author(s)
M Visnupriya and N Muthukrishnan

abstract
Repeated application of insecticides on okra causes in many of the lepidopteran pests may have developed resistance. In general, newly developed compounds are preferred not to show cross resistance with existing insecticides. With this background, field population of Helicoverpa armigera from okra which was known to be resistant to most of the conventional insecticides were collected and subjected to the in vivo toxicity of spinetoram 12 SC to assess whether cross resistance exists or not. After 48 hours, on spinetoram 12 SC treated fruits, LC₅₀s of field larvae were 0.23, 0.84, 3.47 and 6.85 ppm for 2^nd, 3^rd, 4^th and 5^th instars of H. armigera respectively. However, in the laboratory strain, these values were 1.00, 3.54, 31.54 and 85.27 ppm for 2^nd, 3^rd, 4^th and 5^th instars of H. armigera respectively. Laboratory strain of 2^nd instar larvae showed higher LC₅₀ than 2^nd and 3^rd instars of field strain. Resistance ratio was 0.23, 0.24, 0.11 and 0.08 for 2^nd instar up to 5^th instar of H. armigera.