The role of esterase in pyrethroid resistance was studied in the final larval instar of different strains of the
cotton bollworm, Helicoverpa armigera. The resistant strains viz., Nagpur strain and the Delhi strain were
found to have elevated midgut esterase activity in comparison to the susceptible strain. Nagpur strain
and Delhi strain have 2.24 and 1.73-fold higher esterase activity, respectively, than that of the susceptible
strain. The Native PAGE displayed important differences in the midgut esterase isozyme pattern between
the susceptible and the pyrethroid-resistant strains. Out of the 10 esterase isozyme observed, susceptible
strain lacked three bands, E2, E6 and E10 that were found in the resistant strains. The potency of the synergists
piperonyl butoxide (PBO) and dihydrodillapiole (DDA) as esterase inhibitor were also studied both
in vitro and in vivo. The in vitro results clearly show that both PBO and DDA inhibited esterase activity in
the two resistant strains, while there was almost no esterase inhibition in the homogenate of the susceptible
strain. The in vivo inhibition studies (topical application of PBO and DDA followed by biochemical
analysis) illustrated that PBO- and DDA-esterase binding is rather slow and non permanent process.
Esterase inhibition did not occur immediately after the synergist treatment but at 4 and 8 h post treatment
in case of PBO and DDA, respectively. Native PAGE revealed that the in vivo esterase inhibition
caused by both PBO and DDA was due to the binding of the synergist with the E6 isozyme which was
not present in the susceptible strain.