MALATHION RESISTANCE IN TRIBOLIUM-CASTANEUM (COLEOPTERA, TENEBRIONIDAE) - DIFFERENCES BETWEEN DISCRIMINATING CONCENTRATIONS BY TOPICAL APPLICATION AND RESIDUAL MORTALITY ON TREATED SURFACES

Field strains of the red flour beetle, Tribolium castaneum (Herbst), previously tested for malathion resistance by topical application of a discriminating concentration of 500 ppm, were exposed on galvanized steel and plywood surfaces treated with 1.63 g (AI)/0.227 m2 (the labeled rate). These field strains and a malathion-susceptible laboratory strain were exposed for 24, 48, and 72 h at weekly intervals for 12 wk. Residues on steel and plywood killed the susceptible laboratory strain for the entire 12 wk of the test. Initial residue deposits were more toxic on steel than on plywood. Three field strains in particular were highly resistant to the surface residues, two field strains were susceptible, and the responses of the remaining strains varied between these extremes. Knockdown and mortality of the field strains to malathion residues on steel and plywood were not correlated with results from the discriminating concentration tests. We conclude that malathion resistance frequencies, as determined by topical application, cannot be used to predict red flour beetle mortality under simulated field conditions.