Ants "farm" aphids and scale (and others in the homoptera family) for the sweet honeydew they exude. Sometimes you don't notice a problem until you see the sooty mold that grows on the honeydew exuded on the leaves of a plant. The ants we have here (I've seen about 3-4 different types) don't seem to be predatory with other insects.. I see a lot of grain husks outside their nests. Glad we don't have fire ants.
