ECONOMIC THRESHOLD LEVELS FOR CHILO-PARTELLUS (LEPIDOPTERA, PYRALIDAE) CONTROL ON RESISTANT AND SUSCEPTIBLE SORGHUM PLANTS

The effect of various infestation levels of Chile partellus Swinhoe on resistant and susceptible sorghum genotypes was assessed under artifi cial infestation in two field trials, conducted over two seasons. Ln t he first season, infestation levels were 0, 10, 20, 40, 60 and 80 larv ae per 2m plot row. Ln the second season, additional treatments of one and two larvae per plot were included. A series of all infestation le vels on susceptible plants was also included with an insecticide appli cation 5 days after inoculation. Non-linear relationships between infe station level and yield loss were observed. In susceptible plants, sma ll increases in initial larval infestation levels resulted in sharp in creases in yield loss. An opposite tendency was observed in resistant plants where a tolerant reaction at lower infestation levels was obser ved. Gain threshold (GT) value, indicating the yield increase necessar y to compensate for control costs, economic injury level (EIL) and eco nomic threshold level (ETL) at which control actions should be initiat ed were determined 10% reduction in yield loss. However, the yield was still less than that of unsprayed resistant plants. The GT values ind icated that insecticide application was economically warranted at an E TL of 20% and 2% of plants with visible symptoms of whorl damage, for the resistant and susceptible genotypes respectively.