The present work is a trial to study the development and build up of resistance to chlorpyrifos-methyl in a field strain of Aphis craccivora and to evaluate the cross resistance patterns to several conventional and novel insecticides. Additionally, to evaluate the effect of chlorpyrifos-methyl resistance on fitness cost of Aphis craccivora. With the completion of the work, the development of resistance to chlorpyrifos-methyl toxicity in Aphis craccivora as a result of insecticide selection pressure.
The resistance level was amplified over time by continuing exposure to chlorpyrifos-methyl.The resistance ratio (RR) for the selected strain increased from 22.87-fold for the 1st generation to 45.11-fold for the 8th generation and 77.76-fold for the 15th generation. On other word, the oviposition period of the field strain (11.02 days) was longer as compared to resistant strain (6.54) which was shorter than susceptible strain (10.15).The longevity of the chlorpyrifos-methyl resistant strain was (20.68day) and was longer than susceptible aphids (15.24) but field strain was shorter (16.55day). Thefecundity was significantly low in resistant strain of Aphis craccivora. The intrinsic rate of increase (rm) ,the finite rate of increase (λ) and the net reproductive rate (R0) of the resistant strain of Aphis craccivora were significantly decreased comparing by susceptible strain. While, the mean generation time (T) in resistant strain was markedly increased as compared to the susceptible strain of Aphis craccivora. Results indicated that the resistant strain showed high resistance to malathion (27.75-fold) after the 8thgeneration, development of resistance increased to 39.94-fold after the15th generation; and the resistance to imidacloprid was 11.76-fold after the 8thgeneration then developed to 15.28-fold after the 15th generation. The pirimicarb , zeta-cypermethrin and other insecticides with novel mode of action showed no cross resistance with resistance ratio below 5-fold.
So, they can be used in chlorpyrifos-methyl resistance management programs through rotation to reduce the possibility of resistance development in the field. Acetyl cholinesterase (AChE), carboxylesterase activity, Mixed function oxidase (MFO) and glutathione-s-transferase (GST) activity were also measured. Acetyl cholineesterase activity demonstrate a significant role for this enzyme in chlorpyrifos-methyl resistance but other secondary mechanism may be involved.