Favourable factors for outbreak of rice swarming caterpillar in dibrugarh, Assam

Abstract

An outbreak of rice swarming caterpillar, Spodoptera mauritia Boisduval occurred during kharif, 2012 in some parts of Dibrugarh district of Assam that caused considerable damage to rice crop in its vegetative stage. The outbreak was so severe that as high as twleve caterpillar were found at the base of rice plants in the day time during the peak period of outbreak in September and about 805 ha area of kharif rice was infested in Khowang and Tipling area. Moreover, mean larval population density/m2 was observed at 36.80 during outbreak against 0.004 in non-outbreak year. Standard Precipitation Index (SPI) was studied to know whether the favourable factor as identified by the IRRI, Phillipines is playing a role. Pre-monsoon rainfall of 922.75 mm received during March-May, 2012 with intermittent dry spells was identified as the reason behind the pest buildup in alternate hosts and in nurseries of rice. SPI analysis of 35 weeks, including the outbreak period confirmed that spells of drought followed by heavy rain played the key role in outbreak. Altogether seven such favourable spells were identified during 35 weeks of study, of which six spells prevailed before the outbreak; the 1st one prevailing for 11 weeks might have played the crucial role in initiating the population buildup of the pest.