Analysis of oilseed rape stem weevil chemical control using a damage rating scale.

Abstract

Rape stem weevil (Ceutorhynchus napi Gyll.) and cabbage stem weevil (C. pallidactylus Marsh.) can cause significant yield losses to oilseed rape (Brassica napus L.), and chemical control is often needed to protect crops from these pests. The efficacy of six insecticides, chlorpyriphos + cypermethrin, bifenthrin, alpha-cypermethrin, pirimiphos-methyl, thiacloprid and tau-fluvalinate, was tested in a 4 year field trial. Besides the standard efficacy analysis expressed through the number of larvae per stem, a damage rating scale was introduced and modelled using a regression model for ordinal categorical data. Compared with the control, expressed through damage rating and larval number, treatments with chlorpyriphos + cypermethrin and bifenthrin, showed higher efficacy in the control of stem weevils compared with alpha-cypermethrin and pirimiphos-methyl. The lowest efficacy was observed in treatments with tau-fluvalinate and thiacloprid. This study showed that a combined efficacy evaluation expressed through both damage rating scale and the count of larvae, supported by an ordinal regression model for data analysis, is indispensable for obtaining accurate results. © 2017 Society of Chemical Industry.