Onion Maggot

Onion maggot (Delia antiqua) is the larval stage of a small gray fly closely related to the cabbage root fly. Adults emerge in spring when soil temperatures reach approximately 50°F, typically coinciding with early onion transplanting in zones 4 through 7. In most of the northern United States, three overlapping generations occur between late April and September; southern zones may see compressed timing and earlier first-generation pressure.

Egg-laying flies deposit clutches at the base of onion stems or in nearby soil. Larvae hatch within a week and bore directly into roots and the developing bulb. First-generation larvae, feeding on young transplants, cause the most visible damage: wilting, yellowing leaves, and collapse of seedlings that have not yet formed a substantial bulb. Older bulbs can tolerate moderate larval feeding but commonly develop secondary soft rot as bacteria enter through the tunnels. By the time symptoms are obvious, the larvae have typically moved deeper or pupated.

The most cost-effective control window is before and immediately after planting. Floating row cover applied at transplanting and kept sealed through bulb formation physically excludes adult flies during the most damaging oviposition period. Crop rotation of at least three years away from any previous allium bed reduces resident fly populations significantly. Heavy organic amendments applied just before planting attract egg-laying adults; incorporating compost several weeks before planting is preferable when soil amendment is needed. Beneficial nematodes (Steinernema species) applied to moist soil at planting provide moderate suppression of first-generation larvae and are compatible with row cover programs. Insecticide programs targeting adult flies require precise timing relative to local degree-day accumulation and are difficult to execute reliably without regional monitoring data.