(click to enlarge)

What are the black leaf cutters ants (maybe “Acromyrmex spp.”) doing at the lemon tree?

Aphid farming.

“They protect the aphids, and in exchange they get to eat up the waste of the aphids (that has sugar in it). That is definitely a situation to eradicate them for (because the aphids are drinking up the sap of your plant and making it weaker).

Usually all they do with my plants is setup nests at the bottom of the container, they may damage the roots slightly but I usually don’t try to eradicate them unless I’m bringing the plants inside.”