An outbreak ofChrysoteuchia topiaria(Zeller) (formerlyCrambus topiariusZeller), was observed through the autumn and winter to determine the major biotic factors affecting the overwintering larvae in Oregon grasslands. Predation by starlings, killdeer, sandpipers, and blackbirds in October reduced the larval population in a field of bluegrass more than 80%. Birds found infestations and picked larvae out of their hibernacula during most of the winter. The fungusBeauveria bassiana(Balsamo) was present in all field collections but infected only 3.1% of the population; 8.3% of mature larvae were parasitized by the tachinidLydina polidoides(Townsend). Three species of sod webworms that occur in Oregon grasslands were found to differ in nutritional requirements;Pediasia trisectaWalker andTehama bonifatella(Hulst) clearly required leaves for optimum growth, whereasC. topiariagrew equally well on either roots or leaves ofFestuca rubravar.commutataGuad. In nature, the larvae ofC. topiariaremained within the crown of the plant to feed and therefore are probably less susceptible to contact with foliar applications of insecticide than the larvae ofP. trisectaandT. bonifatella. However, the rate of oviposition ofC. topiariasuggests that a quick-killing insecticide applied when the adults emerge could prevent moths from laying most of their eggs.