Mechanisms of insecticide resistance in Spodoptera litura Fab. (Lepidoptera: Noctuidae)

In studies on metabolic resistance mechanisms, there was no significant difference in the activity of carboxylesterase between the susceptible and field populations. However, the esterase zymogram analysis showed differences in isozyme profiles of susceptible and field strains. The specific activities of phosphatases, glutathione-S-transferases and mixed-function oxidases in the insecticide resistant field strain were 2.19-, 1.28- and 2.14- folds higher than the susceptible strain. The activity of acetylcholinesterase (AchE) and its inhibition by organophosphorous and carbamate compounds in the susceptible and resistant populations of S. litura was investigated in order to understand the role of target site insensitivity as a resistance mechanism in the pest. There was no significant difference in the activity of AchE noticed in the susceptible and resistant strains of S. litura. However, slightly higher inhibition of the enzyme by methomyl was noticed in the susceptible population (90.16%) compared to the field population (82.64%) and thus suggesting that AchE in the field population was less sensitive to the inhibition by methomyl. Similarly, the inhibition of AchE by the organophosphorous compound, monocrotophos was lesser in the field strain (21.63%) compared to that in susceptible strain (41.11%), thus indicating the reduced sensitivity of the enzyme to this compound also.