Susceptibility of different instars larvae of cotton leafworm, Spodoptera
Iittoralis (Boisd.) (Lepidoptera: Noctuidae) to Bacillus thuringiensis var. kurstaki was
determined in the laboratory using a leaf dip bioassay. The concentration-mortality
was used to estimate the relative susceptibility among the instars of S. littoralis. The
results showed that there was no trend between successive instars, the second and
fourth instars was the most and the least sensitive, respectively, and the third instar
larvae was the least sensitive at second day. A sublethal concentration of B.
thuringiensis var. Kurstaki was studied, the study proved adversely effect on the
surviving larvae. Treated second instars had significantly weight less than the
controls. Additionally, the pupal weight and adult emergence were significantly
reduced. Moreover, reproductive potential and adult sex ratio of survivors was
affected. Second instars of S. littoralis may be the main target to B. thuringiensis var.
Kurstaki control.