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Cities are ideal jumping-off points for many pests and diseases. Invasive pests have the potential to devastate amenity planting and to threaten our natural ecosystems. The effects of climate change will be felt in cities first, resulting in an environment suited to some exotic pests. Climate change is likely to cause further stress in some of the traditional planting stock, making it yet more vulnerable to pests and diseases.


As the amount of greenspace increases within our cities, we will increasingly rely on importation of plants. More and more, these plants are being imported as mature specimens rooted in soil or potting medium. Greening projects also bring increased movements of humans and machines across the landscape, and ultimately increased connectivity between green spaces. This combination of exotic planting stock and increased movement brings a significant risk of importing and disseminating new pest species (insects and pathogens). Add to this the potential for hybridisation within this new ‘melting pot’ of exotic pests, and we might be creating serious problems for ourselves.

Plant health legislation is in place to control organisms/quarantine pests of economic or environmental importance. Even with this legislation, it is important for growers to be vigilant, to monitor the establishment of their new stock and to contact experts when necessary.

Practical considerations

Increasing levels of urban greening and developing habitat networks is a key mechanism for reversing the effects of fragmentation on biodiversity, and central to delivering a range of other social and environmental benefits. However, this is not only beneficial to the movement of humans in and around the urban landscape – it also offers pests the chance to spread more rapidly. We are already facing major problems. In recent years, we have seen the introduction of a number of pests and diseases, for example:

  • Box blight (Cylindrocladium buxicola)
  • Holly blight (Phytophthora ilicis)
  • Ramorum dieback (Phytophthora ramorum) which affects rhododendrons and viburnums,
  • Oak processionary moth (Thaumetopoea processionea)
  • Horse chestnut leaf miner (Cameraria ohridella)
  • Gypsy moth (Lymantria dispar).

All of these will have been brought in on infected planting stock or via passive transport on vehicles from the continent.

Climate change

The onset of climate change can cause disruption from pests and diseases due to:

  • Physiological changes in the host plant
  • Changes to development and survival of pests and diseases
  • Impacts on natural competitors and vectors
  • Increasing the suitability of the climate for non-native pests and diseases.

Even moderate changes to climate could have a significant and rapid impact on the distribution and abundance of many pests and diseases (Tubby and Webber, 2010).

Current action

Global systems

There are global systems in place that attempt to prevent such introductions. These include:

There are two European and Mediterranean Plant Protection Organization (EPPO) pest lists containing controlled organisms/quarantine pests of economic or environmental importance. The decision on whether a pest should be controlled by quarantine regulations is taken on the basis of a pest risk analysis.

  • A1 list covers quarantine organisms not currently found within the European and Mediterranean Plant Protection Organization (EPPO) region
  • A2 list covers organisms present in some parts of the EPPO region.

All EPPO member governments are urged to take measures against A1 pests whether they are of direct concern, while there is more freedom whether to take action against A2 pests.

Plant Health and Seeds Inspectorate (PHSI)

The PHSI, governed by Fera, executes plant health policy in England and Wales. PHSI inspectors carry out import, export, monitoring and survey inspections, issue phytosanitary certificates, and oversee import controls, plant passport arrangements and eradication campaigns.

Even with this legislation, it is important for growers to be vigilant, to monitor the establishment of their new stock and to contact experts when necessary, as even trained PHSI inspectors have limited ability to pick up potential problems. Only a proportion of the stock is checked, and the system is very list-dependent.

Other issues and problems

Fungistatic compounds are used in many nurseries to suppress, but not to cure, fungal diseases. Therefore as soon as treatment ceases, the apparently uninfected stock becomes diseased. In addition, some diseases make their way in on ‘carriers’ – plants that are unaffected by the pathogen, but that can transmit infection to vulnerable planting stock.

Case studies

Oak processionary moth (Thaumetopoea processionea)

Oak processionary moth is a major defoliator of oak in Europe. The larvae feed on the foliage of many species of oak, including English, sessile and Turkey oaks (Quercus robur, Q. petraea and Q. cerris). Indications from attacks in Kew Gardens are that it will attack many species of oak, grown both for timber and for amenity purposes. It has been recorded on the Continent on hornbeam, hazel, beech, sweet chestnut and birch, although mainly when growing next to severely defoliated oaks. Other species may be vulnerable, so it is important to monitor the situation and collaborate with other European countries.

Outbreak in London, 2006

In 2006, oak processionary moth caterpillars were found in several locations in the London Boroughs of Ealing and Richmond. It is not clear how the pest was introduced, although it is likely that overwintering eggs were brought in with living trees imported for new planting schemes.


Forest Research, Defra, the Health Protection Agency, and Ealing and Richmond Councils were brought together to form an Outbreak Management Team. This team developed a Contingency Plan (PDF-188K) and an Action Plan (PDF-41K) for outbreaks of the Oak processionary moth.


A strong science base is our first line of defence, and is critical to successful plant health outcomes. Forest Research conducts intuitive and strategic research to inform policy and to respond quickly and effectively to new biosecurity threats. Further information can be obtained from Forest Research tree pest and disease alerts and advisory notes.

Diagnostic methodology and behavioural studies

Precise diagnosis of the causal agent is key to managing an emerging problem. We undertake traditional as well as molecular methods of pest diagnosis. This has been particularly important in distinguishing the various species of Phytophthora affecting our amenity plantings. We have a dedicated Tree Health Diagnosis and Advisory Service, which offers verbal and written advice and site visits.

Innovative methods of pest control

Forest Research is a Chemical Regulations Safety Directorate-approved centre for the testing of plant protection products, and has considerable expertise in setting up and evaluating trials of new and existing products. In addition to our familiarity with more traditional chemical products, we are also committed to the reduction of chemicals where possible and have a great interest in biological control.

Further information

Defra (2005) Plant Health Strategy for England. Department for Environment, Food and Rural Affairs, London.

Evans, H. and Webber, J. (2003). Pests and diseases (PDF-139K). Forest Research annual report and accounts 2002-2003, Farnham.

Import regime for plant health controlled material (Fera)

Tubby, K. and Webber, J. (2010). Pests and diseases threatening urban trees under a changing climate (PDF-223K). Forest Research, Farnham.

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