An experimental study of the fitness of susceptible and resistant strains of Sitophilus oryzae (L.) (Coleoptera: Curculionidae) exposed to insecticide

Abstract

Experiments to assess the effects of sublethal doses of two insecticides, deltamethrin and pirimiphos-methyl, upon demographically-important parameters of one susceptible and three resistant strains of Sitophilus oryzae at 30°C and 55% r.h. are described. The effects upon survivorship proved to be more or less as expected, with mortality increasing with dose. The effects upon fecundity and developmental period were more complex and varied with insecticide, dose and strain. All three resistant strains showed higher fecundity levels than the susceptible strain in the control treatments. The susceptible strain showed the most pronounced effects to exposure to insecticide, with a doubling of fecundity in the first week of exposure to the lowest dose of pirimiphos-methyl, followed by progressive reductions to zero. No progeny were produced at the higher doses. Deltamethrin invariably reduced fecundity in this strain. Exposure to the highest dose of deltamethrin, resulted in a 10% reduction in the geometric mean developmental period in the susceptible strain, due to a skewing of the emergence curve. The other strains showed substantially smaller changes. The overall effect of these responses upon population growth rate is investigated and it is apparent that increasing the dose of an insecticide may not always reduce the population growth rate, even though mortality is increased.