Pyrethrins are chemically related to pyrethrin, which is a secondary metabolite found in the flowers of the pyrethrum plant (Chrysanthemum ciner-ariaefolium). Dried pyrethrum flowers were used as an insecticide in ancient China and in the middle ages in Persia. The dried flowers are still used. Current production is around 20 000 tons per annum centered in Kenya and Tanzania. The pyre-thrins are effective insecticides, having very low dose rates and rapid knockdown of insects but being harmless to mammals under all normal conditions. Natural pyrethrins break down rapidly under the influence of oxygen and UV light. This limits their use in agriculture, but recently synthetic analogs have been developed to overcome these problems. Starting from the structure of the natural product a large number of synthetic compounds have been made. It is worth noting how they differ in effectiveness: deltamethrin is a broad range insecticide; allethrin is particularly toxic to house flies (Musca domestica) but much less effective with other insects; flumethrin is active against cattle ticks; while others are acaricides or miticides with little or no insecticidal activity.
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