Onion fly: presentation, damage, treatments

© istock

onion maggot

Also called Delia antique, the onion fly looks like a classic fly. This is a diptera insect, that is to say with only one pair of wings, of the family Anthomyiidae. Gray-yellow in color, this insect measures about 6 to 7 mm when it is adult and differs from the ordinary fly by its yellowish, but transparent wings, which are crossed above its abdomen when it is not flying, by the 5 gray stripes drawn on its thorax and by the intense black of its antennae and its legs on which bristles are visible.

But before you meet the fly itself, you can see its larvae which are 8 mm maggots about or ringed pupae of oval shape and brown color.

Onion maggot life cycle

Depending on the climate, you may be faced with a succession of two or three generations of these flies throughout the growing season. This explains why the growth of the population of this insect can quickly become very important. Here is how the life cycle of these flies unfolds:

  • In the spring, around April or May depending on the regionwhen the temperatures begin to become milder, the flies emerging from the pupae that have spent the winter in the ground mate.
  • A few days later, the females will lay their eggs around or even on the onion plants. Per cycle of 15 days, a female can lay up to 200 eggs and this during the approximately 2 months that her adult life lasts.
  • 2-5 days later, these white and elongated eggs, approximately 1.4 mm in size and positioned in clusters, will give birth to maggots. When they emerge from the eggs, these larvae are cream colored and measure approximately 6 to 8 mm. They have black mouth hooks that will allow them to feed on bulbs, young transplanted plants or roots by digging galleries. Depending on the temperature, this will last between 17 days and three weeks.
  • Once this time has elapsedthe larvae leave the plants that housed them to bury themselves in the ground and pupate.
  • A first generation of adults, the most formidable, will then emerge a fortnight later, then after a new cycle, a second in July and in warmer regions, a third in September. This one will hibernate in the form of pupae until next spring, when the first of the new cycles will begin!

Onion Maggot Damage

As its name suggests, the onion maggot lives in onion seedlings, but also in those of leeks and shallots. Here is the damage it can cause, especially since the attacks are insidious, because it is not always easy to see the presence of these insects:

  • On young plants, you can see yellowing of leaf tips then a withering to finish by a complete decline.
  • When the onion is a little more developed, the fly larva burrows into the tissues of the plant and into the bulb. It thus creates orifices which are open doors for different bacteria which will cause onion rot.
  • The most virulent damage is caused to young plants, because when the plants are larger, they do not necessarily die. Nevertheless the bulb is deformed or infected.
Onion Fly Damage
© istock

Prevent the presence of onion maggot

To avoid these attacks, it can be helpful to adopt a few new habits such as the following:

  • Rotate crops to avoid growing your onions in an area where pupae from the previous year may have hibernated.
  • Install a fine-mesh insect screen to protect seedlings from flies. On the other hand, they must be installed correctly so as not to leave any entry point for onion flies and as soon as the aerial parts develop, you must ensure that the net is not in contact with the plant. Otherwise, flies may lay eggs on the foliage.
  • Focus on beneficial associations : you can thus grow carrots around the onions to protect them.
  • Delay planting your onions as long as possible so as to avoid the first spawning.
  • Spray manure on your plantations tansy, rhubarb or fern during spawning periods.

Controlling onion maggot

If some of your plants show signs of an attack, it is important to act as quickly as possible to preserve the others:

  • Uproot and burn infested plants to prevent the spread. Don’t put them in the compost.
  • It is possible to use yellow glue traps. They will attract onion flies which, after sticking to them, will die.
  • Similarly, you can use pheromone traps of the trade, which, too, will trap the flies.
  • In organic cultivation, pyrethrum-based insecticides can be used. You can buy it commercially or prepare one yourself by diluting a teaspoon of dried pyrethrum flower powder and a teaspoon of liquid black soap in half a liter of water. Be careful to spray this mixture within 2 days of its preparation. However, this type of solution should be used as a last resort, because this toxic substance, pyrethrin, is not selective and will therefore have a negative impact on garden helpers and pollinating insects.


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