Life-history data are presented relating to a common close-wing moth belonging to the recently demonstrated insectan fauna of rushes in the zone of emergent hydrophytes of northern Michigan lakes. Chilo forbesellus Fernald (Pyralididae, Crambinae) bores in the culms of Scirpus occidentalis (Watson) Chase, S. validus Vahl, and S. americanus Persoon. Here the larvae hibernate for eight months beneath the lake surface in silk-lined galleries. The following summer they generally twice migrate to new food culms and construct their pupation chambers beneath characteristic emergence windows in the ultimate larval galleries. Mass emergences of adults occur on calm warm July and August evenings. Eggs are deposited into emergent parts of disused larval galleries of the same species. The imago like the larva is semiaquatic. Egg, larva and pupa are described. Dyar’s law of larval growth is shown to be inapplicable to this species. Two egg parasites and four parasites of the larva were reared.