Bagged maize stored in large stacks on the Kenya coast is subject to reinvasion by stored-products pests following successful fumigation with methylbromide, and treatment of the stack surfaces with insecticidal sprays has been unsatisfactory in preventing reinfestation.

Further tests are described of different methods of applying insecticides to small experimental stacks, each composed of ten 200-lb. bags of maize, exposed within a warehouse, after fumigation, to reinfestation from a large bulk of maize heavily infested with Tribolium castaneum (Hbst.). Populations of adults and pre-adult stages of T. castaneum were recorded from samples of grain obtained by a sampling spear from each bag at intervals.

In the first of two experiments, single applications of malathion in a spray at 100 and 150 mg. per sq. ft. to the entire external surface of each bag during the construction of the stack did not give good control of reinfestation in the six months after fumigation, but were not significantly worse, respectively, than a dust containing 1 per cent. lindane similarly applied to the upper surface of each bag and, four weeks later, to the outside of the stack at the rates of 3 and 6 oz. per bag. When the malathion treatments were followed by 24, 12 or 8 applications, at intervals of 1, 2 or 3 weeks, respectively, of sprays containing malathion to give 100 and 150 mg. per sq. ft. over the whole external surface of the stack, good control of T. castaneum was obtained with all treatments.

Mean residues of malathion in the outer inch of maize in the bags six months after the start of the experiment varied from 3 to about 19 p.p.m., according to treatment, suggesting that little penetrated through the bag wall and that after thorough mixing of the grain before use the malathion content would not exceed the permissible limit.

In the second experiment, an admixture of malathion dust with the maize before bagging to give 8 p.p.m. active ingredient gave almost complete control of larvae and pupae, and application of 3 oz. 0·5 per cent, malathion dust, or a spray affording 400 mg. malathion per sq. ft., to the upper surface of each bag during stacking gave highly significant reductions in numbers of T. castaneum in the pre-adult stages. The last of these treatments, followed, after fumigation, by malathion applied in a spray at 150 mg. per sq. ft. every three weeks to the external surfaces of the stack, is recommended for large-scale trial.