In the UK, the threat from plant pests and diseases is assessed by pest risk analysis (PRA) and managed through the UK Plant Health Risk Register. Each year about a hundred new pests and pathogens are added and currently there are over 1400 entries. Each organism is allocated a score, with and without mitigation, on a scale from 1-125 dependent on a wide-ranging list of factors that includes: likelihood of entry or establishment, rate of spread, impact (economic, environmental, social) and value of hosts or industries risk The greater the risk the higher the score.
Mitigation encompasses a range of different approaches that include the key approaches of regulation, surveillance, industry scheme, contingency plan, raising awareness and research. Not surprisingly, one of the key areas of uncertainty is the distribution of an organism and hence the importance of surveillance.

Observatree volunteers carry out surveillance for named priority pests and pathogens including organisms such as Oak processionary moth, Asian longhorn beetle, and Ash dieback. At the start of the project in 2013, working with plant health policy advisers and pest risk analysts a list of 20 high-profile pests and diseases were selected. Great care was taken when considering these stakeholder suggestions to ensure that the targets selected were suitable for citizen science surveillance and suitable for Observatree. The list includes a mix of organisms not known to occur in the UK such as Emerald ash borer; but several that had just arrived such as Ash dieback. Including a readily found and identifiable pest or disease gives volunteers an opportunity to practice and develop their skills such as tree identification, geographical location recording, and reporting via Tree Alert.

Every year the Observatree priority list is reviewed due to changes in the UK Risk Register. Although the temptation would be to continuously add several new organisms a year, we are mindful of the volunteers and projects’ capacity to cope with too many organisms (30 priority pest is probably the upper limit). The priority list has now expanded to include emerging threats such as Cedar blight (Sirococcus tsugae), European Mountain Ash Ringspot associated Virus, Elm zigzag sawfly (Aproceros leucopoda), Beech leaf disease and most recently in 2024 Agrilus fleischeri.

Working with colleagues from Defra’s horizon scanning team we have drawn up a list of all the additions in 2023 to the UK risk register and have checked other horizon scanning data sources such as the European Plant Protection Organisation (EPPO) Pest Risk Alert website as well as speaking to a wide range of colleagues across the UK Plant Health service to identify potential targets.

For Observatree, we use set criteria to prioritise this list, this includes the pest or pathogen must be on a tree species (focussed on forestry and woodland species), an unmitigated risk register score of 30 or more, and the target must be suitable for citizen science surveillance. The first criterion can be a little subjective and requires consideration as some pests have a broad host range including amenity and orchard species. The third criterion needs to consider whether the pest is ‘conspicuous’ - can it be seen at ground level and not require any invasive sampling, does it have features that allows for recognition in the field and sufficiently different from common native species, could it be confused with a rare or red listed species?

To this end, during our latest review we identified five potential targets of interest:

Following consultation with colleagues within the Plant health service and tree health citizen science network we identified that of the 5 selected pests, the Chinaberry yellow banded longhorn beetle should be taken further.  This pest dovetails in well with the existing priority pests - Asian and Citrus Longhorn beetles and the red-necked longhorn beetle. We will await the formal risk assessment and UK risk rating and based on this decide whether it is appropriate to add this to our priority pest and disease list as a stand-alone pest.