Author(s): K. CHAKRABORTY AND J.GHOSH

Abstract: Field studies were conducted in West Bengal, India, in 2006-07, to determine the egg mass estimation (ETL) of the yellow stem borer (Scirpophaga incertulas; YSB) in relation to seasonal variation. Rice cv. MTU 7029 was cultivated by a standard technique having a fertilizer rate of 60:30:30 kg N:P:K/ha. Fields were examined at fortnightly intervals from the early vegetative to the maturation stages. A total of 25 hills were randomly inspected for YSB egg masses. The plant parts bearing egg masses were cut off and the percentage of hatchability was calculated. Four freshly cut leaves bearing egg masses were collected at different growth stages from the paddy field at 15-day intervals from the vegetative to the maturation phase and placed inside a plastic vial plugged with wet tissue paper. The average percentage of parasite emergence was recorded. Seven parasites were recorded from both the infected eggs and larval stages of YSB (Tetrastichus sp., Stenobracon sp., Apanteles sp.,Chelonus sp., Telenomus sp., Temelucha sp. and Isotima sp.), but their relative abundance and effectivity depended on the prevailing agroclimatic situation and stages of the life cycle of the host occurring in the field. Available egg masses of YSB befitted the field adult incidence. The rate of parasitization and inactivation was found to be density dependent and accordingly varied in different seasons. The higher the availability of egg masses the greater was the rate of parasitization. In general, the least number of active eggs was noted in January and the highest in November. The temperature, relative humidity, rainfall and sunshine hours also had significant impacts on the YSB egg masses.

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