Microplate assay technique for estimation of esterase activity in a single insect was used in combination
with dose mortality bioassays to detect pyrethroid resistance in three strains of Helicoverpa armigera and
to study the frequency of pyrethroid resistant individuals within the population of the same strain at two
larval stages, third and fifth instar. The third and fifth instar larvae of the field strains i.e., Nagpur strain
and Delhi strain that displayed high degree of resistance towards deltamethrin, had higher esterase activity
compared to a susceptible laboratory strain. The frequency distribution of individuals with elevated
esterase activity above 1.00 absorbance unit was correlated with the resistance level of the strains.
The frequency of resistant individuals in the third instar larvae of Nagpur strain and Delhi strain were
29% and 23%, respectively compared to 4% in the susceptible strain. The resistant individuals in the resistant
strains have markedly increased in the fifth instar larvae with a frequency distribution of 63% and
90% in Delhi strain and Nagpur strain, respectively, while only 14% of individuals was found to have elevated
esterase activity in the susceptible strain. The results demonstrated the role of esterase in pyrethroid
resistance in H. armigera. Microplate assay proved to be a rapid and reliable biochemical
technique for monitoring of pyrethroid resistance in H. armigera