Variation in the Toxicity of Organophosphate Insecticides to Field Populations of Spodoptera litura

Spodoptera litura (Fab.) larvae collected from Delhi and Punjab were tested for their susceptibility and resistance levels to commonly used organophosphate insecticides by different bioassay techniques. The toxicity of all the organophosphate insecticides was comparable to both Delhi and Punjab populations and there was not much change in susceptibility of either of the population at LC₅₀ values when evaluated by direct spray and leaf dip method. However, the Punjab population showed 2.27 and 1.36- fold resistance to chlorpyriphos and ethion, respectively than that of Delhi population. At LC₉₀ values, the relative resistance values for quinalphos was 1.32 by leaf dip method, indicating that Punjab population may develop faster resistance against quinalphos as compared to Delhi population.