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Well, Euphranta feeds on the seeds of Vincetoxicum and is termed a pre-dispersal seed predator in several entomological papers (you'll find them if you google the name). May be, "parasite" is used if one feeds on a living organism while seed, though viable, is in a state of dormancy. By the activity of a predator the seed becomes destroyed.
Euphranta loves Vincetoxicum hirundinaria, but we don't have that species in our botanical garden. (It is native, however, and found in the outskirts of Berlin.) We cultivate V. rossicum only, so Euphranta has no choice :D.
Regards, Sundew