Impact of Bt cotton expressing cry1ac and cry2ab on cotton leaf worm, Spodoptera litura (F.)

Feeding assays with the top fully opened young, middle, and bottom leaves, squares, young green bolls and whole plant bioassays were conducted at the Agricultural Research Station, Darsi to compare the relative toxicity of commercially released single and dual toxin Bt cotton hybrids viz., Mallika BG, Mallika BGII with their non-Bt counterpart on survival and extent of damage by cotton leaf worm larvae, Spodoptera litura. Oviposition studies revealed that the number of egg masses laid, the fertile egg masses and number of neonates hatched per egg mass were not statistically different under both choice and no-choice situations. In insect feeding assays, Mallika BGII bottom leaves showed highest mortality (73.75) of neonate larvae followed by young leaves and squares (60.00) at 45 DAS and bottom leaves (81.25) followed by squares (70.00), young bolls (65.00), middle leaves (57.50) and terminal leaves (43.75) recorded at 75 DAS. At whole plant stage the degree of tolerance in Mallika BGII under choice and no-choice situations varied and the minimum leaf damage score (3.42), damage on squares (13.33%) and bolls (1.03%) was observed in no-choice situation when compared to leaf damage score of 3.00, square damage of 15.00% and boll damage of 2.86% by third instar larvae in choice situation.