Onion maggots are the larval form of the Delia antiqua, or onion fly.
Onion maggot description
Onion flies are brownish gray versions of your average housefly, only smaller. The males have a stripe, but females do not. At rest, these flies tend to fold one wing over the other. Eggs are long white ovals. And the larval form, or onion maggots, are creamy white, legless, and a little less than 1/2 an inch long.
Onion maggot lifecycle
Adult flies lay eggs on leaves, shoots, and bulbs of host plants. These hosts include onions, leeks, and, to a lesser extent, garlic. Flower bulbs are also susceptible to onion maggot feeding. When the eggs hatch, onion maggots start feeding and feed they do! You can find up to 50 onion maggots in a single bulb! After they have eaten their fill, the larvae move into the nearby soil where they will pupate. These damaged areas provide points of entry for many other fungal and bacterial pathogens.
How to control onion maggots
Crop rotation is your best defense against onion maggots. Since the adult forms can fly, there is only so much you can do. Row covers may help, but it is difficult to time their use properly. Yellow sticky traps can be used to monitor for adult flies.
Luckily for Bay Area gardeners, our cool, wet winters slow larval growth. In our area, more damage occurs while onions are in storage than in the field. You can protect your onion harvest from damage by thoroughly brushing off onions before putting them into storage, and regularly monitoring harvested onions.
I hope this information inspires you to grow more of your own food. You can ask your garden questions on my Home page.