Flax fly

The flax fly (Nanna armillata) is a member of the Diptera order and the Scathophagidae family. These small flies are typically 4–5 mm long and possess a brownish-yellow body coloration.

The larvae of the flax fly are cylindrical, legless, and reach a length of 6–7 mm. They are usually yellowish-white, making them difficult to spot within the plant tissues they inhabit.

This pest overwintering as a pupa in the soil, often at depths between 5 and 10 cm, which allows it to survive harsh winter conditions and emerge in the spring.

The lifecycle of the flax fly is closely synchronized with the growth stages of the flax plant, ensuring that larvae emerge exactly when food sources are most available.

The adult fly lays eggs under the skin of young leaves or near the apical bud, providing the larvae with an immediate and high-quality food source upon hatching.

The primary hosts of Nanna armillata are fiber flax and linseed. The larvae cause significant economic damage by feeding on the internal tissues of the stem.

By consuming the apical bud or the growing point, the larva prevents the plant from developing a single straight stem, which is essential for high-quality fiber production.

This damage forces the plant to produce multiple lateral branches, significantly reducing the quality of the harvested straw and causing inconsistency in the crop.

In linseed production, the damage impacts the number of seed bolls, leading to a substantial decrease in overall grain yield and oil content potential.

Severe infestations can lead to total crop failure in specific areas of the field, especially if conditions favor a high population density of the pest.

Adult flies typically emerge in the spring once soil temperatures reach 10–12 degrees Celsius, corresponding with the initial germination of flax seedlings.

The most dangerous period occurs during the «fir tree» growth phase of the flax, when the young apical buds are most accessible to the newly hatched larvae.

The larval stage lasts between 15 and 25 days. After this period, the larvae descend into the soil to pupate, completing their primary damaging phase of the season.

A second generation of the fly may appear in mid-summer; however, its impact on the mature crop is generally lower than that of the spring generation.

The fly's activity is highly dependent on humidity levels, as excessive drought or moisture can significantly affect the survival rates of the eggs and young larvae.

The first indicator of infestation is the yellowing and wilting of the top leaves of the young flax plant, often accompanied by a noticeable stop in vertical growth.

Affected stems may appear distorted or bent at the tip, and they often become brittle, breaking easily under mechanical pressure or strong winds.

The plant frequently exhibits abnormal lateral branching, attempting to compensate for the loss of its central growing point, which disrupts the uniform stand density.

Upon closer inspection, small entrance holes or tunnel markings may be visible on the stem, indicating where the larva penetrated the plant tissue.

• Stunted growth of the main stem.
• Yellowing and wilting of the apical buds.
• Deformed and curled upper leaves.
• Increased branching and reduced fiber quality.

Effective crop rotation is the primary strategy to manage the flax fly population, ensuring that flax is not grown in the same field for consecutive seasons.

Deep autumn plowing of fields intended for future flax cultivation is essential to bury the pupae and disrupt their survival during the winter months.

Early sowing allows the flax to reach a developmental stage that is more resistant to larval damage before the peak emergence of the adult flax flies.

If economic thresholds are exceeded, the application of registered insecticides is recommended, focusing on the early stages of infestation to prevent widespread damage.

Maintaining high soil fertility and optimal sowing density supports vigorous crop growth, allowing plants to withstand and recover from minor larval damage more effectively.