Grey squirrels (Sciurus carolinensis) were introduced to Britain from North America in the nineteenth century.
- Damage and impact
- Susceptible tree species
- Government policy and action
- Further information
They are now widely established and many people enjoy their presence, especially in urban parks and gardens.
However, grey squirrels pose a serious threat to our native red squirrels and cause significant damage to trees.
There are estimated to be only 15,000 red squirrels left but over 2 million greys. Greys have been largely responsible for this decline in red squirrels by spreading the deadly squirrel pox virus.
Native red squirrel - a protected species
Threat to trees - grey squirrels also strip bark from trees, causing between £6 and £10million of damage to broadleaved and coniferous woodlands per year in Britain.
This damage is a disincentive to tree planting and management and has a negative impact on biodiversity and on the overall economic value of woodlands. It also limits diversity of woodland planting so reducing potential resilience to pests, disease and climate change.
Bark stripping damage usually starts at the end of April and continues until the end of July (early September in high-risk years). Stripping might occur anywhere on the main stem and branches.
Vigorously growing and dominant trees are generally most affected. Damage levels vary between years and across sites within the same year. Trees younger than 10 years are not normally damaged because their stem and branches are too small (less than 50 mm diameter) to support a grey squirrel.
Bark on the main stem of trees older than 40 years is normally too thick to strip, but grey squirrels will strip the thinner bark on the larger branches in the crown.
Planted or naturally regenerated trees aged between 10 and 40 years, especially sycamore, beech, oak, sweet chestnut, pine, larch and Norway spruce, are most vulnerable to damage.
However, other species (especially broadleaves) and age classes may be damaged, particularly in high-risk years.
Efforts to control grey squirrels in England’s woodlands need to be effective, well coordinated and sustained to both protect and expand our vulnerable red squirrel population and to improve the environmental and economic value of our woodlands.
A review of government policy was carried out in 2013 and an updated national policy and action plan was published in December 2014
Controlling Grey Squirrel Damage to Woodlands provides full details of grey squirrel management strategy an methods.
Data gathered from surveys can be used to monitor how threatened populations of red squirrels are responding to conservation management or to environmental change, and they can also be used to assess the effectiveness of grey squirrel control measures.
Practical Techniques for Monitoring and Surveying Squirrels describes how to plan a survey and gives guidance on which method(s) to use.
More information on the science, current research, and control techniques of grey squirrels is on the Forest Research website.