Studies on yield loss of rice due to mixed populations of major rice pests:yellow stem borer (Scirpophaga incertulas (Walker)), rice leaf folder(Cnaphalocrocis medinalis (Guenee)) and brown planthopper (Nilaparvata lugens (Stal))

Under rice field condition at Pathum Thani Rice Research Center, twenty-day old seedlings of RD 7 were transplanted and covered with nylon net until flowering. The infestation were conducted with brown planthopper, and rice leaf folder, at 35 and 50 days, and 55 days after transplanted respectively. Brown planthopper released on 35 day after transplanted at 0.6 hopper/tiller made hopperburn in 30 days with no yield. The damage was more severe than damage of brown planthopper released on 50 days after transplanted. However, yield on plot of brown planthopper released on 50 days after transplanted were harvested before hopperburn. Yield on plot of brown planthopper at 50 days after transplanted and brown planthopper at 50 days after transplanted plus rice leaf folder compared with uninfested treatment were lossed 72 % and 87 % respectively. Under screenhouse condition at Bangkok. Artificial infestation of the major insect pests;brown planthopper, rice leaf folder and yellow stem borer, were done by single and mixed population at 60, 65 and 75 days after transplanted respectively. Mylar cages were covered each pot. Brown planthopper was the most important insect pest and at 0.6 hopper/tiller made rice in pot hopperburn in 33 days with no yield. The fastest and the most severity of damage was on combination of brown planthopper and yellow stem borer. As similar as this occurrence was shown in the pot of the combination of brown planthopper and rice leaf folder as well as single brown planthopper. Combination of rice leaf folder and yellow stem borer, single yellow stem borer and single rice leaf folder showed less damage respectively. After harvesting, yield and yield components of each treatment were statistical analysis and comparative. Number of panicle per hill was significantly reduced by yellow stem borer. Percent filled grains per hill and number of filled grains per panicle were significantly lower in treatment of the rice leaf folder. The last yield component in weight of 1,000 filled grains was not affected by these insect pests.