A comparison of bioresmethrin, chlorpyrifos-methyl and pimiriphos- methyl as grain protectants against malathion-resistant insects in wheat

Abstract

Impregnated-paper assays and sprayed-grain assays were used to characterise the potency of candidate grain protectants against malathion-resistant and susceptible strains of Sitophilus oryzae (L.), Rhyzopertha dominica (F.), Tribolium castaneum (Hbst.) and Ephestia cautella (Wlk.). Resistance factors for malathion (as measured by impregnated-paper assays) were 4 for S. oryzae, 6 for R. dominica and 39 for T. castaneum. Of 3 synthetic pyrethroids tested, bioresmethrin was more effective than (1R-trans)-allethrin (bioallethrin) or tetramethrin. All synthetic pyrethroids were particularly effective against R. dominica. The classification of organophosphorus materials in descending order of effectiveness was chlorpyrifos-methyl, pirimiphos-methyl and malathion; these were relatively less potent that the pyrethroids against R. dominica. A level of cross-resistance was demonstrated in resistant strains to the newer compounds, but their high potency indicated the possibility of effective pest control at acceptable dosage rates for the present. The stability of the compounds was investigated by exposing treated wheat to conditions in the upper layers of a bulk stored grain for up to 25 weeks. Combinations of chlorpyrifos-methyl or pirimiphos-methyl with bioresmethrin are suggested as grain protectants, subject to the necessary international approval of residues.