Bioefficacy of Insecticides Against Tobacco Caterpillar, Spodoptera litura

The bioefficacy of five insecticides viz., indoxacarb, methomyl, fipronil, thiamethoxam and imidacloprid was determined against 7d old larvae of Spodoptera litura by contact and leaf dip methods. Indoxacarb was the most toxic insecticide at 24h exposure, at all the three concentrations, the values being 7.0 ppm (LC30), 15 ppm(LC50) and 126ppm (LC90) and imidacloprid was the least toxic with 281, 572 and 3313ppm at respective LC levels. The order of toxicity by residue contact bioassay method at LC50 was: indoxacarb > methomyl > thiamethoxam > fipronil > imidacloprid. In leaf dip method also at 24h after exposure, at all the three concentrations, indoxacarb was the most toxic insecticide (LC = 1.0, LC = 2.0 and LC = 13ppm) and imidacloprid was the least toxic, 305090 (LC30 = 2186, LC50 = 3719 and LC90 = 10377ppm). The order of toxicity at all the three LC levels was: indoxacarb > methomyl > fipronil > thiamethoxam > imidacloprid. In residue contact bioassay, methomyl could cause 50% mortality in a shortest span of 5.38h followed by imidacloprid (10.47h) and indoxacarb (12.60h). Thiamethoxam and fipronil were slowest in action causing 50% mortality in 33.37 and 23.93h, respectively. In leaf dip method also methomyl was the fastest acting and thiamethoxam was the slowest acting insecticide with LT50 values 13.82 and 39.02h, respectively. The LT50 values for the other three insecticides viz., fipronil, imidacloprid and indoxacarb were 15.72, 18.53 and 21.59h, respectively. Fipronil proved better than imidacloprid and indoxacarb in leaf dip method. A comparative dose mortality response expressed in terms of relative toxicity (RT) indicated that at 48 hours after exposure (HAE), the RT values for indoxacarb and methomyl were 1.0 and 1.2, respectively. Thiamethoxam, imidacloprid and fipronil were 28.54, 37.45 and 44.36 times less toxic than indoxacarb. The RT values in the leaf dip method (stomach toxicity) were relatively higher owing to the higher LC values. Imidacloprid, fipronil and thiamethoxam, in particular, showed very high RT value indicating far less stomach toxicity to S. litura as compared to indoxacarb and methomyl. The data reflect a general increase in tolerance by S. litura in view of the field recommended doses of the insecticides.