Organophosphorothioates and synergised synthetic pyrethroids were used in duplicate field trials carried out on bulk wheat in commercial silos in Queensland and New South Wales. Laboratory bioassays using malathion‐resistant strains of insects were carried out on samples of treated grain at intervals over 9 months. These established that all treatments were generally effective. Deltamethrin (2 mg kg−1)+ piperonyl butoxide (8 mg kg−1), fenitrothion (12 mg kg−1)+ fenvalerate (1 mg kg−1)+ piperonyl butoxide (8 mg kg−1), fenitrothion (12 mg kg−1)+ phenothrin (2 mg kg−1)+ piperonyl butoxide (8 mg kg−1) and pirimiphos‐methyl (4 mg kg−1)+ permethrin (1 mg kg−1)+ piperonyl butoxide (8 mg kg−1) controlled common field strains of Sitophilus oryzae (L.) and Rhyzopertha dominica (F.). Against a highly resistant strain of S. oryzae, deltamethrin (2 mg kg−1)+ piperonyl butoxide (8 mg kg−1) was superior to the remaining treatments. All treatment combinations completely prevented progeny production in Tribolium castaneum (Herbst), T. confusum Jacquelin du Val and in Ephestia cautella (Walker). Residues of deltamethrin, fenvalerate, permethrin and phenothrin were determined and shown to be highly persistent on stored wheat. During milling, residues accumulated in the bran fractions and were reduced in white flour. They were not significantly reduced during baking.

Duplicate field trials were carried out on bulk wheat in commercial silos in Queensland and New South Wales. Laboratory bioassays on samples of treated grain at intervals over 9 months, using malathion‐resistant strains of insects, established that treatments were generally effective. Fenitrothion (12 mg kg−1)+ (1R)‐phenothrin (2 mg kg−1) was more effective than pirimiphos‐methyl (6 mg kg−1) + carbaryl (10 mg kg−1) against Sitophilus oryzae (L.) and Ephestia cautella (Walker); the order of effectiveness was reversed for S. granarius (L.). Against Rhyzopertha dominica (F.), Tribolium castaneum (Herbst), T. confusum Jackquelin du Val and Oryzaephilus surinamensis (L.), both treatments effectively prevented the production of progeny. The order of persistence was pirimiphos‐methyl> (1R)‐phenothrin>carbaryl or fenitrothion. During processing from wheat to white bread, residues were reduced by 98% for carbaryl, >44% for (1R)‐phenothrin, 98% for fenitrothion and 85% for pirimiphosmethyl.

Methacrifos (22.5 g t-l) and the three protectant combinations chlorpyrifos-methyl ( l o g t-l) plus bioresmethrin (1 g t-l), fenitrothion (12 g t-l) plus (1R)-phenothrin (2 g t-l) and pirimiphos-methyl (4 g t-I) plus carbaryl(8 g t-1) were each applied to grain that was stored in at least 15 silos. Grain temperature and levels of protectant were regularly monitored, and samples f r o m 12 storages using each treatment were taken f o r laboratory assays against Rhyzopertha dominica and Tribolium castaneum. Grain condition did not deteriorate during storage. Grain remained free of insects in 60 of the 63 storages treated; partial failure in the other 3 storages was attributed to low or irregular levels of protectant. The mean and range of residue values of all protectants were recorded as a function of time and the mean observed values corresponded to predicted values. In laboratory 27 1 Pestic. Sci. 0031-613X/87/$03.50 0 Society of Chemical Industry, 1987. Printed in Great Britain

Field trials with various pesticide combinations were carried out on bulk wheat in commercial silos in Queensland, South Australia and Western Australia. Laboratory bioassays on samples of treated grain at intervals over 8 months using malathionsusceptible and malathion-resistant strains established the following orders of efficacy: against Sitophilus oryzae (L.), chlorpyrifos-methyl 10 mg kg-l+ bioresmethrin 1 mg kg-l= methacrifos 15 mg kg-l in aerated storage > pirimiphos-methyl 4 or 6 mg kg-l+ bioresmethrin 1 mg kg-l= bioresmethrin 4 mg kg-l+ piperonyl butoxide 16 mg kg-1; against Rhyzopertha dominica (F.), bioresmethrin 4 mg kg-lf piperonyl butoxide 16 mg kg-l> methacrifos 15 mg kg-l> chlorpyrifos-methyl 10 mg kg-l + bioresmethrin 1 mg kg-l= pirimiphos-methyl 4 or 6 mg kg-l+ bioresmethrin 1 mg kg-1. All treatments completely prevented production of progeny in Sitophilus granarius (L.), Tribolium castaneum (Herbst), T. confusum Jackquelin du Val and Oryzaephilus surinamensis (L.). The biological efficacy of methacrifos was greater and the rate of degradation lower in aerated than in non-aerated storage. Residue levels of all compounds were determined chemically and were below proposed international residue levels to be considered by the Codex Alimentarius Commission.