A northern pest, OM is seldom a problem in dry years. OM can destroy most of crop in wet springs. The maggots tunnel in the bulb and crown. Bulb is rendered worthless if not killed.
Early plantings are most injured.
One maggot can kill a seedling onion. The OM winters in the puparium stage in the soil of onion fields or under piles of onion refuse and cull onions that were discarded at harvest. The flies begin to emerge in May and continue to appear for about a month. Flies coming from refuse piles begin to emerge somewhat earlier than do those in the open field, and emergence continues for a longer period. Refuse piles have long been considered dangerous centers of infestation, producing large numbers of flies and lengthening the period of fly emergency, but currently researchers are questioning the importance of refuse and cull onions.