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Suitable time of insecticide application was tested in relation to the seasonal prevalence of the brown planthopper. Population growth of the brown planthopper in rice field begins with macropterous immigrants from the late June to the middle of July (the 2nd adult period of the year), followed by two generations in which eggs to be source of the next generation are laid by brachypterous females from the end of July to the middle of August (the 3rd adult period) and from the end of August to September (the 4th adult period). Hopperburns appear mostly from the end of September to October along with growing of nymphs produced by the 4th period adults. Controls in later seasons are often little effective because of survival of eggs from insecticides, owing to wide and overlapping generation. It was proposed to advance controlling time and to eliminate the brachypterous females. Times of treatment were selected as follows, the beginning

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