Fertility control research

Field trials of the feeding hopper by APHA


UK Squirrel Accord is fundraising for research into a fertility control for grey squirrels. This will offer a non-lethal management method to control population densities and support eradication efforts.


The study aims to produce an immunocontraceptive that can be orally taken by grey squirrels through a species-specific delivery mechanism. We are now into the fourth of the five-year project, which will end in January 2024, and hopes are high that this innovative work will provide an effective, less labour intensive, non-lethal method for managing grey squirrels. Read the latest blog on research fieldwork achievements in 2021.


Experts at the Animal Plant Health Health Agency (APHA) are carrying out this important research. In 2021, the team gave an online update on progress that can be watched via our YouTube channel and the video below. A 2022 update webinar is being planned for 19 May, 10:00-12:00.




Why use fertility control?

Across the world, fertility control is being seen as a complementary and alternative method for wildlife management. Watch this Botstiber Institute for Wildlife Fertility Control (BIWFC) video of an interview with Dr Giovanna Massei, APHA lead for our fertility control research and BIWFC Europe Director, to find out more…




UKSA progress

APHA is trialling different methods of reducing fertility to create an effective formula that can be taken orally. Three different oral contraceptive candidates were tested during the third year of the five-year research project. Two were based on novel immunocontraceptive formulations and one was a cholesterol inhibitor called Diazacon. One immunocontraceptive and Diazacon are being further refined in year four and the best one will be taken forward to the final stages of development.


APHA is also developing a species-specific feeding hopper. Tests to deliver the oral contraceptive to grey squirrels only are going well. The test bait, without contraceptives, is attractive to grey squirrels and the weighted door of the hopper currently prevents almost all but the very largest red squirrels from accessing it. Alternatives methods are being tested to increase species specificity in red squirrel areas, but weighted doors could be utilised for feeders in grey-squirrel only areas.


In 2019, 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.


Rhodamine B is used as a marker. It is consumed as part of the bait and is incorporated into the animal’s hair. This causes the hair to fluoresce under UV light and identifies squirrels that have ingested the bait.


Hair of squirrels, photographed under UV light, that ingested bait containing the marker Rhodamine B by APHA


Defra is complementing this research through modelling work to compare different management techniques, to predict grey squirrel densities in woodlands to guide management efforts, assess how large a buffer needs to be to prevent reinvasion and predict the rate of reinvasion to areas if management stops.


Modelling suggests fertility control applied in conjunction with culling is most effective at quickly eradicating or significantly reducing densities of grey squirrels to numbers low enough to mitigate most of the economic and environmental problems they cause. This does not prevent fertility control being used on its own, it would simply take longer to significantly reduce numbers.



Fourth year research objectives

Work in the fourth year is focusing on further refinement of the chosen immunocontraceptive and Diazacon formulas, and required dose levels needed to inhibit fertility in grey squirrels. Field studies will assess number of visits to and bait uptake (without contraceptive) from feeders placed in woods. Alongside further development of alternative methods to the weighted door mechanism to ensure red squirrels cannot access feeding hoppers.


There are four phases needed to deliver the grey squirrel fertility control for the UK:

  1. Research and development – ending January 2024
  2. Landscape-scale trials – in development
  3. Testing for registration – in development
  4. Widespread availability of registered methods


Coordinated effort will ultimately be essential for effective roll out of the fertility control in the future. We are exploring the potential to work with stakeholders, projects and partnerships that will enable the oral contraceptive to be trialled at a landscape scale and deployed more widely once registered.



Supporting the research

UKSA is still fundraising to complete the fertility control research and wishes to thank everyone that has donated so far. To complete the research, a further £150,000 needs to be secured. If you are interested in supporting this important work please:



Further information

UKSA fertility control research FAQs 2021 - research - UK Squirrel Accord

Developing a grey squirrel feeder for targeted contraceptive delivery blog - 2021

Fertility research - news from the field blog - 2020

A new hope for grey squirrel management - fertility control research update blog - 2019

APHA Science Blog: Plight of the red squirrel - 2018