Hmm, why not just breed "daughterless" ones?

Ananova is reporting that a British attempt to reduce populations of the non-native grey squirrel (Sciurus carolinensis) has failed. With the goal of limiting the spread of this American species, which has replaced the red squirrel (Sciurus vulgaris) in most parts of the U.K., scientists developed a contraceptive vaccine that could be injected into female grey squirrels, but the vaccine proved too difficult and expensive to manage the squirrels effectively.

Thanks to Conserv@tion for posting a link to this story.