Toxicity of rapeseed meal and methyl isothiocyanate to larvae of the black vine weevil (Coleoptera: Curculionidae)

Abstract

Soil amendments of Brassica spp. tissues display toxic effects to a number of soil organisms, including insects. However, application rates necessary to obtain effective insect suppression have not been determined. We tested the toxicity of soil amended with rapeseed, Brassica napus L., seed meal or methyl isothiocyanate to black vine weevil, Otiorhynchus sulcatus (F.), larvae. Control treatments included untreated soil and soil treated with detoxified rapeseed meal. Larvae were exposed to treated soil for 24 h; those remaining motionless 30 min after then extraction from soil were recorded as dead. Mortality data were analyzed assuming the probit model. We estimated LC50s to be 19 g of rapeseed meal or 6 mg of methyl isothiocyanate per kilogram of soil. Total conversion of rapeseed meal glucosinolates (123 μmol/g) could produce up to 44.3 μmol/g of isothioevanates; however, the actual production was ≃ 15% of the expected value. Lethal concentrations of rapeseed meal necessary for effective control in a nursery or field application may be too high for practical use because of low glucosinolate content in commercially grown rapeseed and catiola cultivars. Consequently, Brassica spp. tissues containing higher concentrations of isothiocyanate-generating glucosinolates would have greater insecticidal potential.