Inert compounds such as insecticidal soaps, oil films on water, and diatomaceous dusts constitute stable insecticides. Active insecticides, such as rotenone, extracted from the roots of derris (Derris elliptica), and natural pyrethrins extracted from the flowers of pyrethrum (Chrysanthemum cineriifolium), are stable insecticides. Rotenone has been used for centuries in the Far East to control body lice, without any suggestion of resistant lice appearing. And pyrethrum flowers have been added to bedding in Dalmatia, for a comparable period, to control fleas and bed bugs, without any suggestion of resistant strains of these parasites appearing.

An extract of natural pyrethrum apparently contains many different pyrethrins, whereas synthetic pyrethroids each consist of a single active ingredient. It is possible that the stability of natural pyrethrins is due to this mixture, this 'cocktail effect'. It may well be the complexity of the mixture that is beyond the capacity for micro-evolution of the parasite. It is not yet clear whether the mixture of pyrethrins occurs within one pyrethrum plant (or clone), or whether it derives from the fact that most pyrethrum crops are genetic mixtures. Both possibilities merit investigation.

Most synthetic insecticides, including the synthetic pyrethroids, however, are unstable. The classic example is DDT, and the appearance of DDT-resistant houseflies and malarial mosquitoes.

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