Toxicity of two pyrethroids (cyfluthrin and fenpropathrin) against cotton leafworm, Spodoptera littoralis, larvae were
collected from heavily sprayed fields in different areas in Dakahlia and El-Beheira Governorate. H+, K+-ATPase (protonpump)
activity was measured in laboratory (lab.) strain and compared with enzyme activities for other collected strains. The
sensitivity of H+, K+-ATPase activity was measured by I50 values. The I50 values of cyfluthrin were 0.24; 0.46; 0.54; 0.65, and
0.68 μM for lab; Abo-Hommos; Shobrakheet; Aga, and El-Senbellaween strains respectively, while I50 values of
fenpropathrin were 0.44; 0.58; 0.66; 0.84, and 0.87 μM for lab and four field strains respectively. Also, proton-pump enzyme
kinetic parameters, Km (Michaelis-Menten kinetics, constant) and Vmax (maximum velocity) values and the inhibition
constant (Ki) were determined. The obtained data proved that cyfluthrin and fenpropathrin compounds were competitive
inhibitors of proton-pump activity. Results indicated that, the pyrethroid have shown potentiality against larvae of S.
littoralis, so, these pyrethroids may be effect of a novel mechanism of action for inhibition of proton-pump activity, these
recommended for S. littoralis larvae control. It could be concluded that the use of pyrethroid, may avoid increasing of
resistance by effect of new mode of action where they disrupt the development of target pest, Therefor proton-pump may play
an important role the mode of action of these pesticides in future.