TOXICITY OF RAPESEED MEAL AND METHYL ISOTHIOCYANATE TO LARVAE OF THE BLACK VINE WEEVIL (COLEOPTERA, CURCULIONIDAE)

Soil amendments of Brassica spp. tissues display toxic effects to a nu mber of soil organisms, including insects. However, application rates necessary to obtain effective insect suppression have not been determi ned. We tested the toxicity of soil amended with rapeseed, Brassica na pus L., seed meal or methyl isothiocyanate to black vine weevil, Otior hynchus sulcatus (F.), larvae. Control treatments included untreated s oil and soil treated with detoxified rapeseed meal. Larvae were expose d to treated soil for 24 h; those remaining motionless 30 min after th eir extraction from soil were recorded as dead. Mortality data were an alyzed assuming the probit model. We estimated LC(50s), to be 19 g of rapeseed meal or 6 mg of methyl isothiocyanate per kilogram of soil. T otal conversion of rapeseed meal glucosinolates (123 mu mol/g) could p roduce up to 44.3 mu mol/g of isothiocyanates; however, the actual pro duction was approximate to 15% of the expected value. Lethal concentra tions of rapeseed meal necessary for effective control in a nursery or field application may be too high for practical use because of low gl ucosinolate content in commercially grown rapeseed and canola cultivar s. Consequently, Brassica spp. tissues containing higher concentration s of isothiocyanate-generating glucosinolates would have greater insec ticidal potential.