Ants

I wouldn't consider ants to be a garden pest unless they become a biting nuisance, or you eat al fresco and they're marching onto your food/drink.

However, ant colonies can be controlled with poison traps (e.g. sold in the UK as 'Nippon') - It doesn't kill instantly, but the worker ants carry it back to the queen. When the queen dies, the whole colony withers away.

The ants going up your tree are probably 'farming' the aphids on the tree. The 'milk' them for the honeydew (sweet plant sap). The ants do no harm but the aphids do. Try putting a sticky bandage round the tree, so the ants cannot get over it.

Hi, vasaline works just as well in quite a thick band round the trunk - will last the summer. They used to run up into the cherry tree and then all the aphids caused a real problem if you wanted to sit underneath it with sticky sap.