A new hope for grey squirrel management - fertility control research update

Field trials of the feeding hopper by APHA


The UK Squirrel Accord (UKSA) partnership is fundraising for research into an oral contraceptive to tackle the negative impacts the invasive non-native grey squirrel causes to native red squirrels and broadleaf trees. As thousands of trees are planted during National Tree Week 2019, we celebrate our success towards developing a new method to help protect those trees from grey squirrel bark stripping damage.


The research into the immunocontraceptive is being carried out by the well-respected Animal and Plant Health Agency (APHA). Researchers are working to turn a proven injectable formulation into an oral contraceptive that will be delivered to grey squirrels via food in a species-specific feeding hopper. Progress at the end of the second of the five-year project is good and hopes are high that this innovative work will provide an effective, less labour intensive, non-lethal method for managing grey squirrels.



Highlights at the end of the second year of research:


  • Significant progress was made in understanding how contraceptives could be effectively deployed in the field. Different densities of hoppers were tested to monitor food (without immunocontraceptive in it) uptake by grey squirrel populations in relatively small woods (6-18 hectares). Results suggest that relatively little effort (four days) is required in summer to ensure the majority of grey squirrels access and consume the food that will ultimately contain the contraceptive.


  • In collaboration with the British Association for Shooting and Conservation (BASC) and the Deer Initiative, a volunteer-led trial was conducted to test food uptake by grey squirrels in a larger wood (30 hectares). Early analysis shows results are very similar to those obtained in summer in the trials in smaller woods. It also demonstrated the feasibility of volunteer groups successfully deploying the hoppers in the field in the future.


  • Physiological indicators of reproduction in grey squirrels were identified. These indicators will be able to show the effectiveness of the contraceptive vaccine.


  • Work is ongoing to optimise the formulation, dose and frequency of dosing required to turn the injectable vaccine into an effective oral contraceptive. These trials are carried out in collaboration with international groups of researchers at the forefront of research and development in this area.


  • Based on camera traps, a method was developed to estimate the number of grey squirrels in a woodland. This provides end-users with a tool to assess the effort required to significantly reduce or eradicate grey squirrels from a given area.


  • A mathematical model was built to quantify the effects of fertility control, applied on its own or after a period of culling, on grey squirrel numbers. Modelling suggests fertility control applied in conjunction with culling is most effective at eradicating or significantly reducing densities of grey squirrels to numbers low enough to mitigate most of the economic and environmental problems they cause.



The oral contraceptive research offers a real opportunity to halt grey squirrel population growth and reduce their numbers and negative impacts. UKSA still needs to raise £250,000 to complete the project. Anyone wishing to support the research can donate online: www.squirrelaccord.uk/donate


Further information on the project can be found on our fertility research web page and our updated UKSA frequently asked questions document in our resources library.



Kay HawKay Haw, UK Squirrel Accord Director. Previously, Woodland Trust Conservation Adviser (started the Wood Wise publication and worked on invasive non-native species issues), Wildlife and Countryside Link Invasive Non-Native Species Working Group Chair, and EU Invasive Alien Species Working Group member.