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Description Adults are gray flies with clear wings. Larvae are white maggots.
Where they live Onion maggots are a serious pest throughout the northern half of the United States and in Canada, especially after a wet spring.
Their life cycle Brown pupae overwinter in the soil, emerging in spring as adult flies. Females lay eggs in soil near the stems of onion seedlings. Newly hatched maggots immediately burrow into the stems and eat the plants beneath the soil. When one onion dies, the maggot moves into the seedling next to it. Plants that survive the first generation of maggots are vulnerable to later generations. Second-generation maggots feed in midsummer on developing onions, deforming bulbs and introducing rot and diseases. Damage from the third generation often isn't evident until after the onions have been harvested. The stored onions rot, becoming inedible.
Plants they attack Onions.
Why they are a problem Clumps of your onion seedlings turn yellow, wilt, and die. Diseases enter the onion through wounds, which promotes rot.
Organic damage control Good cultural practices are essential in controlling this pest. Start by rotating your crops: Don't plant onions in the same place two years in a row. Once the pest's breeding cycle is broken by planting its host (onion) in a different spot, you can use a floating row cover to keep the flies from laying eggs on your fresh plants. Use the cover immediately after sowing seeds or planting sets, and make sure it extends at least 6 inches beyond each side of the planted bed. Plant onions later in the season to avoid the first generation of flies. Mist seedlings with water and then dust each plant thoroughly with diatomaceous earth. Reapply if it rains. Remove and destroy onion crop debris at the end of the season. Beneficials such as ground and rove beetles, birds, and braconid wasps prey on all life stages of the maggot.
