Toxicity and residue half-life (RL50) of three insecticides (chlorpyrifos-methyl, pirimiphosmethyl and cyfluthrin) on wheat grains were determined through time-mortality bioassays and residue analyses
on wheat grains infested with either the rice weevil Sitophilus oryzae L.), or the red flour bettle Tribolium
castaneum (Herbst). Chlorpyrifos-methyl was the most toxic compound to both insect species followed by
pirimiphos-methyl and cyfluthrin with median lethal concentrations (LC50) of 0.003, 0.0011; 0.008, 0.0028 and
0.023, 0.0280 mg/g grain for the rice weevil and the red flour beetle, respectively. Cyfluthrin exhibited the
greater half-life and thus the longest residual effect followed by pirimiphos-methyl and chlorpyrifos-methyl. In
general, chlorpyrifos-methyl and pirimiphos-methyl exhibited greater loss of effectiveness than cyfluthrin. The
results suggest that the application of the pyrethroid cyfluthrin is likely more suitable for long term storage,
while the organophosphates are better suited for short term of storage. Moreover, the study suggests
withholding period after post treatment is 17, 38 and more than 200 days for chlorpyrifos-methyl, pirimiphosmethyl and cyfluthrin, respectively.