The natural history, oviposition, feeding behavior, and parasitism ofColeophora caespititiellaZeller andC. cratipennellaClemens in a Rhode Island salt marsh are described.C. cratipennellais univoltine and feeds only on the rushJuncus gerardiLoisel.C. caespititiellais bivoltine; the spring generation larvae feed onJ. gerardiand the fall generation larvae feed on the annualSalicornia europaeaL. This is the first report of bivoltinism in this genus and the first report of a singleColeophoraspecies feeding on plants in these two plant families. InJ. gerardimonocultures, oviposition frequency increased with increasing flowering culm density and flower density per culm. InS. europaeamonocultures, oviposition frequency initially increased with plant density, but moths appear to be satiated by high-densityS. europaeastands. Overall, larvae infested 22% of theJ. gerardiseed capsules and reduced seed set in these infested capsules by 60%.C. caespititiellalarvae consumed from 4 to 25% of the seeds ofS. europaea. Larvae were rarely seen to move from the flowering culm or plant where they were first encountered. Eighteen percent ofC. cratipennellalarvae and 10% of first-generationC. caespititiellalarvae were parasitized by an unidentified ichneumonid wasp. Because these moths feed on two plant species with known asymmetric competitive interactions and consume more seeds of the dominant competitor (J. gerardi), the moths may mediate the competitive interactions betweenJ. gerardiandS. europaea.