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Hi,
I would be a little bit more careful as to the ID: there are more species of Epistrophe that look like this when they have reached their third instar. In my own experience, also the larvae of E. melanostoma and E. nitidicollis have this appearance. In some (maybe even all) cases the white dorsomedian stripe only appears after the larva has stopped feeding, i.e. at the end of spring when it gets ready for diapausing all through the summer and winter. This is also the time they move away from the aphid colonies on trees and shrubs, to find a more sheltered place among leaf litter etc.
If you want to find more: search in May among aphid colonies on shrubs and trees!