Delaying onion planting to control onion maggot (Diptera: Anthomyiidae): efficacy and underlying mechanisms.

Onion maggot, Delia antiqua (Meigen) (Diptera: Anthomyiidae), is an important pest of onion, Allium cepa L., in northern temperate areas, especially in the Great Lakes region of North America Management of D. antiqua relies on insecticide use at planting, but insecticide resistance can cause control failures that threaten the long-term viability of this strategy. Delaying the time onions are planted was investigated as an alternative management approach for D. antiqua and the ecological and behavioral mechanisms underlying host age and insect relationships were examined in laboratory and field experiments. Delaying onion planting by two to four weeks reduced damage to onions by 35 and 90%, respectively. Onions planted later emerged later and this reduced the period overwintered flies had to oviposit on the plants. Moreover, flies tended to lay few to no eggs on these young, late-planted onions. As anticipated, D. antiqua laid 4-8 times more eggs on older onions than on young onions, and older onions were more resilient to injury caused by D. antiqua neonates compared with younger onions. However, the resiliency to maggot attack lessened as the density of D. antiqua increased from 2 to 10 eggs per plant, which probably explains why greater levels of maggot damage are typically observed in early onion plantings compared with later plantings. Delaying onion planting until mid-May reduced D. antiqua damage without jeopardizing the period required to produce marketable yield, but this cultural tactic must be combined with other management strategies to prevent economic loss.