Ahmed M.A.I., Temerak S.A.H., Abdel-Galil F.A.-K., Manna S.H.M. (2016): Susceptibility of field and laboratory strains of Cotton leafworm, Spodoptera littoralis (Boisd.) (Lepidoptera: Noctuidae) to spinosad pesticide under laboratory conditions. Plant Protect. Sci., 52: 128-133.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 1 st instar larvae, the LC 50 values after 24 h were 12 and 0.275 µg/ml for laboratory and field strain, respectively. In addition, the resistance ratio (RR) of 1 st instar was 43.64-fold. In this interim, the 48 h LC 50 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 6 th instar larvae, the LC 50 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 LC 50 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 6 th instar larvae were the most tolerant instar of all the instars tested. The susceptibility of 6 th , 5 th , and 4 th instar larvae was comparable and significantly lower than that of 3 rd , 2 nd , and 1 st instar larvae. However, the 1 st 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.
