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Abstract: Pyrethroid insecticides contain natural pyrethrins extracted from pyrethrum flowers, and their synthetic derivatives, pyrethroids. The present article provides an overview of the structure of natural pyrethrins, and the discovery and development of pyrethroids with an emphasis on the background of selected compounds. The stereochemical relationships among pyrethroid secondary alcohols, and toxicologic and environmental effects of pyrethroids are also discussed. Finally, the pyrethroid resistance of mosquitoes and future aspects of pyrethroids are addressed.
Keywords: pyrethrins, pyrethroids, insecticide, stereochemistry, pyrethroid resistance
1. Introduction and early studies
Natural pyrethrum extracts from flowers of Tanacetum cinerariaefolium (Fig. 1(a)) are the main insecticides used for household and post-harvest insect control due to their low mammalian toxicity, rapid knockdown activity, and high efficacy against a wide range of insect pests, especially mosquitoes. These flowers are cultivated in Tasmania (Australia), East Africa (Tanzania, Rwanda, and Kenya), and southern China. In 2016, the amount of dried flowers reached approximately 10,000 metric tons.
In Japan, pyrethrum seeds were independently introduced by Tamari and Ueyama in the 1880s. Ueyama cultivated pyrethrum flowers in the Wakayama prefecture and confirmed the insecticidal activity of pyrethrum extracts. He traveled around Japan, even to Hokkaido, and encouraged people to cultivate pyrethrum flowers, which gradually increased the production of pyrethrum in Japan. Pyrethrum extracts were mainly used to control lice (lice powder) in those days, but Ueyama's wife, Yuki, envisaged a mosquito coil (Fig. 1(b)). The development of the mosquito coil enabled the control of mosquitoes for several hours, and is widely used around the world to control mosquitoes.
The first scientific report was published by a Japanese biologist, Fujitani, in 1909.1) His findings had a remarkable impact on many chemists throughout the world regarding the insecticidal activity of pyrethrum extracts. Another Japanese chemist, Umetaro Suzuki, famous for the discovery of vitamin B1 (Oryzanin) in rice bran, became interested in the insecticidal ingredients in pyrethrum and suggested to his students, Yamamoto and Takei, that they clarify the chemical structure of pyrethrum. In 1923, Yamamoto was the first to report that the chemical structures of the active ingredients in pyrethrum contained a cyclopropane ring.2)
In 1924, Staudinger and Ruzicka, both Nobel prize winners, disclosed their extensive investigation during 1910-1916 on the active ingredients in the pyrethrum extracts, which included...