Harvester butterflies are the only carnivorous butterfly in New England. Well actually it's the caterpillars that feed on woolly aphids that are carnivorous. The butterflies only nectar on the honeydew of the aphids.
Woolly aphids suck sugar from the sap of alder trees. The aphids can't process all the sugar in the sap, it's just too rich. So the aphids excrete the excess sugar as honeydew. Ants farm the aphids gathering the honeydew and protecting the aphids from predators.
The caterpillars devouring an aphid colony are on a tree without ants. Some kind of wasp has taken over the aphid colonies and it doesn't recognize the caterpillars as a problem. So the caterpillars have a free run of the colony.
A couple other haverster butterfly photos are found
here and
here.