OPAL Tree Health Survey – The first two years

Why study tree pests and diseases?

Two years ago, trees and their pests and diseases were in the news. Chalara dieback of ash had been found in the UK in the previous autumn (2012), an event of such magnitude that the Government even convened meetings of its Cabinet Office Briefing Rooms (COBR) committee, which is reserved for instances of international, national or regional crises. Never before had it met for a tree disease.

One of the top priorities identified was to better engage people with trees and tree health. The public reaction to Chalara dieback showed the people had a passion for trees and woodlands. Yet it was recognised that more needed to be done to get people involved and supporting official surveillance by keeping a look out for new pests and diseases.

Fortunately, for several months OPAL and a range of interested parties, including Government, its agencies, universities and conservation charities, had been working on a citizen science tree health survey. This innovative survey was launched in the following May (2013).

As well as engaging people with tree health, the survey aims to provide data to help scientists monitor the latest distribution of certain pests and diseases of Oak, Ash and Horse Chestnut (Box 1).

In addition, participants were asked to keep a look out for the “Six Most Unwanted”; six of the most serious pest and disease threats to our trees and woodlands (Chalara dieback of Ash, Emerald Ash borer, Asian longhorn beetle, Citrus longhorn beetle, Oak Processionary moth and Pine Processionary moth).

So what have we learnt in the first two years?

The results require more detailed analysis but a few key outcomes are:

Outreach - Engagement/awareness raising

  • 1,671 trees from 1103 sites were surveyed across England, Scotland and Wales. 
  • 452 educational establishments from primary schools to universities participated.
  • 653 people participated with friends, families or as adult volunteers.

Outreach – Learning

  • 77.0% participants had no previous experience of working with trees.
  • 92.4 % participants felt they learnt something new.
  • 86.7% participants felt they developed new skills.

Outreach - Stewardship/behaviour change

  • 63.7% people felt that taking part in the survey had changed the way they think about the environment.
  • 59.3% of participants said their behaviour towards the environment would change as a result of taking part in the survey.
  • 79.8% people said they will sign-up to the next survey.

Research - Trees

In total nearly 40 different tree species were surveyed, of which 29.1% were Oak, 23.5 % were Ash and 13.6% were Horse Chestnut. Most interesting perhaps was that nearly 2% trees surveyed were Elm – yes, they are still present in the landscape despite the worst ravages of Dutch Elm Disease.

Pests and diseases - incidence

The incidences of the various pests and diseases in Tree Health Surveys conducted between May 2013 and mid-November 2014 are presented in Chart 1. Horse Chestnuts were the most ‘unhealthy’ tree as 73% were affected by one or more pest or disease. Over 40% trees were affected by leaf mining moth and leaf blotch. Perhaps of most interest was the incidence of bleeding canker which affected 25% of the trees surveyed (Chart 1). A previous study by Forest Research in 2007 recorded that 49% of trees were affected by bleeding canker. Further surveillance in 2015 and 2016 will help to determine if this difference is real and then it will be up to the scientists to determine the reason for the decline. The incidences of pests and diseases on Ash (22% were affected by one or more pest or disease) and Oak (20% were affected by one or more pest or disease) were much lower than Horse Chestnut (Chart 1).

Pests and diseases - distribution

The distributions of the various pests and diseases from surveys conducted between May 2013 and mid-November 2014 are presented in Maps 1, 2 and 3. It is early days and more observations are needed but the most striking observation is that most pests and diseases appear to be largely restricted to England. This may simply reflect the increased survey activity in England but it does emphasise the importance of people in Scotland, Wales and Northern Ireland getting out and having a look at their trees. Of particular interest is the distribution of the Horse Chestnut leaf miner. Having been first recorded in Britain in the London area in 2002, it has moved north and west at a steady rate. The finding in Scotland is important. Not only does it help to confirm results of other studies by Forest Research and Conker Tree Science, it again emphasises the need for further data collection in Scotland, Wales and Northern Ireland.

(To view maps 1, 2 and 3 in full-size, please click on the images below)









Six Most Unwanted

There have been 30 suspect sightings of the “Six Most Unwanted”: 22 of Chalara Ash Dieback; 3 of Emerald Ash Borer; 3 of Oak Processionary moth and 2 of Citrus Longhorn Beetle. Fortunately further investigation by experts at Forest Research confirmed that these reports were either ‘false alarms’ or, all except one, were from known infected areas.

For example, in one case, a participant found an iridescent green beetle on an ash tree. Rightly concerned that it may be an Emerald Ash Borer, she posted the beetle to us. Unfortunately, when it arrived, all that remained were fragments of the beetle. However, the clever scientists at the Food and Environment Research Agency were able to identify it as a green dock beetle and not Emerald Ash Borer (Image 1). The one exception to the false alarm was a first finding of Chalara dieback in a 10km grid square that was previously considered by officials to be free from the disease. This sighting of one of the Six Most Unwanted was reported on the Forestry Commission’s official recording site Tree Alert, which ensured that the sighting was flagged to officials and verified swiftly.

What next?

Over the last year all OPAL surveys, including the tree health survey, have been revised. From 18 May 2015, they were officially rolled out across the whole of the United Kingdom. This enables people in Scotland, Wales and Northern Ireland to participate fully and provides scientists with a better picture of tree health across the whole country. Please get out and survey trees across Scotland, Northern Ireland, Wales and England (to see how the OPAL Tree Health Survey is carried out, see the video to the right). Help us to answer the questions:

  • How far north and west has the horse chestnut leaf-mining moth spread, especially in Scotland and Northern Ireland?                                                               
  • How far has Chalara ash dieback spread from the East Coast of England and Scotland?

For those people who have been bitten by the tree health bug and who want to get involved in more specialised work, they might consider having a look at a new project Observatree – a tree health monitoring network of specialist volunteers.

Dr David Slawson
OPAL Director


Additional notes

1These figures are based on OPAL survey data collected between May 2013 and mid-November 2014.  The total numbers of Ash, Oak and Horse Chestnut trees surveyed in this period are 392, 487 and 227 respectively.