Deposition and insecticidal effect of dust formulation on the brown planthopper, Nilaparvata lugens in paddy rice field

For better control of brown planthoppers (BPH), the insecticide susceptibility and deposition on BPH by dust application were examined in laboratory and in paddy rice field, respectively. When carbaryl was topically applied, the LD50 value was 1.9μg/g for the susceptible strain (S-strain) and 14.6μg/g for the Yamaguchi population (Y-population). When a mixture of carbaryl (NAC) and fenitrothion (MEP) (1.5:2) was applied by the bell-jar dusting method, the LD50 value was 23.1μg/g for the Y-population, from which the LD50 value of NAC was estimated to be 10.2μg/g, similar to the value obtained by topical application. When the dust was applied to the Y-population in paddy rice field, the MEP deposit on BPH was 0.1 to 210μg/g, showing logarithm normal distribution. The mean deposits of MEP by different dusting methods were 3.5 to 25.3μg/g, and those of NAC were estimated to be 2.6 to 18.5μg/g. These values are similar to or smaller than the LD50 value of NAC. From the dosage-mortality curves and the distribution of deposits, it could be inferred that the mortality of Y-population was considerably low, while that of S-strain was high. Such experimental results support the assumption that insecticides here become less effective against BPH because the lethal dose has increased as insecticide resistance developed and because the conventional dust application does not deposit sufficient amounts of insecticides.