Distribution of Cartap and Monocrotophos in Rice Plant and Their Toxicity to the Rice Leafroller, Cnaphalocrocis medinalis Guenée

Abstract

Concentrations of cartap and monocrotophos in rice plants treated with granular formulations at different growth stages were measured, and toxicity of each insecticide, applied to rice plants by root dipping method, to rice leafroller larvae was evaluated. Cartap concentrations in the leaf blades of rice plants at the boot stage and the panicle formation stage reached a maximum of 3.2 and 2.1ppm 3 days after application, respectively, while the concentration of the maximum tillerring stage reached a maximum of 0.94ppm 15 days after application. Concentrations of monocrotophos in the leaf blades of the three stages reached a maximum of 1.4, 2.3 and 2.1ppm 3 days after application, respectively. LC₅₀ values were calculated from mortality and insecticide concentration in the leaf blades, and LC₅₀ (24hr) values in 1st, 3rd and 5th instar larvae for cartap were 0.68, 1.4 and 5.6ppm, and those for monocrotophos 0.60, 1.2 and 4.0ppm, respectively. Periods of exceeding LC₅₀ (24hr) levels for 1st instar larvae of cartap and monocrotophos in the leaf blades were 27 and 10 days at the boot stage, 25 and 9 days at the panicle formation stage and 21 and 11 days at the maximum tillerring stage, respectively.