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Identification
Grey fly similar to a housefly; white legless larvae feed in clusters at the base of onion bulbs and roots.
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Life Cycle
Two to three generations per year; adults lay eggs at the base of plants or in surrounding soil in late spring.
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Damage Signs
Larvae tunnel into the bulb base, causing yellowing, wilting leaves and soft rot that kills seedlings and spoils mature bulbs.
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Treatment
Remove and destroy affected plants promptly, as infested bulbs cannot be saved.
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Prevention
Rotate onion crops annually and cover with fine mesh during egg-laying periods in late spring.