Pilot application of residual insecticides as grain protectants for paddy stored in silos

M. Rahim, S.H. Ong, M. Rasali and Y. Sapuan

Abstract

Trials were conducted to evaluate three insecticide combinations to protect paddy stored in concrete silos from insect infestations. The water-diluted insecticides were sprayed to moving grain on conveyor at the rate of 1 litre/t prior to storage. Application of the grain protectants did not adversely affect grain quality based on grain temperature and moisture content. The efficacy of each treatment was tested by assaying treated grain with six species of grain beetles every 6 weeks for 9 months. The effectiveness was based on the ability of the treatment to induce 100% mortality of the test population and the production of F₁ and F₂ generations. The combination of pirimiphos-methyl (12.0 mg/kg) + permethrin (1 mg/kg) + piperonyl butoxide (5.0 mg/kg) was the most effective in providing complete protection for at least 4.5 months from all species commonly encountered in stored paddy. Fenitrothion (20.0 mg/kg) + fenvalerate (0.5 mg/kg) + piperonyl butoxide (5.0 mg/kg) provided a minimum 3 months protection from the primary species Sitophilus oryzae (L.), Sitophilus zeamais (Motsch.) and Rhyzopertha dominica (F.) and one of the two prevalent secondary species, Tribolium castaneum (Herbst.). The effectiveness of chlorpyrifos-methyl (10.0 mg/kg) + carbaryl (8.0 mg/kg) to control primary species was limited to immediately after treatment, but the protection from the gazetted quarantine species Trogoderma granarium (Evert) was 7.5 months. This combination was also effective against T. castaneum dan Oryzaephilus surinamensis (L.) for 6 months. The time taken for the residue on paddy to fall to the MRL permitted by FAO/WHO was as follows: carbaryl after 3 months; pirimiphos-methyl before 3 months; chlorpyrifos-methyl and fenitrothion before 1.5 months. Only a small fraction was detected on polished (milled) rice after 1.5–3 months. Synthetic pyrethroids (SPs) were applied at rates below the MRLs. Milling removed significant amount of residues in husks. All the residues that remained in bran were below the MRLs. Only small quantities of residues were found in bran immediately after treatment. The cost of applying grain protectants in bulk grain is estimated at RM1.82/t; 74% of this is the cost of insecticides.

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