The formation of insecticidal films on building materials: I. Preliminary experiments with films of pyrethrum and D.D.T. in a heavy oil

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Abstract
The relative toxicity to the flour beetle, Tribolium castaneum Herbst, of films of 1‐6% w/v total pyrethrins or 0.8% w/v technical D.D.T. in Shell Oil P31 sprayed on to some common building materials has been investigated. Beetles were exposed on films for 6 days, starting 24 hr. after application of the insecticide, and relatively low kills were recorded even with deposits two to three times as heavy as those normally used in practice. The materials could be placed in order, according to the toxicities of the films formed on them, as follows:1.6 % pyrethrins 0.8 % D.D.T. Rough deal, sacking Rough deal, limewashed brick Films more toxic Fletton brick Fletton brick Wallboard Cement, cement‐sand, limewashed brick ceiling board, Cement, cement‐sand Films less toxic An experiment carried out under practical conditions confirmed the low toxicities of films formed on limewash and wood.The effect has also been investigated of pretreating bricks with six substances, which might influence the rate of sorption of the oily insecticide into the substrate. Pretreatment with polyvinyl alcohol, starch paste, size, and waterglass greatly increased the toxicity of pyrethrum films on brick, but water and calcium chloride failed to do so. The toxicity of a pyrethrum film on cement‐sand blocks was also very greatly increased by a pretreatment with starch paste.