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Tree health in Wales

There are many possible threats to the health of trees in Wales, some of which have the potential to alter our landscapes, change habitats, effect other species and impact on the economy

What are the threats to tree health?

Both native and introduced trees in Wales are potentially susceptible to a wide range of pests and pathogens, as well as abiotic sources of damage such as extreme weather and pollution.

Trees can support populations of many insects and fungi without suffering serious damage. From time to time population explosions, unusual weather conditions or the introduction of a new pest can lead to trees being badly damaged or even killed. Outbreaks of pests and diseases which affect large numbers of trees can have significant effects on our landscapes, natural habitats and economy.

There has been an increase in the occurrence of new pests and diseases in recent years, which appears to be linked to the increase in global trade and travel and the associated movement of plants, wood products, wood packaging and other potentially infected or infested material.

Climate change may alter the area over which conditions are suitable for particular pests and diseases. For example, the pine processionary moth (Thaumetopoea pityocampa), a Mediterranean species, is thought to be moving northwards with climate change. Extreme weather events may potentially change the behaviour of native and introduced pests and diseases in unpredictable ways and climatic stresses such as drought may leave trees more vulnerable to attack.

We have statutory functions relating to tree health, but there are steps everyone in Wales can take to help protect our trees:

  • Be aware of the current and potential threats to trees
  • Know how to identify pests and diseases and how to report them
  • Take simple precautions when visiting the countryside to avoid spreading pests and diseases

Latest news

Current threats in Wales

In the last few years, the outbreaks of P. ramorum (Phytophthora ramorum)and Chalara dieback of ash (Chalara fraxinea) have raised the profile of tree diseases in the UK. Acute oak decline has also been confirmed on oaks in Wales.

While the impacts of Chalara have so far been felt more in England and Scotland than in Wales, its effects here are likely to increase in the coming years. In contrast, Wales has suffered far more from the impacts of ramorum disease which has forced the clearance of very large areas of larch, especially in the South Wales valleys.

We are learning to cope with established pests such as the Great spruce bark beetle (Dendroctonus micans), Green spruce aphid (Elatobium abietinum), Dothistroma and Hylobius.

Potential threats in UK

Some pests and diseases already present or intercepted in other parts of the UK could pose a threat to trees in Wales if allowed to spread.

Oak processionary moth (Thaumetopoea processionea) is currently present in and around London where it has economic impacts because of its effects on street trees but also important human health impacts because of the irritating hairs produced by its caterpillars.  

Chestnut blight has recently been found on sweet chestnuts in England. Although there are relatively few sweet chestnut trees in Wales, they could be threatened if chestnut blight (Cryphonectria parasitica) were to spread here.

Asian longhorn beetle and Oriental Chestnut Gall Wasp have been found in trees in Kent in separate locations, but action was taken to eradicate the pest. There is monitoring in place to determine the success of the eradication program.

Potential Threats from Europe

The following make up a number of pests that are moving closer across Europe towards Britain. These potential threats could easily arrive in Britain via transport links, packaging material and include the following:

For any tree health concerns that are not covered on our web pages please contact the NRW tree health team at

Related document downloads

Biosecurity FAQ PDF [229.0 KB]

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