"Grain of rice" in leeks: how to avoid leafminer?

The leek leafminer fly, which originates from Eastern Europe, first appeared in France in the early 2000s. At present, it is mainly rampant in the northern part of France, but is gaining more and more ground. This diptera also known as Napomyza gymnostoma is harmful to leeks as its name suggests, but also to plants in the same Allium genus such as chives, garlic and onions.

Characteristics of the leek fly

Leeks also have moths or leek worms as enemies, with which the leek fly should not, however, be confused. This fly is in fact a small insect measuring around 3 mm long. The yellowish larvae of the leek maggot will start to make galleries in the leek leaves, from the green part to the white. This weakens the plant considerably and makes it susceptible to bad weather. As they develop, these larvae will become small cocoons with the appearance of a grain of rice, the pupae.

The attacks take place in two stages, in spring and then autumn. After winter, cocoons already present on leeks in the vegetable garden will transform into flies, feed by biting leeks and will also mate and then infest available leeks. The result is malformations on the leaves of the affected leek, with characteristic white spots caused by the fly as it feeds on the plant's sap. Leeks infected in this way cannot be eaten. In autumn, the flies emanating from the spring larvae will emerge and will thus be able to continue their reproductive cycle.

Means of controlling leek maggot

To protect your garden from attacks by this pest, it may be a good idea not to sow or plant leeks in the same place from one year to the next. In other words, it is highly recommended to rotate crops to avoid contamination by larvae that may be in the soil. You can also resort to a physical barrier by using a sufficiently thin culture veil which will prevent flies from coming to infect the leeks whether to feed or to lay eggs. This veil should be put in place at the very beginning of spring, before the flies start to appear in the garden.

If larvae are found in leek leaves, it is essential not to compost them. In fact, the larvae can survive and thus continue their contamination in the vegetable garden. Certain wasps are likely to help eliminate leek maggot, and may be attracted by plants such as yarrow, tansy or St John's wort. Purins made from tansy, lovage or wormwood are also thought to be natural leek fly repellents, although there is no scientifically proven evidence of this.