Research is investigating whether an oral contraceptive for grey squirrels can help keep trees healthy for the future.

Grey squirrels are damaging important broadleaf trees and causing local extinctions of the native red squirrel.

The oral contraceptive being researched by the UK Squirrel Accord (UKSA) is seen as a new hope for protecting tree health from this invasive species.

The grey squirrel situation is so serious that it was part of a key House of Lords debate this week on threats from tree pests and diseases.

A supporter of UKSA's work, Lord Gardiner of Kimble, parliamentary under secretary of state for rural affairs and biosecurity, said “With support from Defra, the Accord’s research to develop an oral contraceptive as an effective method for controlling grey squirrel populations is delivering promising initial results.

"Fertility control does have the potential to reduce grey squirrel populations and the spread of the squirrel pox virus.”

UKSA 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.

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The Earl of Kinnoull, chair of the UKSA and Red Squirrel Survival Trust, said, “The UKSA has commissioned a fertility control project at the Animal and Plant Health Agency, which it is hoped will perfect a suitable active substance and hopper delivery method to allow fertility control to shrink grey squirrel numbers significantly, allowing forestry a chance.”

Progress at start of the third 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.

Intensive bark stripping by invasive non-native grey squirrels creates wounds that pathogens can infect, stresses, weakens and can kill biodiverse and productive tree species.

High densities of juveniles can strip bark from the main stem and branches of trees between April and September. Species particularly susceptible to damage include high-value trees such as oak, beech, hornbeam and sweet chestnut.

Grey squirrels target young broadleaved trees, mostly 10-40 years of age, and repeat the damage year after year if their densities are high and unmanaged.

The oral contraceptive is considered to offer an opportunity to halt grey squirrel population growth and reduce their numbers and negative impacts.