black onion fly

Black onion fly is a medium-sized picture-winged fly. It occurs in the United States from New Hampshire to Minnesota and south to Missouri and North Carolina. It is uncommon in Minnesota, where it is at the western extent of its range. It is a serious pest of cultivated onion and garlic crops in the Northeast and North Central states.

The female is slender, ¼″ to ⅜″ (6 to 10 mm) long. The head, body, and legs are entirely black or brownish-black. The body is shiny and is covered with short hairs. The wings are dark and have three clear cross bands. The outer two bands are slightly arced. The inner band (nearest to the body) is straight and is tapered to a point near the inner margin. The upper side of wing vein R₁ is covered with small hairs (setulose) at the end. The knob-like flight-balancing organs (halteres) are mostly white, black just at the base.

The male is similar but smaller, 3 ⁄16″ to 5 ⁄16″ (5 to 8 mm) long.

Female: ¼″ to ⅜″ (6 to 10 mm)

Male: 3 ⁄16″ to 5 ⁄16″ (5 to 8 mm)

Fields, meadows

Three generations: Early May through late summer

The female lays eggs singly or in groups usually under the lowest leaf sheath, sometimes higher on the plant, sometimes in the soil near the plant. Larvae feed on the bulb. Third instar larvae overwinter within the onion bulb. The following spring they pupate in the soil within twelve inches of the base of the host plant. Adults emerge 5 to 17 days later. There are two complete and one overwintering generation each year. The generations overlap so there are adults present all summer.

Bulbs of onion and garlic

Probably flower nectar