~ 1423 ~ Journal of Entomology and Zoology Studies 2020; 8(5): 1423-1427 E-ISSN: 2320-7078 P-ISSN: 2349-6800 www.entomoljournal.com JEZS 2020; 8(5): 1423-1427 © 2020 JEZS Received: 13-07-2020 Accepted: 15-08-2020 M Baruah Regional Agricultural Research Station, Assam Agricultural University, Titabar, Assam. India BC Dutta Department of Entomology, Assam Agricultural University, Jorhat, Assam, India Corresponding Author: M Baruah Regional Agricultural Research Station, Assam Agricultural University, Titabar, Assam, India Effect of planting dates on stem borer incidence and its natural enemies in relation to weather variables in rice ecosystem M Baruah and BC Dutta DOI: https://doi.org/10.22271/j.ento.2020.v8.i5t.7703 Abstract The field experiment was conducted in the rice field of Regional Agricultural Research Station, Assam Agricultural University, Titabar, Assam in randomized block design sowing the seeds in three dates at an interval of twenty days between each sowing, viz., early (15 th May), normal (5 th June) and late (25 th June) separately. Early sown was the least damaged crop ( 9.6% dead heart (DH) and 10.3% white ear head (WEH) as compared to normal (15.5 % DH and 16.7% WEH) and late (24.6% DH and 25.3% WEH) with a yield of 5.3t/ha in early, 4.5 t/ha in normal and 2.4t/ha in late sown crops. Spider population was significantly higher in early than that of normal and late planting, whereas the population of coccinellid beetles and the egg parasitoid, Trichogramma japonicum were non-significantly higher in early than that of normal sown crop, but the population of these natural enemies were significantly lower in late sown rice. The stem borer infestation was significantly higher in late sown crop with increased maximum temperature, morning humidity having a significant positive effect in all the dates of planting. There were a non- significant positive effect on population with minimum temperature (r = 0.34 for DH and r = 0.22 for WEH) and average rainfall (r = 0.20 for DH and r = 0.07 for WEH) at early sown. Maximum temperature and morning relative humidity had a significant positive effect on borer infestation as dead heart and white ear head in connection with a significant positive effect of spider population in three dates of planting. The variation in the planting times work as a means of cultural control by creating asynchrony between crop and insect pests’ phenology along with crop-weather-pest interaction help in developing forewarning system to warn the farmers in advance to avoid manageable yield loss by stem borer incidences. Keywords: Climate change, biotic factor, abiotic factor, dead heart and white ear head Introduction Rice feeds more than two billion people worldwide and is the number one staple food in Asia, where it provides 40-70% of the total food calories consumed [7] . Rice yield has been stagnant for the last three decades despite the improved varieties and technologies due to several factors, out of which abiotic and biotic stress are the major considerable factors. The biotic stresses which greatly constrain rice productions are insect pests and diseases [3] . The abiotic factors such as temperature, rainfall and humidity greatly influence various growth and developmental stages of a crop and indirectly the incidence of insect pests and diseases. The biotic agents like, predators and parasitoids are the natural regulators of insect pest population. Climate changes associated with alteration in sowing and planting dates have a profound effect on the insect pest incidences as well as the population of natural enemies [10] Rice is attacked by a number of pests, the stem borer being the major one, which cause losses up to 90 percent if not managed timely. Globally, stem borer alone causes yield losses of 10 million tones and accounts 50% of all insecticides used in the rice field. Due to the cryptic behavior of larvae and nocturnal habit of the moth, it is very difficult to be managed by using conventional chemical insecticides and biological methods. Ranjit is one of the most popular and extensively grown mega varieties accounting 50% of the total rice grown area in Assam. Henceforth, the effects of biotic and abiotic factors on the crop losses incurred by stem borer in relation to alteration of planting dates of the rice variety, ‘Ranjit’ was undertaken to combat against the adverse affects of present day’s climatic aberrations.