Sugar ants (the little tiny black ones that don't really bite much but
get into beehives and kitchens) definitely herd aphids and milk them for
honeydew.  Bees will collect honeydew in a pinch but it's inferior to
nectar and they only use it in a pinch.
 
In my opinion, herding aphids is not one of the things that make ants
desireable in the ecosystem.  It falls more in the pest category if
you're a gardener.  It is a pretty fascinating social behavior though.
 
Michael Reddell
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http://www.hotcity.com/~mwr