Resistance to Organophosphorus Grain Protectants in Oryzaephilus surinamensis (L.) (Coleoptera: Silvanidae) from Off‐farm Grain Storages in New South Wales

Infestations of Oryzaephilus surinamensis, sawtoothed grain beetle, from consolidated off‐farm grain bulks in New South Wales were monitored for resistance to organophosphorus grain protectants by impregnated filter paper tests. Resistance to fenitrothion, pirimiphos‐methyl or chlorpyrifos‐methyl was detected in 95%, 95% and 67% of the infestations, and maximum resistance factors compared with a susceptible strain were 85, 55 and 32 times, respectively. Most strains were either resistant to all three insecticides or were resistant to fenitrothion and pirimiphos‐methyl only. Grain treated initially with chlorpyrifos‐methyl 10 mg/kg suppressed strains with strong resistance to fenitrothion or chlorpyrifos‐methyl by >90% after 4 months aging. The usefulness of chlorpyrifos‐methyl in a control strategy for effective grain protection is discussed.