Cotton leafworm, Spodoptera littoralis (Boisd.) is a key pest affecting many field crops and vegetables in Egypt. Therefore, in the current study, the susceptibility of the 2nd instar larvae of S. littoralis laboratory strain to eight insecticides was investigated and the insecticide resistance levels of two field populations were monitored. The two populations were collected from two governorates in Egypt, namely El-Monufia (MS) and El-Fayoum (FS). Resistance monitoring showed that the field population from El-Fayoum was highly resistant to chlorantraniliprole, emamectin benzoate (EMB), spinotram, and spinosad. However, the El-Monufia field population only exhibited high resistance to chlorantraniliprole and spinosad. The relative toxicity showed that the laboratory strain is highly sensitive to EMB (LC50= 0.001 ppm) followed by spinotram (LC50= 0.006 ppm), chlorantraniliprole (LC50= 0.008 ppm), spinosad (LC50= 0.008 ppm), and indoxacarb (LC50= 0.021 ppm), while chlorfenapyr, fipronil, and alpha-cypermethrin exhibited low toxicity to the laboratory strain of S. littoralis. Moreover, the biochemical determinations of detoxification enzymes revealed that carboxylesterase (α, and β -esterase), and AChE activity were significantly increased in the FS population. Thus, Glutathione S-Transferase (GST) showed significant increase in the two populations.
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