Effect of Combinations of Sublethal Concentrations of Chemical and Microbial Insecticides to Different Larval Instars of Spodoptera litura (Fabricius)

The effect of sublethal concentrations of cypermethrin in combination with Dipel or Biobit in simultaneous or sequential combination to different instars of Spodoptera litura was evaluated. Mixtures of low to high concentration of cypermethrin with highest concentration of Bt resulted in mortality higher than the sum of the individual components to younger instars. The mortality resulting from the mixtures was lower in the older instars suggesting their relative resistance. In the sequential combinations, delay in application of cypermethrin upto 5 d after treatment with Bt, resulted in potentiation. However, the degree of synergistic effects differed between instars and concentration of the components. In the reverse sequence i.e., application of cypermethrin followed by Bt, potentiation resulted from the combinations at 5 d lag, involving higher concentration of both the components. Between the simultaneous and sequential combinations, the later application method resulted in greater potentiation, when applied at 5 d lag in all the instars and, also at 3 d lag in 4 d old larvae.