Comparative toxicity of certain insecticides against laboratory and field populations of Aphis craccivora Kock and Rhopalosiphum padi (L.) (Homoptera:Aphididae)

The responsibility of field population of Aphis craccivora Kock and Rhopalosiphum padi (L.) was investigated toward certain insecticides. The role of carboxylesterase activity asa metabolic mechanism was also studied. Based on the comparisons of the LC50 of field and laboratory populations for each species, the resistance factors indicated that the field population of A. craccivora was less susceptible by 5.77, 1.91, I .54 and I .03-fold resistance for malathion, chlorpyrifos, methomyl and cypermethrin insecticides, respectively, while field population of R. padi was less susceptible than the laboratory population by 5.14, 1.26, 9.21 and 1.58-fold resistance for the same corresponding insecticides, respectively. Carboxylesterase activity by using I-naphthyl acetate (I-NA) as a substrate in susceptible and field population for each species revealed that based on the enzymatic half-life (to.5) values, the carboxylesterase activity in field population of A. craccivora and R. padi were higher than carboxylesterase of the corresponding susceptible populations by 1.7 and 2.25-fold,: respectively. These results suggested that carboxylesterase activity plays a role in the insensitivity of the field populations of the two tested aphid species.