ABSTRACT:The relationship between weather and rice leaffolder population in Kamphaeng Phet and Phichit provinces, Thailand from February 2013 to February 2014 was evaluated. The results showed that the numbers of rice leaffolder larvae were significantly correlated with maximum temperature (r = 0.474) and average temperature (r = 0.375). Increasing temperature within the upper threshold of the species generally promoted insect population growth. Although statistically non-significant, relative humidity and rainfall produced negative correlation with rice leaffolder population (r = −0.249 and −0.091, respectively). Both factors might reduce leaffolder population via promoting activity of microbial control agents and physically dislodging eggs and larvae of rice leaffolder. When the insecticide fipronil was tested against populations of rice leaffolder in conventional and organic paddy fields, the lethal concentration (LC 50 ) value from toxicity tests with a population from a conventional field was significantly higher than with that of the organic field (63.3 versus 51.1 ppm). With continued and prolonged use of fipronil in a rice field, there is a tendency of that rice leaffolder to develop resistance to this insecticide. Resistance management and resistance monitoring should to be seriously considered if effective control is desired.