know of any large oak trees planted in the UK in the last few years, please go and inspect the trees. If you suspect OPM you should not attempt to destroy or move infested material yourself, as the nests and caterpillars can pose some risks to human health. You can report tree pests and diseases online using TreeAlert or by contacting your local plant health inspector.

For more on information symptoms, life cycles and import restrictions, visit www.forestresearch.gov.uk/opm.


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