Testing for malathion resistance in field-collected populations of Cryptolestes ferrugineus (Stephens) and factors affecting reliability of the tests

Twenty-three strains of the rusty grain beetle, Cryptolestes ferrugineus (Stephens), were collected in 1982 and 1983 from grain stored on farms in the prairie provinces of Canada and in Minnesota. Fifteen more stains of C. ferrugineus were collected from primary grain elevators within a 48-km radius of Winnipeg, Manitoba, in July and August 1984. A tentative discriminating concentration of 0.0091 mg (AI)/cm super(2) malathion on filter paper based on probit regressions for four laboratory strains was used to test the strains collected from farms. A reliable discriminating concentration of 0.0130 mg/cm super(2) with 5-h exposure at 25 degree C gave complete knockdown of all 38 strains. None of the strains was resistant to malathion. Subsequent studies on the reliability of the resistance tests indicated that treated filter papers wrapped in aluminum foil could be stored for 24 weeks at -25 or 5 degree C with only slight loss of insecticide as determined by bioassay with the red flour beetle, Tribolium castaneum) (Herbst), and chemical assay.