Oviposition, feeding preferences and distribution of delia species (Diptera: Anthomyiidae) in eastern Canadian onions

Root maggots are a major pest of many crops around the world. In Canada, root maggots can cause economic losses in Allium crops, particularly onion. In Eastern Canada, the root maggot of greatest concern was the onion root maggot, Delia antiqua. Pest management strategies focused on this species but recently the discovery of two other species, the bean seed maggot, Delia florilega and seedcorn maggot, Delia platura have raised the question of their role in the damage observed in onion. The Delia species composition was examined in Ontario (Holland Marsh and Harrow), Quebec and Nova Scotia in 2016 and 2017 where onion are produced. Laboratory studies compared the preference of onion growth stages for onion root maggot and seedcorn maggot feeding and egg-laying. Each onion growing region was found to have a different proportion of the three Delia species. Ontario (Holland Marsh) and Quebec were the only two areas where onion root maggot (D. antiqua) was captured. The predominant species in Nova Scotia and Quebec was seedcorn maggot followed by bean seed maggot. Ontario (Harrow) had the lowest number of seedcorn maggot flies captured. In the laboratory, seedcorn maggot flies would lay eggs on onion plants that were young, not yet with 2 true leaves developed, while onion root maggot would lay eggs on older plants, with 5-7 true leaves developed. Feeding damage was similar in that seedcorn maggot flies would feed on younger plants while onion root maggot preferred to feed on older plants. Plants fed on by the onion maggot were not marketable while those fed on my the seedcorn maggot showed minimal damage. Our results point to potential options for managing these pests in Eastern Canada. This could include sowing seed to avoid synchrony between the emerging adult flies and the preferred plant stage. Development of improved trapping methods is recommended as the impact of seedcorn maggot is greatly reduced compared with onion root maggot. Knowing which species is present in the field will ensure growers are applying the appropriate control measures to the species that is causing damage.