Parathion is widely used in controlling rice stem borer, Chilo suppressalis, in Japan. However, since parathion is so toxic to mammals, it is desirable to use some other insecticides which are less toxic than parathion but effective to control rice stem borer. Many kinds of less toxic organo-phosphoric insecticides are being tested, but organo-chloric insecticides such as BHC and DDT which were previously evaluated as less effective than parathion should be re-examined on their effectiveness and methods of application.<br>YAMASHINA (1955) and FUKUDA (1955) have found out that only a little of parathion deposits on rice plants by means of present method of spraying. Therefore, the method of application seems to be very important to be improved in controlling rice stem borer.<br>SUGIMOTO & HATAI (1957) reported on the effects of the wetting power and the methods of spraying parathion emulsion to control rice stem borer larvae. Authors have studied more on the wetting power and the methods of spraying insecticide liquids to obtain fundamental references in improving application methods of organo-chloric insecticides.<br>The present paper deals with the comparison of the effectiveness of emulsions of parathion, BHC and DDT, and the influences of the wetting power and volume of spray liquids to the effects of killing rice stem borer larvae.<br>1. Insecticide emulsions used:<br>Commercial emulsifiable concentrates containing parathion 47 percent, lindane 10 percent and DDT 20 percent were used. Triton X-100 was added as a wetting agent when the wetting power of emulsions was to be strengthened. Commercial emulsifiable concentrates named “Specially formulated” containing BHC-γ 15 percent and DDT 20 percent were also used in some of the experiments.<br>2. Methods of application:<br>The egg masses of rice stem borer were attached on the leaf blade of potted rice plants on the previous day of hatching. Insecticides were applied in several methods such as follows:<br>A. Emulsions were injected into the leaf sheaths or into the crevices of leaf sheaths of rice plants using a small syringe.<br>B. Emulsions were sprayed on the leaf sheaths of rice plants by an air blast nozzle (SUGIMOTO & HATAI, 1957). A potted rice plant was sprayed twice, once each from the front side and the opposite side, without using a turn table. Low volume (4cc per pot in a distance of 1.0m or 5cc per pot in a distance of 1.5m) of emulsions were sprayed to deposit insecticides on the outer surface of leaf sheaths. High volume spray (10cc, 20cc and 40cc per pot in a distance of 1.5m) were also done.<br>3. Results:<br>A. The ratio of median lethal concentrations (MLC) of parathion, BHC and DDT to the first brood rice stem borer larvae bored into the leaf sheaths of rice plants was about 1:8:20 when the emulsions were injected into the leaf sheaths, and this ratio did not increase when the emulsions were sprayed on the outer surface of leaf sheaths. Therefore, the effectiveness of such insecticides to rice stem borer larvae are greately different, but the penetration of these insecticides into the inside of leaf sheaths of rice plants from the outer surface of leaf sheaths seems not to be so different.<br>B. The longevity of residual effectiveness of parathion, BHC and DDT to the first brood rice stem borer larvae placed on leaf blades after applications of insecticides did not differ much when the emulsions were sprayed on the outer surface of leaf sheaths. However, BHC and DDT kept their residual effectiveness longer than parathion when the emulsions were injected into the crevices of leaf sheaths.<br>C. The effects of killing rice stem borer larvae obtained by spraying a same dose of an insecticide changing the volume of spray liquids were small in case of low volume spray but became remarkably large in case of high volume spray, and the effect of high volume spray became larger when the wetting power of emulsions were strengthened.