The rapeseed stem weevil, or Ceutorhynchus napi , is a major pest of rapeseed crops in Europe. Every year, it causes considerable damage, leading to yield losses and a deterioration in the quality of the crops. The fight against this pest is based on an understanding of its life cycle, its characteristics and the appropriate means of prevention. Our advice and biological solutions to fight against this weevil, a rapeseed pest.
Description of the rapeseed stem weevil
Ceutorhynchus napi belongs to the Curculionidae family. Native to Europe, it has established itself as one of the most damaging pests of rapeseed crops. The adult is about 3 to 4 mm long, with a long rostrum and is distinguished by a dark gray-black body .
As for the larva, it is legless and light-coloured , measuring between 5 and 7 mm. Its head has a yellowish-brown hue. It feeds exclusively on the pith of rapeseed stems, which explains the considerable damage it can cause to crops.
What are the host plants?
The rapeseed stem weevil is closely associated with brassicas. While the adult can feed on many spontaneous brassicas, oviposition occurs only on certain host plants. Among these, rapeseed ( Brassica napus ) is the preferred target.
Other brassicas such as rape ( Brassica rapa var. silvestris ), cabbage ( Brassica oleracea ), turnip and radish ( Brassica rapa ) are also affected.
Development cycle of the rapeseed stem weevil
Ceutorhynchus napi has a single generation per year . Adults overwinter in the soil of the previous year’s rape fields, remaining in their pupal case until early winter. They usually emerge between late January and late March.
Upon emergence, adults feed on nearby brassicas before flying to new rapeseed fields, where they spread rapidly . Egg-laying begins 10–20 days after adult emergence. The female lays eggs inside young stems , just below the terminal bud. Embryonic development lasts several weeks.
The larva goes through three stages of development over a period of 30–40 days, feeding on the pith of rapeseed stems. When mature, it bores a hole near the petiole of a lower leaf, drops to the ground and burrows to pupate. Pupation lasts about 25 days, after which the adult emerges to repeat the cycle.
Damage to rapeseed crops
The main damage caused by the rapeseed stem weevil is related to laying eggs in the stems. These punctures form small holes of about 1 mm in diameter , often surrounded by whitish mucus.
The presence of eggs in the stems causes necrosis of the growing tissues. This phenomenon leads to significant deformations, such as dwarfism of the plants, twisting and splitting of the stems. Affected plants suffer premature drying, which compromises their development and yield.
This damage promotes the entry of diseases, such as phoma ( Leptosphaeria maculans ), which increases the risk of losses for crops.
Cold weather conditions increase the damage. During periods of frost, stems already weakened by egg-laying and necrosis become even more fragile, leading to even more severe crop losses.
How to fight against the rapeseed stem weevil?
Control of the rapeseed stem weevil is based on a combination of preventive measures, monitoring and targeted interventions.
Prevention involves encouraging rapid start-up of vegetation . In fact, a rapeseed stem that reaches 20 cm in height becomes less attractive and less susceptible to weevil attacks. Vigorous growth can thus limit the sensitivity of the stems to egg-laying and subsequent damage.
Monitoring is essential to detect the arrival of the first adults. Trapping helps identify the right time to intervene. It is recommended to treat within 8 to 10 days of the first captures, in order to neutralize the adults before they begin to colonize the plots.
Agrobiotop solutions against the rapeseed stem weevil:
If you would like advice on applying these solutions, please contact us .
