Susceptibility of field and laboratory strains of Cotton leafworm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae) to spinosad pesticide under laboratory conditions

The susceptibility of field and laboratory strains against all instars larvae of S. littoralis to spinosad pesticide after a 24- and 48-h exposure and under laboratory conditions was investigated. As a result against 1st instar larvae, the LC50 values after 24 h were 12 and 0.275 µg/ml for laboratory and field strain, respectively. In addition, the resistance ratio (RR) of 1st instar was 43.64-fold. In this interim, the 48 h LC50 values were 8.7 and 0.18 µg/ml for laboratory and field strain, respectively and the RR was 48.33-fold, which revealed the field strain was more susceptible to spinosad than the laboratory strain. Distinctly similar trend was shown for later instar larvae stages. For instance, in 6th instar larvae, the LC50 values after a 24-h exposure to spinosad were 1100 and 105 µg/ml for the laboratory and field strain, respectively, and the RR value was 10.48-fold. Furthermore, after a 48-h exposure, the LC50 values for laboratory and field strains were 500 and 42 µg/ml, respectively, with RR value being 11.90-fold. On the other hand, according to relative tolerance values, the 6th instar larvae were the most tolerant instar of all the instars tested. The susceptibility of 6th, 5th, and 4th instar larvae was comparable and significantly lower than that of 3rd, 2nd, and 1st instar larvae. However, the 1st instar was the least tolerant. The results implied that spinosad may play a potential role in the control of S. littoralis and, therefore, it is considered a promising tool in integrated pest management program to control Cotton leafworm which is becoming resistant to conventional pesticides in Egypt.