The onion bulb fly (Eumerus strigatus) is one of three closely related syrphid fly species known as the lesser bulb flies (the others are the narcissus bulb fly, Eumerus narcissi Smith and the lesser bulb fly, Eumerus tuberculatus Rondani) whose larvae can cause much damage for onion and flower bulbs (for example narcissus, hyacinth, lilies, etc.) as well as potato tubers. The lesser bulb flies were introduced to the United States from Europe, probably in contaminated bulbs. Like most hoverflies (family Syrphidae) the adults eat pollen and nectar, are generally thought of as beneficial pollinators. They are metallic bronze-colored flies about 5-10mm that lay masses of up to 40-50 eggs on the soil near plant bulbs or on the skin covering or neck of the bulb itself. The larvae that hatch out burrow into the bulb and develop inside, causing the inside of the bulb to decay and liquefy and usually completely destroying the bulb. Many individual larvae can inhabit one bulb. Although they can attack healthy bulbs, larva prefer to infest bulbs that are already damaged because in order to develop they require the presence of other organisms to help promote decay, such as stem nematodes or root rot fungi. Two generations of flies occur each year, the second generation overwinters in the bulb as an almost full-grown larva and crawls out to pupate in early spring. While pesticide controls are not available, cultural and sanitary practices can help control this maggot. Planting bulbs deep and keeping them covered with dirt as well as removing damaged bulbs help to prevent larvae infestation from propagating.
(Barbour et al. 2008; Murray; University of Florida 2003; Wilcox 1926)