Aphids are equipped with piercing mouths designed to slurp up juices from new growth. They especially like rose buds but will wreak havoc with all types of plants including citrus.
Aphids are little slobs. When they taste the sweet nectar of their prey they spit half of it out! Those poor table manners causes those juices to rot causing a condition called black sooty mold.
In turn, that black sooty mold becomes like caviar for ants. Now you have an aphid problem, black sooty mold problem and an ant problem.
To fix these problems all you need to do is wash the plants off with soapy water. It'll wash the ants and black sooty mold off while the soap scum clogs up the aphids so they can't breathe.