Identify and treat Onion Fly in the UK


When the maggots emerge they tunnel into the developing onion bulbs and feed on them. After three weeks or so the maggots then move a small distance away from the onion bulb, pupate and produce a second generation of maggots. There may be several generations of maggots in a growing season, the last one occurring around the end of August. This last generation overwinters in the soil and plant debris to start the lifecycle again next spring. The Latin name of this pest, for those researching further, is Delia antiqua. A very detailed article on the onion fly, by Cornell University, can be found here.


The first signs become visible on younger developing onions. The leaves wilt and turn brown, these symptoms also then occur on more mature plants. The top of the bulb may smell slightly and will begin to rot. Maggots may well be visible in it. Gardeners sometimes confuse onion fly with white rot because the above ground symptoms are similar. However onion fly has has maggots in the the top of the bulb whereas white rot does not and white rot damage first affects the bottom of the bulb not the top.


There is no treatment for affected bulbs. Dig affected plants up as soon as the pest is identified and burn them. Affected onions are inedible. Watch out for maggots in the surrounding soil as you dig the bulbs up, remove and destroy the maggots.


Other common pests and diseases which affect onions include:


Date: 13 July 2016 From: Ronald Maggs
I have previously been affected with growing leeks with the onion fly pest and I have covered them up this year with fleece. When do you think I should remove the fleece?

ANSWER: Onion fly have two or more generations within a growing season. The last generation tends to be in August when the onions are ready for harvest. This means that the fleece needs to stay on all season for maximum protection. The last generation in the year overwinters in the ground so if you have completely eradicated them in one year they will not re-appear from your ground the next year. Unfortunately the flies can spread from neighbouring crops.


Onion fly

Q What is onion fly?

A The onion fly (Delia antiqua) is a widespread pest of onions and related crops. It attacks bulb onions, salad onions, shallots, leeks and garlic, as well as the closely related ornamental alliums. This pest is more common in some seasons than others, but when severe attacks occur it can make growing these crops impossible.

Caption: Onion flies ruin affected onions

Q What do affected plants look like?

A The onion fly lays its eggs near the base of the plant. The larvae feed inside the bulb or lower stem just above the bulb of seedlings. When they have finished with one seedling they move on to an adjacent one, so seedlings tend to be killed in patches. In older plants they feed in the bulb and work upwards. Eventually, the lower part of the bulb is so damaged that the resulting pale, wilting foliage is easily pulled off. The outer leaves tend to fall to the ground, while the inner leaves remain vertical, but are soft and no longer crisp.

When you look closely at the bulb it is rotting, smelly and can have as many as 30 maggots in it. Even lightly affected plants are unfit for harvesting.

Q When do attacks develop?

A Flies overwinter as pupae in the soil. They’re on the wing in May and lay their eggs in the young leaves or in the soil around the host plant. The eggs hatch after three days and larvae enter susceptible plants by tunnelling into the base. It takes three weeks for the maggot-like larvae to reach full size, when they are 8mm long. Then they burrow about 75mm into the soil and pupate. The chestnut-brown pupae hatch into the second generation of flies about 17 days later. There are up to two more generations in July and, to a much lesser extent, in August and September. The last generation overwinters in soil as pupae.

Onions sown in August for overwintering are especially vulnerable to later generations.

Q How do I recognise them?

A The flies are very similar to houseflies. In fact, telling them apart is a job for an expert.

Q Can I confuse them with anything else?

A White rot produces similar symptoms. This disease destroys roots, causing the plant’s foliage to yellow and die. When you pull a plant affected with white rot, however, there are usually no maggots. Instead there is a whitish mould at the base where the dead roots are. In the mould, or nearby, are black, pinhead-sized spheres. These are the sclerotia, or resting bodies, of the white rot.

Q What early-warning signs should I look for?

A Typically, the first sign of onion-fly attack is that the leaves of established plants begin to yellow. Affected seedlings tend to die in patches. These patches can spread along the row.

Q Can affected plants be saved?

A No, but, where seedlings are affected, removing the attacked ones along with nearby soil may stop the pest spreading.

Q Can I re-sow affected sites?

A If there is no other land available you could re-sow or replant, as long as you are sure you have removed the infested plants before the maggots pupate. It is safest to re-sow or plant elsewhere, protecting the new crop with horticultural fleece or insect-proof mesh.

Q Will crop rotation help?

A You should always practice crop rotation where onions and their relatives are grown, as there are several soil-borne diseases and nematodes that can attack these crops. However, as onion flies are so mobile, they can seek out and fly to crops within a wide radius, it is unlikely that rotations will control them, unless used in conjunction with insect-proof netting.

Q What preventative measures can I take?

A Where onion flies are a recurring problem, on allotments for example, covering with fleece or insect-proof mesh to protect against the early generation is worthwhile. Similarly, covering August-sown onions with insect-proof mesh will protect them. Using fleece after the end of May can lead to the plants getting too hot. Mesh, which is better ventilated, is less likely to cook the plants.

Q Is companion planting effective?

A We know of no suggestions for companion plants.

Q Are there any resistant varieties?

A Unfortunately, we know of no resistant varieties.

Q Are there any biological controls?

A There are at least two significant natural parasites and predators of the onion fly: a parasitic wasp and the rove beetle. These may occur in sufficient numbers to give some control, but they are not available to buy.

Q What should I do with affected plants?

A They should be lifted and consigned to the dustbin or burned, to prevent the larvae overwintering and adding to the following season’s problems.

Q Can I reduce the risk to next year’s crop?

A There is some evidence that digging in the autumn, rather than the spring, will expose the pupae to the pest’s natural enemies and the effects of the weather.

Learn more about how to grow onions.

Suppliers of insect-proof mesh and fleece

How to Fight Onion Flies

by Matt Gibson and Katie Olson

Control onion flies to protect your crop. 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 carrots, parsnips, garlic, and leek plants as well.

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.

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.


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.

Onion Fly — Information, Control, Treatment




ONION FLY (Hylemia antiqua).

The onion fly is a two winged fly, grey in colour and is not unlike the common house fly.

It attacks young onions, shallots, garlic and leeks.

It lays its eggs around the young onions when they are about 2—3 inches high.

The resulting onion fly maggots, after eating their way into the developing bulb and completely hollowing it out, pass on to the next.

One onion fly maggot or larvae often destroys three or four onions before it becomes full grown and pupates.

A second generation of onion flies appears in late July or August. These onion flies again lay eggs which hatch into maggots.

Instead of the plant dying right out as when attacked at the young stage, the stem falls over and becomes yellow and limp.

At the same time bacteria may enter the bulb, causing a rapid rotting of the root.

When full-grown the larvae leave the bulbs to pupate in the soil.


Once onions are attacked by onion fly there is nothing that can be done, except to pull them up and burn them.

As a safeguard against attack, bring the onion plants through the early stages as rapidly as possible by applying suitable cultural methods. This can be done most easily by planting onion sets rather than growing from seed.

Gamma dust and calomel dust are the best insecticides to use to prevent trouble from an onion fly infestation.


Onion Fly — Information, Control, Treatment, Advice about onion flies

Onion fly Delia antiqua

Look for

Dead and wilting young plants, softening and decaying of mature plants. Also the larvae are often found within the root tubers and they are 8-10mm long white coloured maggots.

Plants affected

  • Mainly Onions but other members of the Allium family can also be attacked.

About Onion fly

  • Adult Onion Flies emerge from pupae that have over-wintered within the soil.
  • Pupal cases are brown, oval shaped and about 10mm long.
  • Onion flies have several generations a year, the first generation usually emerges about May/June time.
  • The first generation of adults to emerge are able to reproduce within a few days and females then lay batches of eggs onto or nearby onion seedlings.
  • The eggs laid are small, white and elongated, with darker stripes that run longitudinally.
  • Second and third generations attack the onion sets which will have developed from the seedlings.
  • The damage is caused not by adult flies but by their grey, legless maggot-like larvae (8mm long) feeding on the onion sets and moving from one to another in the soil.
  • Adult onion flies are similar in appearance to house flies however they are grey, slightly smaller and have longer legs.
  • Larvae can often cause secondary damage such as rotting when they bore in and out of the roots.



Products containing the following chemical ingredients are all effective on Cutworms

  • There are no approved insecticides currently available to amateur gardeners.

Note: It is important to read manufacturer’s instructions for use and the associated safety data information before applying chemical treatments.

Onion Maggot Control – How To Get Rid Of Onion Maggots

In some parts of the U.S., onion maggots are without a doubt the most serious pest of plants in the onion family. They infest onions, leeks, shallots, garlicand chives. Find out about the identification and control of onion maggots in this article.

What are Onion Maggots?

Onion maggots are the larval form of a little gray fly that looks a lot like a common housefly except that it is only one-fourth inch long. The small, cream-colored maggots infest the bulbs, riddling them with tunnels. The damage leaves the bulbs susceptible to invasion by bacteria.

The maggots have about three generations each year. The first generation is the largest and causes the most damage. The last generation attacks just before harvest. This generation leaves the bulbs susceptible to rot during storage.

The parents of onion maggots, which are small, gray flies, are hard to distinguish from any other fly. The females lay their eggs in the soil where onions grow in order to get their offspring off to a good start in life. When they hatch, the maggots feed on the underground onion bulbs for about three weeks before they leave the bulb and move out to the soil where they pupate. They later emerge as adults that begin the process all over again.

How to Get Rid of Onion Maggots

Onion maggot damage includes a poor rate of germination and survival in young plants. Older plants may have limp, yellow leaves. The bulbs may show signs of soft rot while they are still in the ground, but they sometimes don’t begin to rot until after harvest.

Crop rotation is one of the most important aspects of onion maggot control. The maggots feed only on members of the onion family. If newly hatched maggots don’t find a food source, they won’t survive. When you thin your plants, remove and destroy the culls, which also serve as a food source. You should also completely remove any crop debris remaining at the end of the year.

The insects like to pupate and overwinter in areas with large pieces of organic matter. To avoid providing them with a comfortable hiding place, make sure all compost is completely decomposed before adding it to areas where you are growing onions.

For the most part, insecticides available to home gardeners are ineffective. Contact insecticides never reach the maggots, which are hidden away inside the bulbs. The insects have developed resistance to systemic insecticides.

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