Control of Ceutorhynchus napi and C. pallidactylus
Ceutorhynchus napi and C. pallidactylus appear in winter oilseed rape in early spring. Depending on the temperature, the females of C. napi need only a few days of maturation feeding before they start oviposition. With their oviposition, the damage is initiatied and stems may become s-shaped or can burst open. In addition, the larvae damage the plants by their feeding by disruption of xylem and phloem tissues and the transport of water and nutrients, respectively within the stems. During flowering, the larvae complete their development and leave the plants to pupate in the soil, where they stay until the next spring. Compared to C. napi, females of C. pallidactylus need some more time before they start with oviposition (depending on temperature approximately 10-14 days). The larvae also feed within the stems. After finishing development, larvae drop down from the plants to pupate in the soil. In contrast to C. napi, the next generation of C. pallidactylus emerges as soon as June/July from the soil and searches for their overwintering sites in the upper soil layer and leaf litter in edges of forests and hedges. Immigration of both weevil species can be monitored in spring with yellow water traps (YWT). After exceeding the thresholds (> 5 C. napi / > 15 C. pallidactylus per YWT within three days), the only control option in Germany are insecticides from the class of pyrethroids. While C. napi is still sensitive against this insecticide class, C. pallidactylus shows a beginning of resistance to pyrethroids. For effective control, the timing of insecticide application is very important. This requires a better knowledge of the biology of the weevils. Results from field trials near Braunschweig will be reported.