My first reaction to the question, I have to admit, was "Ooh goody, this makes a change from questions about pruning the darned things!"
Karen had noticed ants congregating on the tip of each leaf, in vast quantities. She noted that none of the leaves appear to have been eaten or damaged, but obviously no-one wants to have ants all over the place. She'd tried traditional bug spray, and she'd tried blasting them with the hose pipe but they kept coming back: so her question was in three parts:
1) how to get rid of them
2) will they cause any harm to the tree
3) is it safe to use ant powder on a tree
3) yes, you can use ant powder on trees.
2) No, it won't cause any harm to the tree, as long as you use it in moderation.
1) see below for detailed instructions!
Right, firstly, ant killers: ant powder is the usual stuff, it comes in a flexible plastic puffer pack: it looks like talcum powder, and you puff it out where the ants are to be found. They walk through it, as they march around on their unfathomable business (where are they all going? Aren't they supposed to have a nest in once place, then send out troops to find and bring back food? Why are they going in both directions yet not carrying anything?), then carry it back to the nest where it kills them, the others, and the queen, thus getting rid of your ant problem for good.
Ant powder is what I always recommend for ant in the lawn, and ants in patios. As a non-chemical alternative, you can use boiling water on patios, but I would not suggest doing this on plants!
However, ant powder is nasty stuff, and being a powder, it can blow around all over the place, so a better option for trees is to find the gel version. It's sold for getting rid of ants in the house: you squeeze out a thin line of the gel along a threshold, or across a marching line, and it works in the same way as the powder, ie they get it on their feet and trample it back into the nest.
(At this point I always have a vision of the Doorman Ants fussily saying "Wipe your feet! You don't know what you've been walking though, honestly, my nice clean nest, harruumph")
Secondly, why are there ants in our trees anyway? Generally, the ants only colonise a tree if there are aphids, or certain types of insects, in it. Ants like aphids (and those other insects: if you're that interested in knowing exactly which ones, you can look up a list yourself!) because they produce a sweet sticky fluid called honeydew, which ants really enjoy. So if the aphids are there, the ants will move in and "farm" them: they'll guard them and look after them, and encourage them to produce more honeydew.
So having used ant powder to get rid of the ants, it's worth taking a close look at the tree to see if there are any other inhabitants that need removing. As far as I can tell from personal experience (and a quick internet search), ant powder does not kill aphids, so you'd need to address them as a separate issue.
Generally speaking, my advice about ants and all creepy crawlies on my small trees is to get the hose out, close the nozzle well down, and jet wash the plant, to blast off as many of the pesky little blighters as you can, then pouf a ring of ant powder around the main stem, fairly close to the ground, so that everything marching up or down has to go through it.
Refresh the ant powder every couple of days, particularly if it rains. In a couple of weeks, you should be ant-free.