As tiny flying insects that have survived on the earth for hundreds of millions of years, mosquitoes spread across many continents and have thousands of hosts. Most male mosquitoes feed on plant juices, while female mosquitoes ectoparasite on the surface of other organisms, piercing the host's skin with piercing-sucking mouthparts to suck blood. Generally speaking, mosquitoes sucking blood will not have much impact on the host, but in the process of sucking, the mosquito's saliva will cause symptoms such as skin rash on the host. In addition, mosquitoes are the vectors of many diseases. It is known that many harmful diseases such as dengue fever, malaria, yellow fever, and Japanese encephalitis are transmitted through mosquitoes.
In view of the continuous threat of mosquitoes to human health, especially in areas with relatively backward medical and sanitation conditions, mosquito repellent and mosquito control have become one of the important topics of human concern. There are many methods of repelling and killing mosquitoes proposed in the market, but in fact, many of them are folk remedies, and there is not enough scientific evidence and experiments to prove their effectiveness. In daily life, mosquito nets, mosquito killing lamps, electric mosquito swatters, mosquito coils, etc. have already become cheap, easy to use, and effective methods for repelling and killing mosquitoes in ordinary people's homes. Mosquito coils are ignited to release smoke due to incomplete combustion to achieve the effect of killing mosquitoes, which is inseparable from the unique chemical composition in the smoke. Interestingly, this kind of substance actually originated from a unique type of plant-pyrethrum.
Pyrethrum is a perennial herb belonging to the Asteraceae family, native to the Mediterranean region and found in Serbia. As early as the 19th century, people discovered that its flowers had a strong poisonous effect on insects, so smart people collected the flowers of pyrethrum and dried them, and mixed them with wood chips and spices to make fumigants. It has also become the earliest prototype of natural "mosquito coil". In 1885, pyrethrum was introduced to Japan via the United States and was cultivated locally. Around 1955, Eiichiro Ueyama (うえやま えいいいちろう), the founder of Dainippon Pyrethrum Co., Ltd. (Dainippon Pyrethrum Co., Ltd.), invented the spiral mosquito coil by using pyrethrum. Since the 1920s, chemists have been interested in the insecticidal substances in pyrethrum and tried to extract and separate the chemical substances. In 1922, chemists Hermann Staudinger and Leopold Ružička first isolated a large number of pyrethrums to obtain the insecticidal active substance-pyrethrin. Through chemical degradation, they proposed that pyrethrins are actually composed of two substances with very similar chemical structures, pyrethrin I and pyrethrin II. Inspired by this, they decided to modify the structure of pyrethrins to obtain substances with stronger insecticidal effects, but failed many times, especially they were still unable to synthesize the five-membered ring in natural pyrethrins and the rings on the ring. The reason for allene branching is that their early research on the structure of pyrethrins was not deep enough.
According to common sense, pyrethrin, a natural insecticide derived from plants, should attract more scientists' attention, but it encountered the "famous" synthetic insecticide DDT in 1939. In 1939, Swiss chemist Paul • Paul Hermann Müller discovered that DDT can quickly kill mosquitoes, lice and crop pests and has high safety. In 1942, commercialized DDT was launched and greatly reduced the incidence of infectious diseases such as malaria, typhoid and cholera in the latter part of World War II . Since then, DDT has been occupying the pesticide market, and there are relatively few studies focusing on natural pyrethrins. It was not until the 1960s that scientists discovered that DDT was extremely difficult to degrade in the environment and would accumulate in animals, seriously disrupting the ecological balance. In 1962, American marine biologist Rachel Louise Carson discussed pesticides and environmental pollution in her book "Silent Spring", which greatly awakened the public's awareness of ecological and environmental protection.
In fact, when the research on DDT was very hot, a small number of scientists in the world still stood firm to conduct a more in-depth exploration of pyrethrins, including its precise chemical structure. As mentioned above, the early studies of Hermann Staudinger and Leopold Ružička were relatively rough, which was limited by the imperfect scientific theory and experimental conditions at that time. With the increasing maturity of separation technology and analysis methods, scientists from all over the world worked together, and it took decades to finally "correct" the structure of pyrethrin proposed in 1924.
More in-depth studies have shown that pyrethrins are actually quite complex mixtures. In addition to the most important components of pyrethrin I and pyrethrin II, there are four compounds with less content in the combination of different "fragments". Although these compounds are structurally different , but have insecticidal effect. Pyrethrin has a wide insecticidal spectrum, can kill many sanitary and agricultural and forestry pests, has no drug resistance, and has little toxicity to humans and other warm-blooded animals, but because of its poor stability to light, it is only suitable for indoor use. It is used to control mosquitoes, flies, and cockroaches and other sanitary pests. It is not suitable for spraying in the field as a pesticide.
In view of the excellent insecticidal effect of pyrethrin, people try to modify its core structure in order to obtain better insecticidal effect. So far, dozens of pyrethrin derivatives have been successfully "modified" and widely used in commercial insecticides. In addition, it has been found that many compounds themselves have no insecticidal effect or have weak insecticidal effect. Once mixed with pyrethrins, they can greatly improve the insecticidal ability. Such substances are called synergists (synergists), such as 40mg of pyrethrins and 420mg of amide (N-isobutylundylenamide) mixed can exert more than 100mg of individual pyrethrin insecticidal ability.
In view of the excellent insecticidal effect of pyrethrin, people try to modify its core structure in order to obtain better insecticidal effect. So far, dozens of pyrethrin derivatives have been successfully "modified" and widely used in commercial insecticides. In addition, it has been found that many compounds themselves have no insecticidal effect or have weak insecticidal effect. Once mixed with pyrethrins, they can greatly improve the insecticidal ability. Such substances are called synergists (synergists), for example, 40mg of pyrethrins and 420mg of amide (N-isobutylundylenamide) mixed can exert more than 100mg of individual pyrethrin insecticidal ability.
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