How to Fight Onion Maggots / Onion Flies (Delia antiqua)

by Matt Gibson and Katie Olson

Onion maggots (Delia antiqua) can cause significant damage to your home vegetable garden, specifically targeting plants in the Allium family, such as onions, garlic, and leeks. Identifying these pests early is key to protecting your crop. The adult onion fly resembles a greyish housefly, with a narrower abdomen, longer legs, and overlapping wings when at rest. Keep an eye out for these flies in your garden, as they lay their eggs near the base of onion plants.

The eggs of onion maggots are small, white, and elongated, often found in the soil near the plant’s stem. Upon hatching, the larvae emerge as creamy white, tapered maggots that measure about 8-10 mm in length. These larvae are the culprits behind the damage to your plants, as they use their hooked mouthparts to penetrate the base of the plant and feed on its internal tissues.

To spot the damage caused by onion maggots, watch for wilting or dying onion seedlings, as well as distorted bulb growth in more mature plants. By learning to identify these pests and the signs of their activity, you can take timely action to protect your valuable Allium crops and ensure a bountiful harvest.

Onions are a popular seasonal vegetable that presents itself in many colors and can be grown in the spring and the fall. This vegetable can be eaten in multiple ways with different dishes, and it provides a great source of nutrition. The quercetin in red onions is even known to help fight against cancer. However, onion flies, also known as onion maggots, can put a damper on your vegetable garden if preventive measures aren’t taken against them.

These pesky insects seek out onions in the ground in which to lay their eggs. Those eggs grow into larvae that feast on the inside of the plant’s bulb and root system. Onion flies do not target onions alone. These pesky insects also terrorize other plants in the Allium family, such as onions, garlic, and leeks. 

All throughout the winter, the onion fly pupae stay warm inside the soil. When they finally come out of hiding as adults in the spring, they often travel up to a mile away in search of a good onion plant to feast on. When the yellow rocket weeds are in bloom along the road, you can put money on the chance that onion flies have arrived along with them. If the soil near onion crops stays cool and moist, the onion flies are destined to thrive. If soil temperatures rise above 95 degrees Fahrenheit, the onion flies will die off.

How Can Gardeners Control Onion Flies?

Although these flies can cause severe damage to your budding onion crop, there are steps you can take to keep your vegetables healthy and edible. The following tips can help you maintain a strong supply of onions from your garden in any growing season.  

Nematodes

Beneficial nematodes are an effective, natural way to control onion maggots in your garden. These microscopic, soil-dwelling organisms, particularly the species Steinernema feltiae, actively seek out and parasitize the larvae of various pests, including onion maggots. To use nematodes in your garden, simply mix them with water according to the package instructions and apply the solution to the soil around your Allium plants.

Nematodes are most effective when soil temperatures are between 60-90°F, and the soil is moist but not waterlogged. Keep the soil consistently damp for the first week after application to ensure the nematodes establish themselves and start hunting down the onion maggots. As an added bonus, beneficial nematodes are safe for humans, pets, and beneficial insects, making them an eco-friendly choice for keeping your garden healthy and thriving.

Sticky Cards

One tool for prevention that’s low maintenance for any gardener is using sticky fly traps. It is best to purchase yellow sticky cards because the flies are attracted to bright colors. You can then attach the cards to small wire stakes and plant them in the soil near your onions. The flies will be trapped when they land on the card, lured in by the bright shade. For best results, change out the sticky cards twice a week.

Floating Row Covers

Floating row covers, also called reemay fabric,  are another tool gardeners can use to keep adult onion flies from being able to lay their eggs on your batch of growing onions. This method is best used after the onion seeds are first planted in the ground. Leave the covers up from the time you first plant your crop until you harvest the fully grown onions.

Crop Rotation

You can also ight onion flies by rotating your onion crop. After each growing season, it is also important to till the soil in your garden to destroy any larvae still underground, get rid of plant debris, remove any leftover onions and culled bulbs, and make sure there aren’t any places for onion maggot pupae to hibernate in your garden. Your future self will thank you once spring rolls around.

Bury or Haul Away Cull Piles

Culled bulbs, also called culls or cull piles, stack up at the base of onion plants naturally. They problem is, they provide a perfect place for onion flies to make babies and a cozy place to lay their eggs. Remove these naturally occurring creature comforts as often as possible, keeping your garden beds clean and safe from pests like the onion fly that like to burrow and set up camp in cull piles. 

Raised Beds

To help ensure further protection of your onion plants, place your seeds in raised beds instead of flat plots or containers. The onion maggots prefer slower draining soil, and by using raised beds, you can take the opportunity to replace the soil as needed and keep it well drained.

Time Your Planting

The first generation of onion maggots is always the largest, and therefore it can cause the most damage to your plant. To keep the first generation from attacking your onions, plant them in your garden as late in the season as possible. This way, it will be too hot for the onion flies to survive by the time your onions begin to grow.

You can also set out your onion seeds before spring if the weather permits. Then by the time the onion flies attempt to launch an attack against your vegetables, they will already be ready to harvest already. It’s also important to note that white onions are more prone to the flies invasion. Instead, a pest-savvy gardener may choose to plant red onions and Japanese bunching onions, which are somewhat more resistant to damage from the maggots.

Organic Methods

You can also sprinkle the areas in your garden where you’ve planted your onions with ground cayenne, pepper, ginger, dill or chili powder as a repellent against the females that can prevent onion fly eggs from being laid. You can apply sand, wood ash, or diatomaceous earth to the base of the plants to naturally deter the flies as well.

Insecticides

While insecticides may appear to be a surefire way to cure your onion fly problem, gardeners should proceed with caution. The insects have developed some resistance to insecticides, so consult your local gardening expert before purchasing chemicals for your garden or applying them to your plants. Insecticides are also somewhat counterproductive when it comes to getting rid of onion flies, as the chemicals tend to also drive away the beneficial insects that feed on onion flies and their larvae, such as the celebrated onion fly connoisseur, the predatory ground beetle. 

Purge Your Soil

If you discover that an area of your garden has been infected by onion maggots, the experts recommend getting rid of that soil. However, if the area is too large for easy removal, you can try the following recipe to nix the flies. Puree peppers, garlic and onion in a blender. Then add water and organic soap, and let the mixture sit for a day. Finally, strain out the solids, and use the liquid to drench the infected areas of soil well.

If onion flies are wreaking havoc on your garden, there are plenty of steps you can take to nip the problem in the bulb. We rounded up these helpful tips to provide you with the most foolproof ways for every gardener, regardless of skill level, to harvest a successful onion crop they can then dry to preserve freshness, store, and enjoy at any time of the year.

Category	Information
Common Name	Onion Maggot, Onion Fly
Scientific Name	Delia antiqua
Order and Family	Diptera: Anthomyiidae
Size and Appearance	Larva: 8-10 mm, creamy white and tapered; Adult: 3-6 mm, greyish, narrow abdomen, long legs, overlapping wings
Type of Feeder	Chewing (hooked mouthparts)
Host Plants	Serious pest of onion and related Allium crops such as garlic and leeks
Life Cycle	Egg (1.25 mm, white, elongated) -> Larva (feeds on plant tissues) -> Pupa (7 mm, chestnut brown) -> Adult
Damage Description	Larvae cause damage by entering the base of the plant and feeding on internal plant tissues, distorting bulb growth
Organic Control	Crop rotation, proper sanitation, use of floating row covers, beneficial nematodes
Chemical Control	Insecticides (use with caution and follow label instructions)
Monitoring	Check for eggs on soil near plant stems, larvae infestation on plant base, and distorted bulb growth

Want to Learn More About Fighting Onion Flies?

National Onion Association covers All About Onions

University of Massachusetts Amherst covers Onion Maggot

Organic Gardening News & Info covers Organic Control of Onion Maggots

Weekend Gardener covers Onion Maggots

Plantwise Knowledge Bank covers Onion Fly