Field Efficacy of Certain Microbial Insecticides against Plutella xylostella Linnaeus and Pieris brassicae Linnaeus under Cabbage-Crop-Ecosystem of Manipur

A field trial was experimented during Rabi, 2014-15 at the Vegetable Research Farm of College of Agriculture, Central Agricultural University, Imphal to evaluate certain microbial pesticides against the diamond back moth, Plutella xylostella Linnaeus and the cabbage butterfly, Pieris brassicae Linnaeus and their effect on the population of lady bird beetle, Coccinella septempunctata Linnaeus in cabbage -crop-ecosystem of Manipur. The results on the efficacy of insecticides against P. xylostella and P. brassicae revealed that all the insecticidal treatments resulted in significantly suppression of both the pests' incidence. However, bioasp (Bacillus thuringiensis var. kurstaki) @ 1000 g ha^-1 was the most effective insecticide against P. xylostella with a record of minimum mean leaf damage of 13.32 per cent as against 69.18% in untreated control, closely followed by spinosad 2.5 SC applied @ 500 ml ha^-1(14.22% LD) which did not differ significantly from each other. Against P. brassicae spinosad 2.5 SC registered significantly the lowest mean leaf damage of 24.30 per cent as against 87.38% in untreated check, closely followed by myco-jaal 10 SC (Beauveria bassiana) with a record of 26.59 per cent leaf damage but, differed significantly between them. The highest mean leaf damage incidence (37.47% LD) was noticed in the plots treated with brigade (Vertcillium lecanii) applied @ 1250 g ha^-1. The results on toxic effect of insecticides on the population of C. septempunctata revealed that bioasp (B. thuringiensis var. kurstaki) @ 1000 g ha^-1 proved to be the safest insecticide with significantly highest population of 1.99/ 5 plants as against 3.04/5 plants recorded in untreated control, followed by myco-jaal 10 SC (B. bassiana) @ 500 ml ha^-1 (1.61 beetles/ 5 plants) and spinosad 2.5 SC @ 500 ml ha^-1 (1.51 beetles/ 5 plants) which did not differ significantly from each other. The lowest population C. septempunctata (0.55/plants) was observed in the malathion 50 EC @ 500 ml ha^-1 treated plots. The highest mean cabbage yield (24.77 t ha^-1) was harvested from the plots treated with spinosad 2.5 SC with the maximum yield increase over control of 9.37 t ha^-1 which showed non significant difference with the plots of bioasp (B. thuringiensis var. kurstaki (23.70 t ha^-1) with its yield increase over control of 8.30 t ha^-1, whereas verticel (V. lecanii) @ 1000 g ha^-1 treated plots accrued significantly lowest cabbage yield of 19.27 t ha^-1 with a record of lowest yield increase over control of 3.87 t ha^-1. The avoidable yield loss was computed to be 37.82 per cent in the untreated control plots. Application of insecticides resulted in reduction of the mean avoidable loss, which ranged between 4.32 and 22.21 per cent in different insecticidal treatments, the lowest being in bioasp (B. thuringiensis var. kurstaki) and highest in verticel (V. lecanii). The net profit of the insecticidal treatments varied from Rs. 17860.60 (verticel) to Rs. 45621.60 (spinosad) with the cost : benefit ratios ranging between 1:11.99 and 1:43.09, the minimum and maximum being with verticel and myco-jaal 10 SC, respectively.