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The cabbage fly is an insect pest of a large number of wild or cultivated plants of the Brassicaceae family (ex-Cruciferae). We could also call it the "cabbage" fly, because it can attack all types of cabbage, as well as turnips, rutabagas, rapeseed, radishes, mustard and horseradish plants. It is the larvae that are responsible for crop damage, by digging galleries in the roots, causing secondary rot and dieback of the plant.
Description of the cabbage fly
The adult fly, Delia radicum or Hylemia brassicae, is small and measures approximately 6 mm. She is gray, speckled with black. The female has clearly separated eyes, unlike the male who brought them very close together. The maggots, whitish, have the distinction of having a posterior end crowned with ten membranous black tips, two of which are bifid (split). The nymph, called pupa in Diptera, 7 to 8 mm long, is brown and ovoid in shape.
Biology of the Delia fly, the disease it causes and its consequences
It is at the stage of the pupa, whose physiology is slowed down (it is said to be in diapause) that the fly spends the winter, buried in the ground. Depending on the region's climate and the gradual warming of spring, the first adult flies hatch between late March and early May. The female lives a dozen days and lays from the fourth day. Attracted by the sulfur compounds of the Cruciferae, it deposits its eggs in bundles (up to 150 eggs in total), generally in the soil near the host plant, or exceptionally in the axils of the first leaves. The eggs take 4 to 6 days to hatch at a temperature of 15-20 ° C. The maggot can then move to the root of the host plant and begin to tunnel in the tenderest parts. After three weeks, the maggot leaves the root to pupate in the soil. It takes 20 days for the pupa to turn into a fly. Under favorable temperature conditions (15-20 ° C), the insect's reproductive cycle of around six weeks can occur two to three times during the growing season. During the summer, if the temperature exceeds 25 ° C, there is a high mortality of eggs and larvae. The pupae put themselves "at rest", we speak of quiescence, while waiting for the return of more favorable conditions. First signs of the disease, the plants seem to lack water, then the peripheral leaves gradually redden. Cabbage fly larvae exclusively attack the root system which eventually rots. This results in the death of young plants and the decline of older plants. The crucifers whose roots are eaten (radishes, turnips, rutabaga, etc.) become unfit for consumption.
Cultural precautions and chemical and biological control
Vegetable producers tend to use chemical means of control. The seeds are coated with insecticidal substances, or the roots of young plants are previously soaked in an organophosphorus-type substance. We avoid too early sowing because we noticed that the first generation of maggots seems the most destructive. The first adult cabbage fly outbreaks are closely monitored by agricultural warning stations, in order to optimize chemical control. There is no longer any need to treat when the soil temperature exceeds 22 ° C. The insect comes to rest in the form of a pupa. Eggs and larvae die from heat. You can also use insect nets to prevent theft and lay. But there are several drawbacks to creating such protections on crops (difficulty of installation, problem of solidity, soiling resulting in a lack of light, greenhouse effect favoring fungal diseases). There is no effective biological control today. But we know that certain natural auxiliaries (certain hymenoptera and beetles-staphylins), present in the ecosystem attack the larvae of the fly and could control the population. In the vegetable garden, some ecological means can limit the action of the pest. For example, cardboard collars are placed at the base of the cabbage to prevent the larvae from sinking towards the roots. Plants such as marigold, clover, tansy repel females from the cabbage fly. You can also water Cruciferous plantations with an infusion of tobacco, tansy, horsetail, plants that have an insect repellant action. By C. Schutz Croué