230 Population build up of leaf folder in paddy in relation to weather parameters June 2019 Journal of Agrometeorology 21 (2) : 230-232 (June 2019) India is the second largest producer of rice in the world after China, producing about 105.48 million tonnes from an area of 44.11 million hectares during 2014-15 (Annon., 2016). Assam has its climatic and physiographic features favourable for rice cultivation and the crop is grown in a wide range of agro-ecological situations. Sali or winter rice is dominant crop of the state covering 19 lakh hectares (75 % of rice area) followed by boro rice of 4 lakh hectares (16 %) and ahu rice of 2 lakh hectares (9 %) (Annon., 2014). Though diverse agro climatic zones of Assam offers a great potential for cultivation of rice but, its production is severely affected by various biotic stresses.Rice ecosystem acts as a habitat for different pests which drastically diminishes the economic harvest of the crop (Dhaliwal et al., 2010). Nearly, 300 species of insect pests attack paddy at various stages of the crop growth and among them 23 species causes notable damage (Pasalu and Katti, 2006). Cnaphalocrocis medinalis, rice leaf folder, which was considered a minor and sporadic insect pest of rice in several Asian countries, has become a major threat to rice production in tropical and subtropical Asia (Heinrichs et al. ,1985). C. medinalis infestation influences photosynthesis of the crop and may lead to as high as 60-70 per cent leaf damage inflicting yield loss up to 80 per cent (Saikia and Saikia, 1999). Hence, need was felt for thorough understanding of the seasonal incidence of leaf folder and its relationship with the meteorological parameters so as to predict and plan for effective forewarnings and efficient protection measures. The present study was conducted at farmers’ field near Regional Agricultural Research Station, Titabor, Jorhat,(Lat: 26°60' N, Long: 94°20' E andAlt:90 m)during kharif season of 2013 and 2014. ‘Ranjit’ variety of rice was chosen for the present experiment as it is widely grown in Assam and standard agronomic practices were followed with zero pesticide application.The seedlings were transplanted to main plots on 19 th July, during both the years, 2013 and 2014. Plant inspection method was used to record leaf folder for which 20 plants were randomly selected and folds were opened to record the pest population.Observations were started 15 days after transplanting (DAT) and continued at fortnightly interval till harvesting to estimate their fluctuation during various phases of the crop. For assessing the association of leaf folder with weather, fortnightly average meteorological data viz., mean temperature (Tmean), total rainfall (RF), mean relative humidity (RH), and bright sunshine hours (BSSH), preceding each sampling date were collected from RARS, Titabor for the whole cropping season and the data on seasonal incidence of leaf folder was obtained from the field. Two year pooled data were used for working out Karl Pearson’s correlation and t-test was used to understand its statistical significance. An attempt was made to develop a multiple regression equation for identifying the weather parameter playing major role in determining occurrence of leaf folder using fortnightly averaged pooled meteorological data preceding each sampling date as the independent variable and pooled leaf folder population at fortnightly interval as the dependent variable. The best fit regression equation was obtained using XLSTAT software and was selected on the basis of adjusted coefficient of determination (Adjusted R 2 ) and root mean square error (RMSE). Seasonal incidence of C.medinalis in kharif paddy under field condition The activity of leaf folder started soon after establishment of the crop in the main field (Fig. 1). The occurrence of the pest was observed from the first sampling date i.e., 15 DAT to 105 DAT. The population was initially low with a population level of 1.6 larvae/20 plants at 15 DAT. It increased gradually and the peak population (8.2 larvae/ 20 plants) was recorded at 60 DAT i.e., during mid of September. Maximum population at 60 DAT might be due to highest foliage cover resulting in higher leaf area index during that phase (Ko et al., 2017) in combination with favourable weather factors such as reduced rainfall coupled with high humidity, high temperature and diminished Short Communication Seasonal population build-up of leaf folder (Cnaphalocrocis medinalis Guenee)in paddy in response to meteorological parameters RITURAJ SAIKIA 1 , DILIP KUMAR SAIKIA 1 , ATHAR NISHAT ISLAM 2* and ANJUMONI DEVEE 1 1 Department of Entomology, Assam Agricultural University, Jorhat, Assam-785013 2 Department of Agrometeorology, Assam Agricultural University, Jorhat, Assam-785013 * Email: athar.islam790@gmail.com