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Well, going by Tschorsnig (1994): bristle comb is regular without any larger bristles mixed into it (Masicera comes off a couplet which says it has irregular bristles on the hind tibia and Townsendiellomyia has a comb but 1 large bristle too); it looks like it has 2 inner orbital bristles but actually the first is smaller (in Masicera there would be 2-3 inner orbitals, the anterior one being strong). Shouldn't be Blepharipa either because the second antennal segment seems black (Blepharipa is yellow). :)

So, it would be nice to have the specimen in-hand (blah, blah, blah) but ... it looks pretty damn good for Sturmia bella :)

The only problem is the host (as you said) ... I don't think I'd take a record for a non-nymphalid host (or at least a non-butterfly) without a lot of proof that the collector didn't accidentally mix his pupae in the same cage. :D