Description, Biology, Life Cycle, Damage, Common Names, Images
- Host plants: wild and cultivated brassicas: radish, turnip, swedish turnip, cabbage, cauliflower, fodder cabbage, rape, etc.
- Adult: the females have quite a short lifespan (12 to 15 days at 20°C) and a fecundity of about 150 eggs. Laying begins about 3 days after the adults have emerged. The eggs are deposited in the ground in small clusters, nearly always in the immediate vicinity of the plant's stem, exceptionally at the axil of the petioles of lower leaves.
- Egg: embryonic evolution lasts 4 to 6 days at 15-20°C.
- Larva: once the egg has hatched, the larva buries itself in the ground and penetrates the roots where it forms galleries in the most tender parts. Its development completed, after 3 weeks, it leaves the plant and pupates (*) .
- Pupa: development lasts 20 days.
- 3 to 4 generations per year, from the beginning of April to the beginning of October.
- The adults emerge from the ground at the beginning of spring at dates which vary from year to year and from one region to another, from the end of March to the beginning of May, according to the incease in temperature.
- The time of larval presence and, therefore, evidence of the damage, vary according to the region, from the end of April to September. The speed of development of maggots and their activity rise rapidly with temperature. On the other hand, high temperatures lead to a high mortality of eggs and young larvae. In general, the emergence of winter Rapessed corresponds to the last generation.
- Two pauses in development are observed:.
. the first in summer when the ground is above 22°C: the pupae then remain in a state of rest (quiescence) not developing until the temperature returns to below 20°C.
. the second in winter, beginning in September-October: the pupae enter diapause and resume their development in spring, when the ground warms up, after being subjected to cold conditions.
It is seen in nurseries and at all stages of plant development. On the root vegetables, the larvae penetrate the fleshy parts and form galleries which render the plant unsuitable for consumption (*) .
On leaf-vegetables and rape, the root is more or less destroyed (*) .
In both cases, the presence of larvae is often accompanied by moulds which attract saprophogic Diptera. Destruction of young plants can occur or the growth of older plants can be slowed down. The attacked plants can be recognised by the appearance they have on warm days; the leaves are often tainted purplish red, turn yellow and sometimes dry up.
The damage can manifest itself suddenly in spring under the influence of a substantial rise in temperature. Under good growing conditions, moderately attacked plants can send out new roots and produce a normal harvest.
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