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Update 8/05/05: Turns out that there is a species of aphid, Myzus lythri, that feeds only on purple loosestrife. It is thought to have been introduced from Europe around the same time purple loosestrife started showing up in the U.S. I have sent samples off to be identified and will post here when I find out what species they are.
Update 9/30/05: The ants in this photo were identified as Crematogaster cerasi by a scientist from the Systematic Entomology Laboratory. I am still waiting on the aphid id.