P. orphisalis Walker is a foliage feeder of spearmint (Scotch spearmint (Mentha spicata) and native spearmint (M. cardiaca)) and peppermint (M. piperita) in Washington during its larval stages. Three generations occur per year, with typically 5 larval instars per generation. The life history of each generation is illustrated and various biological data are provided, i.e., information on flight behavior, mating, adult longevity, ovipositional behavior, fecundity, immature development, larval habitat, feeding damage, overwintering and natural enemies (ichneumonid, braconid and tachinid parasites and heteropteron and thomisid spider predators).