I’ll note that the ants are only there because of the aphids. I’d recommend you focus on the removal of the aphids. Once the aphids are gone, the ants will leave.
If you can refrain from spraying chemicals in your garden, you might find that predator insects arrive to solve the aphid problem for you.
The main issue with this is the waiting. Predator insects in my experience usually take 7-14 days to arrive on the scene after the problem insect has begun its damaging activities.
While you wait there are a number of actions that you can take that will greatly assist.
If you have only one or a few trees, you could try any one or a combination of the following:
- every 2-3 days, wash the aphids from the plant using a hose with a jet-like spray pattern;
- prepare a homemade batch of chilli spray (soak chillies in water with a small amount of dishwashing liquid) and apply using a spray bottle - the detergent breaks the surface tension of water and the chemicals from the chillies are disliked by most sap-sucking insects;
- on the worst affected stems/ branches, run your finger along to literally brush as many of the aphids off.
This last action also has the added bonus of leaving the smell of dead aphids on the branches, which I have found works better than anything, with the exception of a sustained hoverfly attack.
We attribute the very low damage from aphids in our garden this year to three factors:
- we refrained from spraying any chemicals (including the relatively innocent “white oil”);
- we enjoyed a constant hoverfly presence in our garden - the hoverfly lays its eggs near the aphids and the resulting larvae voraciously consume the aphids;
- we planted a number of sacrificial flowers near plants we wanted to protect, including Calendula and Cosmos - the aphids tend to gravitate towards these plants.