The insecticides MIPC, BPMC, carbofuran and diazinon were treated to the brown planthopper (BPH) , Nilaparvata lugens STAL, at the dose levels of $LD_{10};and;LD_{50}$(by topical application)/at the rate of 40kg/ha (product base) (by dusting or broadcasting on potted rice). The BPH population in the first generation was significantly decreased in the topical treatments compared to the untreatment. The number of BPH offsprings was relatively greater at the dose level of $LD_{10};than;of;LD_{50}$. The number of nymphs, however, were greatly varied with the insecticides. The offsprings from the BPH treated with the rate of 40kg/ha showed longer nymphal periods and higher adult emergence, but except diazinon treatment, rather less egg-numbers were observed. The BPH population density was significantly decreased in general up to 38 days after treatment(DAT) in the treated pots. Among the insecticides tested, however, only diazinon induced greater number of insects at the 54 DAT compared to the untreated pots.