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132 History of the International Rice Research Instituteter of stems and other morphological characteristics. Resistant varietiesshowed lower percentages of larval survival, lower larval and pupal weights,and slower rate of larval growth. There was a strong negative correlationbetween silica content of the rice varieties and the number of deadhearts in theplants (the first sign of stem borer damage). It was shown that the mandiblesof larvae feeding on varieties with a high silica content became badly worn,whereas those of larvae feeding on low-silica plants appeared to be unaffected.Plants with thin culms seemed to have lower infestation rates.Pathak concluded that the nature of varietal resistance to the stem borer wascomplex. An initial screening of more than 9,000 varieties from the worldcollection found none to be immune to stem borer attack, despite considerablevariation in the degree of susceptibility. To this day, no modern rice variety hasbeen bred that has more than medium resistance to the rice stem borers.By 1966, the entomologists, although still working with BHC, discoveredthat diazinon was a more effective insecticide against the pink stem borer, thegreen leafhopper, and the brown planthopper. Unfortunately, however,within a few years the brown planthopper developed resistance to thatinsecticide.In the same year, Pathak discovered that an Indian variety, TKM6, wasconsistently more resistant to stem borer attack than were most other varieties.It showed considerable resistance as well to the brown planthopper, the tungrovirus disease, and bacterial blight; however, it had a weak stem and was talland leafy. It was used, nevertheless, by the entomologists and the plantbreeders as a parent in crosses with short, stiff-strawed varieties or lines.One cross that turned out to be particularly successful was that betweenTKM6 and (Peta/Taichung Native 1). In tests in 1967, many of the selectionsfrom that cross in the F 4 and F 5 generations proved to be resistant to stem borersand other insects and to several diseases as well. In 1969, one of those selectionswas designated as IR20 and was the first IRRI-named variety that had a trulybroad spectrum of resistance to insects and diseases. Although it inherited arather weak stem from its TKM6 parent and thus could not stand heavyapplications of nitrogen, its disease and insect resistance and its superior grainquality nevertheless made it popular in South and Southeast Asia for severalyears. Indeed, it was a signal advance for IRRI.In 1966-67, Pathak screened more than 1,000 varieties for resistance to thebrown planthopper, which was becoming a major pest wherever 2 or morecrops of rice were grown annually. Although TKM6 showed significantresistance to the brown planthopper, in 1967 the entomologists discovered thatanother Indian variety, Mudgo, was essentially immune to attack by thatinsect. Because Mudgo was tall and leafy and had poor grain quality, it was notuseful as a variety, but its high resistance to the brown planthopper made it amost valuable parent in IRRI’s rice breeding program. When Pathak crossedIR8 with Mudgo, he discovered that 75% of the F 2 population were resistant tobrown planthopper attack and 25% were susceptible. Thus, resistance, likeshort stature, conformed to the Mendelian ratio of 3:l.