Department of Entomology, J. N. Krishi Vishwa Vidyalaya, Jabalpur (M. P.)
The most injurious fifth-instar larvae of Mythimna separata (Walker) were released in five fixed densities of 1 to 10 larvae/bill under the cages at the most vulnerable panicle stage of Kranti’ variety of rice in the field (zero larva as the control) in the rainy season of 1980–81. The leaf area consumed, number of spikelets and grains fallen, weight of total grains fallen, and yield loss were more due to higher larval densities of 7 and 10 than in the lower ones of 1 and 3/hill. The avoidable yield loss ranged from 12.82 to 35.89 percent, i. e. 125 to 35.0 q/ha, by 1 to 10 larvae/hill.
With an increase of every one larva/hill, there was an increase of 2.78 per cent foliage loss, 3.11 fallen spikelets. 2.71 fallen grains and 0.51 g in the weight of total fallen grains, and a yield reduction of 1.24 g/hill. With an increase of every one per cent of foliage loss by the larvae, there was a yield reduction of 0.41 g/hill.
The economic threshold (cost-benefit ratio 1:1) of the pest was calculated to be 0.423 larva/hill (10 earheads) at the panicle stage of rice crop, causing an estimated yield loss of 5.12 per cent, while the economic injury level was regarded as one larva/hill causing 5.43 per cent foliage loss and 12.8. per eent yield loss. Use of insecticide is suggested when the average larval population level ot M. separata exceeds 0.423 and is likely to increase to one per hill at the panicle stage of rice crop.
