AbstractDifferential resistance to yellow sugarcane aphid, Sipha flava (Forbes), exists among Digitaria species. Differential resistance among species is expected on the basis of interspecific variation. Differential resistance to aphid damage exists among clones within species. Differences in insect resistance among clones within species are expected on the basis of intraspecific variation. Clones superior in insect resistance were delineated in this trial. The superior performance of phenotypes within species is masked by the average performance of the species. Significant negative correlations were found between insect resistance ratings and the number of rainy days and average temperature during a 28-day interval prior to rating. A model was calculated for the average behavior of species; it includes infestation expected on the basis of the number of rainy days and average temperature during the 28-day interval before rating. The model accounted for approximately 66 percent of the variability present.
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