Book gives "golden-haired flower longhorn" specifically. In and near forests. Larvae live in wood, and the adults live on nectar and pollen. Didn't see any mention of them preferring white flowers particularly.
Incidentally -- since I know you care, O nonexistent one! -- the book said Leptura chrysocoma....a recent book, color photos, but it looks like contemporary classification is Cosmosalia chrysocoma. chrys- is flower, I think. as for -coma, I can only contribute the fact that comer is eat in Spanish. Probably coincidence.
on cow parsnip. I've been watching c.p. all year because I'm impatient for swallowtail caterpillars.