I suspect fruit fly.
The ants are farming sap suckers, the sap suckers obviously suck sugars from the plant and then excrete honeydew which the ants harvest.
You can put a sticky band around the trunk (stops the ants but change frequently and dont strangle the tree), spray the tree with white oil to kill them sap suckers but the fruit fly, if it is that, mate you are in for a battle.
CDFA > PHPPS > PDEP > Mediterranean Fruit Fly (Medfly) Pest Profile
And this is for you.
Quote:
To the same end, bait sprays or mass trapping
should be applied in and around hosts on which ßies
congregate, such as apricots in spring in northern
Greece. These practices may help to maintain a low
population level until autumn. Alternatively, eliminat-
ing early maturing hosts from the area entirely might
prevent breeding of the ßyearly in the season and may
hence reduce population densities later in the season.
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From here.
Seasonal and Annual Occurrence of the Mediterranean Fruit Fly (Diptera: Tephritidae) in Northern Greece