Adult onion maggots resemble brown, hairy humpbacked flies which in turn are similar to the seed corn maggot. Adults generally remain hidden. The eggs are laid at the base of a plant. The maggots are 1/3-1/4" long and legless. They are found near the plant bulb or neck. They are whitish in color and don't have stripes. Maggots overwinter as pupae in soil. Usually 3 generations a year appear. They prefer cool, wet conditions.
Generally two species of maggots attack onions: the onion maggot and the seed corn maggot. The onion maggot is the worst pest that bothers onions.
Maggots burrow into the underground part of the stem in young onions. They also like the developing bulb of larger onions. The lower stem gets destroyed and the neck damaged. There should be some wilting and stunting of plants as well--a first sign. Onions will not store well and rotting usually occurs very quickly.
Not only onions are attacked, but radishes as well.
Best control is avoid close spacing of plants, don't plant in rows to confuse the maggots, and practice fall cleanup.
Also you can use barriers such as wood ash and diatomaceous earth. Plant a trap crop of culled onions to attract maggots then destroy them.
Plant resistant varieties such as yellow or bunching, including japanese bunching onions. Yellow and bunching however, only sustain minor damage. Japanese are completely resistant.
Move your onion patch every other year. Maggots live, mate, eat and die in a very small area. Spread onions throughout the garden, not just in any one area. Attract birds, parasitic wasps, the usual beneficial nematodes, and other beneficials. Also some beetles, predatory flies, spiders, and ichneumonid wasps enjoy a meal of maggots. Yum!
I did not find anything on whether or not rotenone, sabadilla, or bt would work on this pest. I'm willing to bet that maybe bt will, but would have to test it out first. I don't usually grow onions much so if any of you know if it works, let me know!
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