The seasonal life history of the cereal stem moth (CSM), Ochsenheimeria vacculella Fischer von Roslerstamm, (Lepidoptera : Ochsenheimeriidae) was investigated in a commercial field of annual ryegrass in the Willamette Valley, OR from January 1993 to August 1994. Phenology of life stages, effect of temperature on eclosion, and development of larvae on potential hosts of economic importantce were also studied. First records of parasitization and effects of cultural practices on this potential pest in annual ryegrass were reported. CSM is a recent introduction to North America from a monobasic family of the Palearctic region. Its life cycle is functionally univoltine in commercial ryegrass seed fields. Eggs are characteristically deposited on interior wooden walls, ceilings and straw bales or piles in outbuildings from June through September. Eclosion occurs bimodally with approximately fifty percent of current season eggs hatching in late June and July. The remainder overwinter and hatch in February and March. Larvae ballooned from oviposition sites to potential hosts and were found in annual ryegrass from February to early June. First instar larvae typically mine leaves; later stadia are stem borers. Variance to mean ratios of larvae sampled in annual ryegrass described a clumped population. Pupation occurred from late May to early July. The pupa was enclosed in a flimsy cocoon usually located on the inside of a flag leaf's sheath. Shortly after emergence in early June and July, adults fly to outbuildings preferentially remaining within those where grass straw has been stored. Migration from the field and subsequent flight, copulation and oviposition within buildings occurred only on bright days from approximately noon to 4:30 pm (PDT) through September at which time most adults have died. Two species of larval parasitoids in the Eulophidae and Ichneumonidae were very abundant in the annual ryegrass field under study during June and July 1994. Their combined parasitization rate of CSM larvae exceeded ninety percent. A frequency distribution of head capsule widths indicates CSM larvae probably develop through five instars. However, inter-instar ratios of head capsule widths did not conform to Dyar's hypothesis. Eggs deposited by females collected in the field and allowed to oviposit in the laboratory did not hatch at either room temperature or 6��C. However, cohorts of eggs hatched readily during incubation at either 10�� or 14��C when observed after two and three months exposure. Nine varieties of six commercially important species of grasses and cereals were evaluated for suitability as larval hosts. Annual ryegrass, Lolium multiflorum Lam., consistently supported the greatest population densities in the field and resulted in greatest survival of larvae in laboratory and greenhouse tests. Removal of annual ryegrass foliage in plots during late winter or early spring to simulate the effect of sheep grazing significantly reduced subsequent larval populations relative to plots without vegetation removal. / Graduation date: 1995