Agroscope, Institute for Plant Production Sciences IPS, Switzerland

Rape-stem weevil: effect on yield and action threshold

Rape-stem weevil (Ceutorhynchus napi Gyll.) has proven harmful for oilseed rape in Switzerland since the introduction of this crop during the Second World War. The impact of this pest on yield and the possibility of establishing an action threshold were determined during tests, from 1981 to 2008. Weevils invade oilseed rape crops in winter, once soil and air temperatures exceed 5 °C and 11 °C respectively. Damage is mainly caused by egg-laying punctures, which cause a physiological disorder in stem growth. The good correlation between the rate of plants presenting symptoms of oviposition when the rape stem is between 5 and 10 cm long and the number of larvae per plant at the time of flowering allows to set an action threshold. At the current price of oilseed rape (0.80 CHF/kg), an increase of yield of approximately 2 kg/are is required to cover treatment costs. Losses of this order are caused by 1 to 3 larvae per plant, corresponding to an action threshold ranging between 45 and 65 % of plants with symptoms of oviposition. The yield increase generated by a treatment is nil if the threshold is not exceeded and on average 2.7 kg/are when it is exceeded. From an economic point of view, it is therefore important to treat only once the threshold is exceeded. Moreover, the premium of 400 CHF/ha (equivalent to 5 kg/are of oilseed rape) currently awarded for extensive crop farming in Switzerland must be taken into account during decision-making.

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