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Many factors directly and indirectly influence pest dynamics in cultivated crops and identifying the suite of factors associated with a pest's population dynamics is an important component of pest management. Onion maggot, Delia antiqua (Meigen) (Diptera: Anthomyiidae), is a significant pest of onion in temperature regions worldwide. In the Great Lakes region of North America, D. antiqua is the most important early-season pest of onion and causes considerable damage by killing plants. Despite growers employing the same management and cultivation practices, plant damage by D. antiqua across the region is highly variable. The commonality of production and management suggests that other factors may be important in explaining the disparities in damage across the region, and previous studies have indicated that temperature and precipitation, soil organic matter, surrounding landscape composition, planting date, and plant size at peak fly activity are all important factors affecting D. antiqua. In a two-year study (2018–19), the aforementioned factors were monitored in commercial onion fields (2018: n = 15; 2019: n = 13) in central and western New York, USA. Fly activity was positively associated with plant damage in both years of the study. Onion fields surrounded primarily by forest rather than agricultural crops and those planted late rather than early in the season had higher levels of damage by D. antiqua. Plant damage also was negatively associated with soil temperature and positively associated with soil organic matter content, but these effects were context dependent and were only observed in the first year of the study. These results will be used to inform management decisions by growers and have important implications for the development of predictive models for D. antiqua infestations. • Landscape and planting date explained most of the variation in onion plant damage. • Forested habitat and later planting dates were associated with higher plant damage. • Soil temperature and soil organic matter occasionally impacted plant damage. • Plant size at peak D. antiqua fly activity had no effect on plant damage. [ABSTRACT FROM AUTHOR]