Damage loss assessment and control of the cabbage seedpod weevil (Coleoptera: Curculionidae) in winter canola using insecticides

Abstract

Experiments examining the efficacy, timing, and number of applications of various insecticides were used to assess cabbage seedpod weevil, Ceutorhynchus assimilis (Paykull), yield loss relationships in winter canola, Brassica napus L. Typically, the pyrethroid insecticides bifenthrin, esfenvalerate, permethrin, and zetacypermethrin were more effective than the currently registered insecticides endosulfan and methyl parathion in reducing adult numbers and preventing pod infestation by larvae. Two insecticide applications during flowering usually were needed to effectively reduce adult numbers and to prevent seed injury. Larval injury primarily affected grain weight by reducing seed weight and number of seeds per pod. One, 2, and 3 larvae per pod reduced seed weight per pod by 20.2, 38.1 and 52.2%, respectively. Larval injury did not consistently affect kernel weight or grain oil content. Yield loss increased linearly by ≃1.7% for each 1% increase in percentage of infested pods, when larval infestation of pods exceeded 23% infested pods. These results support findings from Europe that canola can tolerate pod infestations of ≥26% without measurable yield loss. However, depending on control costs and commodity value, preemptive insecticidal control was not justified until pod infestations exceeded 26-40% infested pods. These results provide a quantitative basis for the development of decision rules for C. assimilis which will minimize unnecessary insecticide use on canola in the United States.