Evaluation of insecticides in field-control simulators and standard laboratory bioassays against resistant and susceptible Bemisia tabaci (Homoptera: Aleyrodidae) from Sudan

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Abstract
Cypermethrin, profenofos, and a mixture of these insecticides were evaluated against resistant and susceptible whitefly, Bemisa tabaci Gennadius, from Sudan using (a) laboratory technology which simulates the aerial spraying of whitefly infestations on cotton crops, and (b) standard residue-bioassays. Simulator tests in which adult cohorts were given a single insecticide treatment produced results that were unexpected on the basis of standard bioassay data. Resistance to cypermethrin, for example, was expressed in the bioassays but not in the simulator, resistance to profenofos was expressed both in bioassays and in the simulator, resistance to the mixture was expressed in the simulator but in the bioassay the chemicals acted synergistically and there was no resistance. Repeated treatment of resistant populations, comprised of all life stages, over three generations gave further unpredicted results: cypermethrin failed to contact larvae on the under surfaces of leaves and hence did not prevent population growth; profenofos sprayed at field rates controlled all life-stages (despite the expression of resistance) provided the insecticide vapour was not dissipated. An alternation of cypermethrin and profenofos – cypermethrin timed against maximum adult emergence and profenofos timed to coincide with high larval densities – gave the best control. The disparities between the simulator experiments and standard bioassays suggest that techniques simulating the field treatment of all life-stages are better suited for evaluating the practical significance of resistance and potential curative strategies.