Assessment of non-uniform malathion distribution for insect control in a commercial wheat silo

Experimental (non-uniform) and conventional malathion treatments of wheat, stored in 2000 tonne vertical steel bins were compared. For the experimental treatment, grain pretreated with malathion at 750–800 ppm was added to an untreated grain stream during inloading of a silo, so that pretreated grain comprised about 1% of the bulk. Conventional treatment consisted of continuous spraying of a similar grain stream with an emulsion of malathion. Mean overall malathion levels of approximately 5–6 ppm were obtained in both silos.Distribution of insecticide in the two treatments was compared by assaying groups of 25 grains taken from each bin. Significantly increased non-uniformity of distribution of insecticide was found in the experimental treatment which was considered to account for the greater persistence of malathion residues.Bioassays to indicate the potential of adult Tribolium castaneum (Herbst) and Sitophilus oryzae (L.) to develop infestations were conducted on wheat from the experimental and conventional treatments throughout their respective 10 and 8 months storage periods. Results indicated effective suppression of T. castaneum infestation throughout the storage periods in both silos. After 6 months storage, the conventional treatment did not prevent infestation by S. oryzae, whereas the experimental treatment remained effective throughout the 10 months storage period; suggesting that the proposed method has the economic advantage of eliminating retreatment.