Seham Ismail - Department of Insect Population Toxicology, Central Agricultural Pesticides Laboratory, Agriculture Research Center, Dokki, Giza, Egypt

In insecticide resistance research on cotton leafworm multiple resistance mechanisms, including behavioral resistance, some types of physiological resistance (e.g. increasing activities of esterases), were identified as conferring organophosphate insecticide resistance in Spodoptera littoralis (Boisdüval, ۱۸۳۳) after decades used in Egypt. Enzyme kinetic parameters (Km and Vmax) and in vitro inhibition were used to detect variations in Cholinesterase (ChE) and Aliesterase (Ali-E) activities among different field populations and determine tolerance levels of field populations to chlorpyrifos using the dipping technique. Results revealed that, S. littoralis resistance levels in fourth-instar larvae exposed to chlorpyrifos were ۹- to ۱۲۰-fold in field populations compared with an insecticide-susceptible population. Activity of esterase preparations of larval head homogenates varied, that is the relative activity of ChE and Ali-E were ۱.۷۱ and ۴.۲۳-fold respectively in heavily-sprayed field populations while recorded ۱.۲۷ and ۲.۵۳-fold in desert field populations. The kinetic studies revealed that the Michaelis Constant (Km) and Maximal velocity (Vmax) values of ChE and Ali-E were higher in all field populations than susceptible population. The affinity of ChE, toward alpha-naphthyl acetate (α-NA) as a substrate, in S. littoralis was higher than that of Ali-E in all field populations. The ChE and Ali-E level was higher in the field-populations than its level in susceptible-population. Results revealed that the Ali-E was much more sensitive to in vitro inhibition by chlorpyrifos than ChE in field populations, meaning that ChE had an antagonistic effect on chlorpyrifos.

Aliesterase insensitivity, Cholinesterase insensitivity, esterase enzymes, organophosphorus insecticides resistance, Spodoptera littoralis