The Gulf of St. Lawrence aster, Symphyotrichum laurentianum (Fernald) G.L. Nesom (Asteraceae), a small annual halophyte endemic to disturbed and highly transient habitats in the Gulf of St. Lawrence, is classified as “threatened” by the Committee on the Status of Endangered Wildlife in Canada. Lepidopteran larvae that are predispersal seed predators of the Gulf of St. Lawrence aster are reported for the first time from populations in Prince Edward Island National Park. DNA barcoding was used to identify the seed predators tentatively as larvae of the casebearing moth Coleophora triplicis McDunnough (Lepidoptera: Coleophoridae), which is typically associated with a related halophyte, Solidago sempervirens L. (Asteraceae). These larvae were found to consume a large proportion of seeds from one of two aster populations in Prince Edward Island National Park and may be yet another risk to the survival of this threatened species.