Onion maggots are very small measuring around 9mm. The adult is a fly that is about 9mm long, greyish coloured with large wings.
Onion Maggot-Later’s, Maggots & Pupae-Jack Kelly Clark
Onion maggots are very small measuring around 9mm. The adult is a fly that is about 9mm long, greyish coloured with large wings. They lay their eggs around the base of onions and garlic in the spring after they emerge for overwintering pupae. Eggs hatch and the larva feed on the onions. The maggots can feed for up to 3 weeks before they become a soft brown pupae.
Onion Maggot Flies-Manitoba Agriculture, Fly, Larva and Pupae-University of Maine
They are found in cooler damper areas. Cool coastal climates are where they are prevalent. You can find these guys near dandelions and edge weeds around the garden.
Plants turn yellow and the maggots hollow out the whole onion if given a chance. Large bulbs will rot. Destroy all affected plants.
Maggot damage on onions-University of California
1) Sticky Traps
Yellow sticky traps will capture the flies in the spring. Tanglefoot is a good organic substance that can be safely sprayed or spread on stems and catches anything that climbs up.
2) Plan Your Planting Times
When planting onions and garlic in cooler climates, like the Pacific Northwest, it would be best to either plant in february when the flies are not present or wait until after the spring hatch of the flies. (mid-May) If you live in the Pacific Northwest there are several types of green onions that will thrive in the winter.
3) Rotate Your Crops
Pupae overwinter in the ground where the affected plants are, so rotating will give you a head start on controlling these guys. Clean up all garden debris before you close your garden for the winter.
4) Wood Ashes
Sprinkle wood ashes and cayenne pepper around the plants to discourage egg laying. Be very careful what ashes you use. Ashes from your wood stove or fireplace are the best choice. Be sure that wood has not been treated with chemicalss and that nothing other than newspaper was also burned. Coated papers, cig butts, and other things tossed in the fire can contaminate your crops. Know where your ashes came from before you apply.
5) Build Raised Beds
These guys love poorly drained soil, so a good idea would be to plant your onions and or garlic in raised beds.