Onion maggot

Identification:
Onion root maggot flies are rather delicate, hump-backed gray-brown flies, about 5-7 mm long. Onion, seedcorn and cabbage maggot flies are difficult to distinguish with the naked eye, but each will only be found on and near their appropriate crop family. 

Life Cycle:
Onion maggots spend the winter as small brown pupae in the soil. Adults emerge in spring and can travel up to one mile in search of host plants. A good indicator of the start of root maggot flight is blooming of the common roadside weed, yellow rocket. Female flies seek out their host crop to lay eggs at the base of the stem. Cool, moist soil conditions favor survival of the eggs, and soil temperatures over 95°F in the top ½ to 1 inch kill them. There are three generations each year in New England.

Crop Injury:
When eggs hatch, larvae feed on roots and can cause complete destruction of the root system. In onions, newly hatched larvae crawl behind the leaf sheath and enter the bulb, and feed on the roots, stem, and developing bulb. Feeding damage also encourages entry of soft rot pathogens.