CONTROLLING THE HERD. “We believe that ants could use the tranquillising chemicals in their footprints to maintain a populous ‘farm’ of aphids close their colony, to provide honeydew on tap. Ants have even been known to occasionally eat some of the aphids themselves, so subduing them in this way is obviously a great way to keep renewable honeydew and prey easily available.”
http://www.sciencedaily.com/releases/2007/10/071009212548.htm