Physiological and behavioral resistance to esfenvalerate + fenitrothion in populations of the maize weevil, Sitophilus zeamais

The maize weevil, Sitophilus zeamais (Coleoptera: Curculionidae) is considered the main pest of stored maize in Brazil and its control is achieved mainly by insecticides. The massive and intensive use of these compounds may lead to selection of resistant populations and consequently compromise the control efficacy of this insect pest in Brazilian storage facilities. Therefore, we surveyed physiological and behavioral resistance to the insecticide mixture esfenvalerate + fenitrothion in 27 populations of S. zeamais collected in several Brazilian counties and Paraguay, and also investigated possible costs associated with this phenomenon. The insects were subjected to concentration-mortality bioassays to determine the lethal concentrations LC50 and LC95. The populations were also subjected to two walking trials on surfaces fully-treated and partially-treated with dried insecticide residues for detection of behavioral resistance. We also determined the instantaneous rate of population increase (ri), and body mass of individuals of each population. The concentration-mortality bioassays indicated resistance ratios (at LC50) ranging from 1.00 to 5.02x for the insecticide mixture esfenvalerate + fenitrothion compared with the susceptible standard population (Sete Lagoas). Although the resistance ratios were modest at LC50, they reached up to 232x at LC95 (Votuporanga county, São Paulo, Brazil). The behavioral trait of walking in treated arena varied among populations and sex, but there was no significant avoidance to the insecticide mixture. There was no correlation between physiological and behavioral resistance, indicating that physiological resistance is independent of behavioral resistance in the populations tested. There was no significant difference in the instantaneous rate of increase (ri), and body mass among the insects. Therefore, we conclude that there was no fitness cost associated with the levels of resistance observed in the populations studied.Keywords: Insecticide resistance, Locomotion, Behavioral avoidance, Fitness cost, Rate of population growth