Influence of Formulation on the Effectiveness of Malathion, Methoxychlor, and Synergized Pyrethrum Protective Sprays for Stored Wheat

Laboratory tests were made in a study to determine the degree of toxicity and repellency to the rice weevil, Sitophilus oryzae (L.) , and the confused flour beetle, Tribolium confusum Duv., of selected dosages of malathion (5 p.p.m.), methoxychlor (15 p.p.m.), and synergized pyre thrum (pyrethrins-1.5 p.p.m. +piperonyl butoxide-15 p.p.m.) applied to wheat in sprays formulated as solutions, emulsions. and wet table-powder suspensions; the residues remaining on wheat at various intervals after application of sprays; the proportional amount of residue on the wheat related to each formulation that is removed by the usual handling and cleaning practices followed at milling; the residues finally resulting in the milled fractions; and the effect on quality of bread as evaluated by taste and odor panels. Two temperature levels and two levels of moisture content were incorporated in the series of tests composed of 36 primary lots of treated wheat and an equivalent amount of unsprayed wheat for appropriate controls. Differences between formulations in the effectiveness of insecticides applied in sprays as protectants against insect infestations in stored wheat were not clearly defined, but differences between the two species of test insects in their response to protective treatments were of particular significance. S. oryzae . appeared to be more susceptible to the toxic effects of insecticides, and T. confu8um appeared to be extremely sensitive to repellent action. In tests with malathion, S. oryzae was more susceptible than T. confusum as indicated by results from toxicity tests. The influence of moisture content and storage temperature on the persistence of biologically effective malathion deposits on wheat was recognized by reduced mortalities of test insects with increases in temperature and moisture content. Repellency indicated for malathion applied in the tetrachloroethylene solution formulation was considered to be due primarily to the solvent used. Residues of malathion persisting on whole grain at various intervals after application of spray decreased with time; the influinence of moisture content and storage temperature on the persistence of residues was indicated by the results from chemical analyses for residues on whole grain. In milled samples the larger amounts of malathion residue were recovered from bran (including germ), shorts, and wheat middlings. Very little malathion carried over into the flour. Methoxychlor treatments failed to kill insects in toxicity tests and demonstrated little or no repellency toward the rice weevil, but they were highly effective in repelling confused flour beetles. larger amounts of methoxychlor residues were recovered from bran than from other milling fractions, but considerable methoxychlor was recovered from flour. Negative results were obtained in toxicity tests with T. confusurn exposed to wheat sprayed with the synergized pyrethrum formulations. Erratic results, but recognizable toxic effects, were recorded in tests with S. oryzae . Mortalities of S. oryzae declined rapidly in tests made after storage of synergized-pyrethrumtreated wheat for more than 1 month after application of sprays. Results from repellency tests conclusively illustrated repellency of the synergized pyre thrum sprays to both species of test insects. Residues persisting on whole grain at various intervals after application of spray decreased with time. Results from chemical analyses for piperonyl but oxide indicated that cleaning operations used in this study were not highly efficient in removing chemical residues from treated wheat. probably because most of the insecticide retained by wheat was in the per carp and endosperm. Residues recovered were greater in bran than from the other milled fractions, but recoveries from flour were considerably higher than expected. Small differences observed between treatments in flavor and odor evaluation of bread made with flour milled from wheat treated with the insecticides tested were not considered of significance.