Pyrethrins derived from

Natural pyrethrins, derived from pyrethrum, contain six insecticidal components. Due to their excellent insecticidal potency against insects in small amounts and their high safety for mammals, they are the only natural insecticidal components used for more than 100 years to date throughout the world as a household insecticide. 

Pyrethrins are naturally occurring pesticides derived from chrysanthemum plants. They are active against many insects and mites. Over-the-counter liquid and gel preparations of pyrethrins with piperonyl butoxide are available for the treatment of pediculosis (piperonyl butoxide inhibits the hydrolytic enzymes that metabolize the pyrethrins in the arthropod). A synthetic pyre-throid, permethrin (Elimite), is available by prescription. A lower concentration of permethrin (Nix) is available without prescription. Pyrethrins and permethrin are quite safe. 

Pyrethrins and pyrethroids are probably the best known and safest classes of natural or synthetic insecticides, widely used in domestic and agricultural applications (1-7). Pyrethrins are natural insecticides derived from the Chrysanthemum cineraria flowers the plant extract, called pyrethrum, is a mixture of six isomers (pyrethrin I and II, cinerin I and II, jasmolin I and II) which was first used in China in the century AD, during the Chou Dinasty. The world pyrethrum market is worth half a billion US dollars [main producers are East Africa highlands (Kenia, Tanzania and Rwanda) and Australia] however, its availability is subject to cyclical trends, due to rains and relations with farmers, who face high harvest costs also due to the fact that the flowers have to be... 

Traditional insecticides were derived from plants. Key amongst them are nicotine and rotenone. Increasing interest is being shown in azadirachtin from the neem tree. The classical natural insecticide pyrethrin is derived from the flowers of Pyrethrum cinerariaefolium, from which the major insecticide group the pyrethroids were derived. 

The natural pyrethrin insecticides are esters derived from the alcohols cinerolone, jasmololone, and pyrethrolone (Fig. 6). Extracts of pyrethrum, the dried flowerheads of Chrysanthemum cinerariae-folium, contain a mixture of six of these ester (Fig. 6) and are commonly available with the addition of the pyrethrum synergist piperonyl butoxide. Variations on these basic structures have... 

All manner of pollutants are considered under the discipline of terrestrial ecotoxicology including pesticides, persistent organic pollutants, other organic substances, and metals and metalloids (e.g., selenium and arsenic). Naturally occurring toxins such as those produced by poisonous plants, snakes, or invertebrates generally are not included unless people intentionally apply them for pest control. For example, the pyrethroid pesticides are derived from the naturally occurring pyrethrin toxin that is found in chrysanthemum. 

Insecticides such as permethrin, benzyl benzoate, lindane, and pyrethrin have been used to treat lice. Pyrethrins are insecticides initially derived from certain species of chrysanthemum flowers. Synthetic pyrethroid insecticide is more stable with a similar activity and low mammalian toxicity and is called permethrin. Lindane (1,2,3,4,5,6 hexachlorocyclohexane) and benzyl benzoate have also been used as pediculocides. Lice infection is spread by direct contact or by wearing the clothing of an infected person. The control of secondary bacterial infection may require an antibiotic. 

Pyrethrins are naturally occurring insecticides derived from the chrysanthemum plant. Pyrethroids (Table 11-48) are synthetically derived compounds. Acute human poisoning from exposure to these insecticides is rare however, they can cause skin and upper-airway irritation and hypersensitivity reactions. Piperonyl butoxide is added to these compounds to prolong their activity by inhibiting mixed oxidase enzymes in the liver that metabolize the pyrethrins. Common pyrethrin-oontaining pedi-culicides include A-200, Triple-X, and RID. 

Synthetic pyrethroid insecticides are photostable analogs of the natural pyrethrins of botanical origin they consist of a series of related esters derived from alcohols and acids that maintain critical isosteric relations with the natural product prototype. Small changes in substituents and stereochemistry are sufficient... 

Among the worldwide total of 30000 known natural products, about 80% stems from plant resources. The number of known chemical structures of plant secondary metabolites is four times the number of known microbial secondary metabolites. Plant secondary metabolites are widely used as valuable medicines (such as paclitaxel, vinblastine, camptothecin, ginsenosides, and artemisinin), food additives, flavors, spices (such as rose oil, vanillin), pigments (such as Sin red and anthocyanins), cosmetics (such as aloe polysaccharides), and bio-pesticides (such as pyrethrins). Currently, a quarter of all prescribed pharmaceuticals compounds in industrialized countries are directly or indirectly derived from plants, or via semi-synthesis. Furthermore, 11% of the 252 drugs considered as basic and essential by the WHO are exclusively derived from plants. According to their biosynthetic pathways, secondary metabolites are usually classified into three large molecule families phenolics, terpenes, and steroids. Some known plant-derived pharmaceuticals are shown in Table 6.1. 

Certain bacteria are able to form C3Q, C45, and CgQ-isoprenoids of the phytoene type in addition to phytoene by head-to-head condensation of appropriate precursor molecules. In higher plants certain monoterpenes probably are synthesized in a similar manner. Chrysanthemyl alcohol, occurring in Compositae, is an analog of presqualene and prephytoene. The C-skeletons given in Fig. 97 are derived from chrysanthemyl pyrophosphate. Chrysanthemum carboxylic acid is a constituent of the pyrethrins, compounds with marked insecticidal activity (E 5.5.3). 

Pyrethrins are a group of closely related, naturally occurring compounds that are the active insecticidal ingredient of pyrethmm and have been used for centuries. Pyrethmm, which has been used as a pesticide in commercial applications since the early nineteenth century, is extracted from the flowers of Chrysanthemum cinerariaefolium and Chrysanthemum cineum [1]. Most of the insecticides derived from plants traditionally have been considered safe for use on animals, and pyrethmm is an example of such material. There are six active insecticidal compounds that comprise the natural pyrethrins pyrethrin 1 and 11, cinerin 1 and 11, and... 

In the last century, the development of organophosphorus, carbamate, and organochloride insecticides was followed by synthetic pyrethroids. As a result, pyrethroids are now used frequently in the domestic milieu. Pyrethroid insecticides are synthetically derived from the molecular structure or sharing the same mechanism of action of natural pyrethrins that have broader spectmm of activity, more stability, and residual activity (persists longer than that of natural pyrethrins) and include the following allethrin, cyfluthrin, cyhalothrin, cypermethrin, deltamethrin, fenvalerate, flumethrin, fluvalinate, tau-fluvalinate, and permethrin (see structure in Fig. 137.2). They are lipophilic compounds and generally of low acute oral toxicity to mammals but are very toxic to aquatic organisms. When synthetic pyrethroids are administered to mammals parenterally, the synthetic pyrethroids are neurotoxic. 

Cyclopropanes are occasionally found in Nature, and one class of naturally occurring cyclopropanes, the pyrethroids, are useful insecticides. These compounds are highly toxic to insects but not to mammals, and because they are rapidly biodegraded, cyclopropanes do not persist in the environment. The naturally occurring pyrethroids are found in members of the chrysanthemum family and are formally derived from chrysanthemic acid. Many modified pyrethrins not found in Nature have been made in the laboratory and several are widely used as pesticides. The molecule known as pyrethrin I has the systematic name 2,2-dimethyl-3-(2-methyl-l-propenyl)cyclopropanecarboxylic acid 2-methyl-4-oxo-3-(Z-2,4-dipentadienyl)-2-cyclopenten-1-yl ester. See why the shorthand is used ... 

Several significant classes of insecticides are derived from plants. These include nicotine fi om tobacco, rotenone extracted from certain legume roots, and pyrethrins (see structural formulas in Figure 12.5). Because of the ways that they are applied and their biodegradabilities, these substances are unlikely to be significant water pollutants. 

Loss by enzymic destruction can be overcome through use of a synergist. Thus pyrethrins in fly sprays are commonly formulated with a methylene-dioxybenzene synergist, often derived from piperic acid. One of the most... 

A newer class of insecticides is the pyrethroids. These are synthetic derivatives of pyrethrins, which are natural extracts from chrysanthemums. Pyrethroids have been developed to be more stable (and thus more effective as insecticides) than the pyrethrins, which are particularly instable in light. Pyrethroids are frequently used as broad-spectrum insecticides. They have high insect toxicity, but lower mammalian toxicity than their organophosphate or carbamate counterparts. Pyrethroids are still limited in effectiveness due to their environmental lability, their high cost, and their potential for resistance development. 

Several nerve toxins produced by plants are interesting because of their insecticidal properties. Insecticidal nicotine is extracted from tobacco. Rotenone (Figure 19.2) is synthesized by almost 70 legumes. This insecticidal compound is safe for most mammals, with the notable exception of swine. The most significant insecticidal plant derivatives, however, are the pyrethrins, discussed below. 

The mechanism of carbamate-resistance, formerly considered to be enhanced hydrolysis (e.g. carbaryl to 1-naphthol), was found to derive almost exclusively from hydroxylation at various points on the molecule, not only the aromatic leaving group but also the N-methyl on the carbamate (Fig. 3), as well as some desmethylation for good measure (14). Pyrethrin-resistance, at first considered to be due to hydrolysis of the alcohol-acid linkage, was also found to be due to an oxidation, occurring at the transmethyl group of the isobutenyl side-chain of the chrysanthemic acid (15). 

Plant-derived insecticides for example, pyrethrum, which is a crude extract from flowers of the pyrethrum plant Chrysanthemum cincerariaefolium. Pyrethrin is a more refined extract containing the six naturally occurring pyrethrins. The greatest activity resides in pyrethrin I. Pyrethrum is regarded as one of the safest insecticides, at least in terms of primary toxicity. It resembles DDT in mode of action, and works at least in part by opening sodium channels in excitable membranes, so causing paralysis. See SODIUM-CHANNEL ACTIVATORS. 

However, it is not only the first generation derivatives of individual cresols but secondary and even tertiary derivatives that have proved to be very important plant growth regulators or agrochemicals apart from their uses as household insect and pest repellents. Synthetically made pyrethroids which are replacing more costly natural pyrethrum or pyrethrins are one such example. 

Chrysanthemol from the leaves of Artemisia ludiviciana (Asteraceae) belongs to the cyclopropane monoterpenes Cinerins, jasmolins and pyrethrins (all including derivatives 1 and 11) are esters of /rara-chrysanthemic and pyrethric acid with terpenoid hydroxypentenones such as cinerolone, jasmolone and pyrethrolone. These are the active insecticidal constituents of pyrethrum recovered from dried flowers of several Chrysanthemum species (e.g. Chrysanthemum cinerariaefolium, Asteraceae). Some synthetic esters of chrysanthemic acid are also applied as insecticides. 

The insecticidal properties of pyrethrum result from six esters that are collectively called pyrethrins. After the isolation and identification of these compounds, a number of derivatives have been synthesized that are more effective than the natural pyrethrins. For example, dimethrin is effective against mosquito larvae and is safe to use (oral LD q for rats is greater than 10,000 mg/kg). The basic pyrethrin structure is shown in black, and the groups that vary in different pyrethrins are shown in yellow. 

---