Onion maggots (Hylemya antiqua) are tiny maggots that are a cosmopolitan pest of onions and related plants. They feed below the ground on onion bulbs, producing tunnels and possibly introducing disease organisms into the plant. Once established, onion maggots are very tough to get rid of, particularly where continuous production is practiced. Onion maggot damage is more prominent in cool and moist soils.
The onion fly has an ash-grey body and resembles a housefly. They have humped backs, large wings and are ¼ inch long. Their wings overlap over their bodies when at rest.
Onion fly eggs are elongated and white and are generally laid in groups on the shoots, leaves and bulbs of host plants and on the ground nearby. Eggs hatch in 3-8 days. Maggots tend to keep together and feed in groups.
The larvae or maggots are white and cylindrical and have 3 stages that develop over the course of 2-3 weeks before turning into brown, ringed, oval pupae.
Onion maggots overwinter as pupae in soil associated with previous host plant cultivation. Adult onion flies emerge around mid-May, after which females start laying eggs. The larvae, upon emergence, crawl beneath the leaf sheath and enter bulbs. After weeks of feeding they pupate and the next generation of adults emerges 3-4 weeks later. There are usually 3 generations per year, the 1st being the largest and often most damaging. A complete cycle takes about 45 to 65 days.
Onion maggots are highly host-specific to plants in the onion family including onions, leeks, shallots, garlic and chives. Stunted or wilted onion plants are the 1st signs of onion maggot damage. Light infestations may not kill onion plants but make them more susceptible to bacterial rots. Larval feeding may kill seedlings, thus poor plant growth may indicate onion maggot problem. In larger plants, larvae may tunnel into the bulb causing plants to become flaccid and yellow.
Organic Control and Prevention Methods:
1. Use yellow sticky traps in spring when onion adult flies emerge. Tanglefoot can be spread safely on stems and leaves to catch anything that sits it. Unfortunately, this method will bring in some beneficial insects too.
2. Crop rotation is a good practice. Grow onions and related plants in a different spot from previous seasons.
3. Cover seedlings and plants with floating row covers to keep adult onion flies from laying eggs on host crops. This is a highly effective method and has to be implemented immediately when seedlings are planted.
4. After each season of growing onions and related crops, the ground has to be worked thoroughly. Till the soil to destroy pupating larvae, remove plant debris, remove all onions and culls to ensure there aren’t areas for onion maggot pupae to overwinter.
5. Since the 1st generation of onion maggots are the largest and most damaging, plant onion sets into the garden as late as possible. Or set out onion plants before spring, weather permitting so that when the onion flies emerge your onions will be ready to harvest.
6. White onion varieties are prone to onion maggot attack. Grow red onions and Japanese bunching onions, which are somewhat resistant to onion maggot damage.
7. Sprinkle onion planted areas with ground cayenne pepper, ginger, dill or chili powder to repel females and prevent them from laying eggs.
8. If a small area of your garden is infected with onion maggots, get rid of that soil. If the area is too large, drench the area with the following mixture. Puree peppers, garlic and onion in a blender. Add water and organic soap and let it stand for a day. Strain out the solids and use this liquid to drench infected soil.
9. Buy insect parasitic nematodes that control onion maggots and apply to the soil as directed. Seek advice from a local gardening expert before buying.
10. The onion maggot has many natural enemies such as ground beetles, birds, parasitic wasps, nematodes and a parasitic fungus that is most effective in cool, wet weather.
11. Sand, wood ash and diatomaceous earth can be applied to the base of plants to deter adult flies and onion maggots.
12. Growing onions in raised beds with fresh soil or in containers is a good idea as onion maggots love poorly drained soil.