Major outbreak of plant pest - Agrilus planipennis

Impact 5
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risk indicator
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ID 57b
Risk theme Human, animal and plant health
Impact & Likelihood
Impact key
5 Catastrophic
4 Significant
3 Moderate
2 Limited
1 Minor
Likelihood key
5 >25%
4 5-25%
3 1-5%
2 0.2-1%
1 <0.2%


Larvae of the emerald ash borer beetle (Agrilus planipennis) bore into the inner bark and outer sapwood of ash trees, weakening the trees and causing them to die. There have been no previous outbreaks in the UK, but as the beetle spreads across Europe, there is an increasing likelihood that it will enter the country. This would result in significant damage to both the environment and economy. Import restrictions have been introduced to mitigate the risk of entry on host plants, host wood, wood chips and bark. The Forestry Commission also carries out annual surveillance for the beetle.


The reasonable worst-case scenario is based on an outbreak in a mature, mixed woodland, which has remained undetected for 5 years. Initial surveillance would show that the beetle has spread beyond a 100x100m area, with the spread having occurred over multiple other sites. The beetle would have been present at these sites for 2 years. Damage could be partially reversed through the replanting of trees, although this is likely to have significant economic costs. The economic cost of the outbreaks could be over a billion pounds in environmental losses from impacts on air quality, biodiversity loss and carbon release (from burning).

Key assumptions

It is assumed that the beetles would remain undetected for a long period of time, meaning that they spread a significant distance and cannot be eradicated. It is also assumed that resources would be drained if multiple outbreaks or outbreaks of other pests occurred simultaneously, with increased levels of impacts being seen.


A high-impact variation involves a beetle found in southern England (with a more favourable, warm climate with ash trees within flight distance), allowing the beetle to spread further and impact a wider area. In comparison, an outbreak of the beetle in Scotland, which has a less favourable climate, will not grow as quickly and would have lower impacts on ash trees.

Response capability requirements

Surveillance to monitor the spread of the beetle, capabilities to diagnose the pest and procedures to report suspected cases would be needed. Additionally, capability to destroy the worst- affected trees would be required, including tree-felling services to remove seriously damaged trees and dispose of them via chipping or incineration. Response capabilities also include research and development to improve detection and management methods, including research to approve biological control agents.


It is likely that the majority of ash trees would be impacted. However, there are some long-term management options that could be introduced to reduce the impact of the beetle, including chemical trunk injections for valuable trees and the release of biological control agents that parasitise the beetle. Where ash trees have been killed, replanting with resistant ash species (if identified) or other tree species would be possible, although regeneration could take many years.