Abstract
Liquid insecticide containing 20% isoxathion which was spread over the paddy water showed high toxicity to the rice plant weevil for three days after application. This insecticide was thus applied to the paddy water for various periods of time (0∼6 times) at three-day intervals during the period of migration of the rice plant weevil into the paddy field, and the percentage of injured leaves caused by the insect was compared among the treatments. The results indicated that the application of this insecticide twice suppressed effectively the occurrence of injured leaves. The lethal period in the plot where the application had been carried out twice was estimated to be about seven days after the treatment based on the relation between the amount of isoxathion in water and insect mortality. These results suggest that the period for controlling the rice plant weevil to keep a low injury level was only one week although this insect invades the paddy field for more than one month. This period coincided with the time when the population density of the insect in the paddy field reached about 60% of the peak.
