Resistance to Chlorpyrifos-Methyl, Pirimiphos-Methyl, and Malathion in Brazilian and U.S. Populations of Rhyzopertha dominica (Coleopera: Bostrichidae)

Resistance to chlorpyriphos-methyl, pirimiphos-methyl, and malathion was detected in lesser grain borer, Rhyzopertha dominica (F.), collected from 8 sites in the sates of Mynas Greats and São Paulo in Brazil and from 7 sites in northeast Kansas. These populations were screened for resistance by a discriminating concentration technique, which used vials impregnated with dried insecticide residues. Concentration-mortality regression lines were estimated to compare the resistance ratios for each insecticide and population. Malathion resistance ratios ranged from 2.1- to 12.2-fold at LC50; pirimiphos-methyl resistance ratios ranged from 2.4- to 9.2-fold; and chlorpyrifos-methyl resistance ratios ranged from 5.6- to 167.9-fold. Such high resistance levels to chlorpyrifos-methyl in lesser grain borer have never been reported before either in the United States or in Brazil. Because chlorpyrifos-methyl is not used against stored grain pests in Brazil, we conclude that chlorpyrifos resistance has evolved as a result of selection to other oregano phosphorus insecticides in Brazil.