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Studies were conducted to investigate the performance of eight Bt and five non-Bt cotton genotypes against whitefly and thrips and impact of abiotic factors on the population fluctuation of these sucking pests, at Cotton Research Station, Multan, during 2010 and 2011. The results exhibited that Bt genotypes found more susceptible host for the whitefly and thirps than non-Bt genotypes, during the course of years of study. Among Bt genotypes, maximum and minimum temperature showed significant and positive effect on whitefly population whereas relative humidity exerted negative effect during 2010. During 2011, the effect of all the factors was non significant. On cumulative basis, there was positive correlation between population of whitefly and minimum temperature. But in case of non-Bt, it has negative with maximum temperature whereas relative humidity had a positive effect on whitefly population. Similar trend was observed for thrips population on Bt varieties during both years but on non-Bt varieties only minimum temperature exerted strong positive impact on thrips population. Hierarchical regression models for whitefly and thrips revealed that minimum temperature was the most important factor (Bt and non-Bt varieties). Maximum temperature was the major contributing factor for whitefly fluctuation on Bt varieties during 2010. [ABSTRACT FROM AUTHOR]