They have been the scourge of trees and the native red squirrel in Britain since their introduction from the United States in the 1870s.
But government scientists are now planning drastic measures to reduce gray squirrel numbers – by doling their food with an oral contraceptive.
Before going this far, the Animal and Plant Health Authority (APHA) conducted trials using special feeding boxes in forests in northern England and Wales.
About 70 percent of the gray squirrel population have used the boxes, which have a weighted gate and keep most other animals out.
The Department for Environment, Food and Rural Affairs’ senior scientific adviser Gideon Henderson said the trials had great potential for non-lethal management of gray squirrel numbers.
“It will help red squirrels… to expand back into their natural habitats, protect UK forests and increase biodiversity,” he added.
Vanessa Fawcett of the Red Squirrel Survival Trust said research into developing an oral contraceptive for gray squirrels is progressing.
“Without effective conservation management, red squirrels could face further local extinctions across the UK.”
No contraceptive has yet been used in the studies, but APHA researchers said it would be effective in both male and female gray squirrels.
There are now 2.7 million gray squirrels in the UK and their numbers are increasing compared to just 140,000 of the smaller red squirrels.
Grays compete with reds for food and also carry the squirrelpox virus. They are immune, but reds are not, and infection is almost always fatal.
High densities of gray squirrels also threaten the health and survival of young trees as they strip bark, weakening and killing them.
Conventional culls of gray squirrels have proven ineffective because they can breed quickly and recover numbers quickly.
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