Composition and relative abundance of Delia spp. (Diptera: Anthomyiidae) infesting onions in selected onion-producing regions

In the Great Lakes and Pacific Northwest regions of North America, maggots (Diptera: Anthomyiidae) threaten the production of dry bulb onion (Allium cepa L.). Onion maggot (Delia antiqua Meigen) is considered the primary Delia spp. pest in the Great Lakes, whereas seedcorn maggot (Delia platura Meigen) is considered most common in the Pacific Northwest. Bean seed maggot, Delia florilega (Zetterstedt), is also reported to infest onions, but has not been detected widely in these regions. Where these Delia spp. co-occur, D. platura/D. florilega are presumed to infest onions before D. antiqua, but this also has been poorly documented. To address these assumptions, larvae from infested onion plants were identified to species from 2022 to 2024. We hypothesized that D. antiqua and D. platura would be the predominant Delia spp. in the Great Lakes and Pacific Northwest, respectively, and that D. platura would infest onions before D. antiqua where they co-occur. Results indicated that D. antiqua was most common in the Great Lakes (87% of Delia spp.) and in eastern Oregon/western Idaho (USA) (79% of Delia spp.), but sample sizes were too small to make inferences about northern California and eastern Washington. Delia florilega was not detected in our study. Delia platura was most prevalent infesting onions early in spring, whereas D. antiqua was dominant in late spring and early summer. Our findings document Delia spp. composition and phenology in selected onion production regions and will inform regionally tailored monitoring and pest management strategies.