Population fluctuation of rice leaffolder, (Cnaphalocrocis medinalis) in two consecutive rice seasons

Larvae of rice leaffolder, Cnaphalocrocismedinalis attack rice crop at all three phases (from tillering until maturity stages). Their population throughout season was influenced by abiotic and biotic factors. Study on population fluctuation is important to understand the population dynamic and its factors in natural field condition. Thus, a study of population fluctuation of C. medinalis was conducted at rice field in Semanggol, Perak. Larvae were collected from 10 rice hills per plot in three 15m x 4 m plots started on April 5, 2015 until June 7, 2015 (off season)and from October 25, 2015 until December 27, 2015 (main season). Abiotic data namely temperature, relative humidity (RH) and rainfall were obtained from Department of Meteorology Malaysia. Comparison of C. medinalis population between two seasons at different weeks was analysed using two-way ANOVA. The relationship of C. medinalis population with abiotic factors was analysed using Pearson correlation and stepwise regression. There was a significant interaction effect between season and week (F₉,₄₀=2.19; p<0.05). The highest population was recorded at week 55 day after transplanting(DAT) in main season comprised of 17% of total collection, followed by week 62 in off season(14%), week 48-main season(13%)and week 55-off season(13%). There was a positive correlation between C. medinalis population and RH in main and off season with RH was the key factor in regulating population in both seasons. Our results revealed that population of C. medinalis at different weeks was influenced by season. It is due to differences of weather condition between seasons, management practices and plant stages.The highest population recorded during reproductive phase is due to the morphology of rice plant that offers great suitability for larvae feeding and development. The abiotic factors also influenced C. medinalis population. Therefore, the abiotic factors should be considered in management of C. medinalis besides the plant stage and fertilizer effect.Future research on effect of natural enemies on C. medinalis population in field is needed in order to gain a better understanding of the factors that influence the population.