Veratrum false hellebore
Dichotomous table for leafminers
1a irregular blotch with semi-deliquescent greenish frass, containing several larvae: Delina veratri
1b corridor, with black frass in strings or grains, containing a single larva => 2
1c galls, etc => Tables for all parasites per species
2a frass in discrete grains: Liriomyza wachtlii
2b frass in strings (occurrence on this plant needs confirmation): Liriomyza bryoniae