Synergistic effects of suppress resistance of cypermethrin and chlorpyrifos in the cotton leafworm, Spodoptera littoralis (Boisduval) populations in Egypt.

Insecticide resistance in Egypt is a major barrier to the successful management of Spodoptera littoralis (Lepidoptera: Noctuidae). One way to overcome insecticide resistance could be the formulation of insecticides with synergists that counteract metabolic resistance. Therefore, the effect of three synergists on the efficacy of cypermethrin and chlorpyrifos was evaluated in three S. littoralis populations. The Marsa Matruh (MAR) and Damietta (DAM) populations were more susceptible to cypermethrin and chlorpyrifos than the Kafr El-Shaikh (KAF) population, which showed an increase in resistance levels (ranging from 48.49- to 170.86-fold). Resistance levels in all field populations decreased markedly when piperonyl butoxide (PBO) synergized cypermethrin, with a synergism ratio (SR) of 2.47 and significantly suppressing 59% of resistance in the highly resistant population of S. littoralis collected from KAF. In addition, S,S,S-tributyl phosphorotrithioate (DEF) suppressed chlorpyrifos resistance in the KAF population by 24% with an SR of 1.32-fold higher than chlorpyrifos alone. On the contrary, diethyl maleate (DEM) did not cause a statistically significant change in the toxicity of both cypermethrin and chlorpyrifos in all field populations. From these findings, it is recommended to use synergists as a countermeasure to the problem of insecticide resistance in S. littoralis and to restore the efficacy of these insecticides in Egyptian fields. [ABSTRACT FROM AUTHOR]