Biology of Scaptomyza apicalis Hardy (Diptera: Drosophilidae) was studied in relation to its host, meadowfoam, Limnanthes alba, a recent oil seed crop grown in the Willamette Valley, Oregon. Populations of flies and larvae were monitored weekly over three consecutive crop-years beginning in 1996. Yellow sticky traps gave relative population estimates of adults. Absolute estimates of larval populations were derived using Berlese funnels to extract immatures from whole plant samples. Weather and crop phenology are key factors in population regulation. Meadowfoams, Limnanthes species, were the only observed hosts for S. apicalis in this study. This has four to five overlapping generations per year. Adults of a small founder population colonize commercial fields coincident with fall rains and seedling emergence. Females deposit eggs in or on plant tissue. Larvae mine leaves and stems. They also bore into crown tissue and flower buds later in the season. Second generation flies arising from the larvae of the founder population first appear in late winter. Successive generations peak during the rapid vegetative growth stage of meadowfoam (mid-April). A steady decline in adult and larval numbers occurs as daily temperatures rise and plants develop flower buds. Last flies are detected in early July when meadowfoam is harvested. Temperatures below 0�� Celsius during December were a key mortality factor for S. apicalis in 1998. Three often major weather components analyzed, accounted for up to 60 percent of the trap count variability. These components were temperature, solar radiation and relative humidity. S. apicalis larvae fed only on plants within the Limnanthes in feeding studies. They accepted nine native meadowfoams but with varying survival rates. The commercial meadowfoam cultivar, Floral, was the most suitable larval host. An increase in supplemental nitrogen fertilizer rates generally resulted in increased infestations of S. apicalis and decreased seed yields. / Graduation date: 2001