Developmental rates and population growth of insecticide-resistant and susceptible populations of Sitophilus zeamais

Two experiments were carried out to assess the developmental rate and population growth of four populations of Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae). Two of these populations are resistant to pyrethroid insecticides (Jacarezinho and Juiz de Fora), while the other two are susceptible (Bragança Paulista and Sete Lagoas). The first experiment, with progeny removal and assessment every other day, allowed the estimation of the developmental rates of the four populations. The second experiment, without progeny removal and with monthly assessments, allowed the estimation of population growth and the consequent grain loss caused by each population. Both susceptible populations and the resistant population from Jacarezinho showed similar developmental rates, population growth and grain loss. In contrast, the insecticide-resistant population from Juiz de Fora showed reduced and delayed emergence. This population also had a smaller rate of population growth and high mortality at high densities associated with reduced grain loss. The Jacarezinho population was maintained without insecticide selection for over a decade suggesting that resistance is fixed or nearly so in this population. This explains its good demographic performance as opposed to the Juiz de Fora population, which was recently collected in a grain mill. Pyrethroid resistance in the Juiz de Fora population seems unstable without insecticide selection, which will likely prevent its fixation.