Honestly, I wouldn't worry about the ants. They aren't harming anything. They have a job to do and will always be there anyway.

I'll bet that the Holly is full of aphids which the ants will be 'milking' for their honeydew (actually their droppings). So the answer is kill the aphids. Put a grease band round the trunk to stop the ants from climbing up.

Been a lot of ants nests this year....

Do you have a good reason for wanting to kill the ants? They are doing no harm. If you have ants in the garden, as many of us have, killing one lot won't make a lot of difference anyway.

Forget the ants - inspect the tree itself - as Owdboggy suggests, there'll be an infestation, either of aphids or scale, and that's why the ants are on the tree, they're harvesting the honeydew produced by these insects, not harming your tree directly - they're just a symptom of another problem. You can treat for scale infestations by spraying pesticide, aphids with gentler treatments such as oil spray treatments, but that may not be easy to do if its a large tree. Note that some forms of scale are impervious to treatment at different times of the year, so inspect backs of leaves closely, along with stems and branches, for signs of scale infestation, to decide which type of scale you have (if it is scale and and not aphids). More info on scale, and treatments, here

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