Onion fly

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The about six to seven millimeters large onion fly (Delia antiqua) belongs to the family of flower flies (Anthomyidae). It resembles the slender, somewhat lighter housefly (Musca domestica) in appearance.

Way of life of the pest

The egg leaves of the onion fly are placed directly next to the shoot base of the affected bulbous plants. This happens around the beginning of May, around the time of the dandelion bloom, as the animals previously feed on its nectar and pollen. After about a week, the whitish maggots, about eight to ten millimeters long, hatch from the eggs. They feed on the soft tissue of the young bulbous plants and hollow them out from the inside. A maggot can severely damage several plants. After the maturation of the animals they pupate in the soil. In the course of the summer usually follows a second generation whose maggots then damage the crop. Winter onions and late leeks can even be affected by a third generation in warm years. Most of the time, however, the damage is not that great anymore, as the plants are already larger and more robust at this time. The overwintering of onion flies occurs in the pupal stage in the soil.

Damage to the onion fly

Maggots of onion fly

The maggots of the onion fly eat their way through the leek and leave feeding passages

Shortly after emergence of the young onion and leek plants, the leaves turn yellowish gray on infestation with the pest. The plants slowly begin to wither and are easily pulled out of the ground. Some plants have arcuately curved leaves. On older onion and leek leaves the hobby gardener detects the infestation on the basis of feeding. The onions are eaten from the inside and often you can still see the bright maggots of the onion fly.

Frequently affected plants

Onion flies occur mainly as pests on leeks (Allium ampeloprasum), chives (Allium schoenoprasum) and onions (Allium cepa). In some cases, the small pest spirits also on Tulips (Tulipa) are up to mischief.

Prevent onion flies

Vegetable bed with cultural protection net

With cultural protection nets you can effectively prevent onion flies in the vegetable patch

A cultural protection net provides a safe and lasting protection against damage by the onion fly in your garden. The net is laid in such a way that all bulbous plants are completely covered and there are no gaps at the edges of the net. Cover the beds with the females until shortly before the harvest. You should gradually loosen the net so that the onion plants can push it up as it grows. The application of a cultural protection network works better than the use of pesticides when used correctly and also provides protection against infestation with the leek moth (Acrolepiopsis assectella).
Preventively promote natural enemies such as parasitic wasps, ground beetles or spiders. Such beneficials can attract you by setting up an insect hotel, organic gardening and a generally near-natural garden with great plant diversity.
Dispose of all rotten onions to prevent the development of a strong onion fly population from the outset.

Do not sow your bulbs in the same place in the garden next year and keep a proper crop rotation in your vegetable patch. Above all, a mixed culture with carrots (Daucus), cabbage, bush or runner beans effectively counteracts the onion fly.
A possible infestation can be avoided by sowing the onion plants late. Do without a strong-smelling fertilizer or fresh manure. Preventively spray your bed with a self-made tansy or vermouth tea.

Fight onion flies

A direct control of the pest is unfortunately not possible with natural preparations. If your bed is already infected, you should remove and burn all affected plants as soon as possible to prevent further spread of the pest. In case of emergency, you can use insecticides (litter or spray).

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