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.

Occurrence of new pests and diseases

There has been an increase in the occurrence of new pests and diseases in recent years. This 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. Climatic stresses such as drought may leave trees more vulnerable to attack.

Pests and diseases currently in Wales

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

The impacts of ramorum disease 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 the 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.

Find out more about potential threats to Wales in tree pests and diseases on

Plants, seeds and fruit from outside the UK

Plants, seeds, fruit, flowers or vegetables you find overseas should not be brought back to the UK as they can carry pests and diseases that would destroy UK plants, trees and crops.

Read more about the risks of bring plant pests and diseases into the UK on

How we all can protect our trees

There are steps everyone in Wales can take to help protect our trees:

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