Development and Application of Botanical Insecticides in China
BackTime：2016-06-06
The plant resources in our country are extremely abundant and the species, distribution and system relationship of over 30 thousand kinds of higher plants have been ascertained and nearly a thousand kinds of plants contain insecticidal active substances. There is a long history of pest killing with plants in our country and as early as over 2000 years ago, “Jianshi removed the moths by hand, attacked them with coils and fumigated them with illicium lanceolatum” was recorded in the Rites of Zhou; Stellera chamaejasme, radix stemonae and derris radix are also recorded in the Compendium of Materia Medica; 220 kinds of botanical pesticides distributed in 86 families are recorded in the Indigenous Pesticides in China and over 1300 kinds of poisonous plants are listed in the Poisonous Plants in China and many of them are utilized as botanical pesticides. Although there are abundant medicinal plants in our country and there is a long history of botanical pesticide application, the really systematic study began in 1930s (Huang Ruilun, 1937) and the progress is very slow. The studies on botanical insecticides in our country have been carried out widely since 1980s with some important achievements obtained and have approached or reached the international advanced level on certain aspects. As the botanical insecticides have such features as not polluting the environment, safe to human and livestock and that the pests are not liable to develop resistance, so when people increasingly recognize that the ecological environment on which human relies for existence is seriously polluted by chemicals substances and requires to be purified urgently, it is imperative to know and study the biological pesticides again. Therefore, in recent years the studies on botanical insecticidal active substances in our country, especially those on insect deterrent, antifeedant, growth and development inhibitor, etc. are being carried out widely.
1 Type of insecticidal active substances in natural plants
The insecticidal active substances in natural plants are extremely abundant and can be generally grouped as follows in accordance with the chemical structure:
1.1 Alkaloids
In ancient times, people knew that botanical alkaloids could be used for pest control (Zhao Shanhuan, 1983). There are a variety of action modes of alkaloids on insects, e.g. poisoning, deterrence, antifeedant, growth and development inhibition, etc. and there are also various alkaloids having these functions (Luo Wanchun, 1995), mainly including nicotine, physostigmine, wilforine, stemonine, matrine, veratrine, coptisine, berberine, camptothecin, cephalotaxin, hyoscyamine, conine, aconitine, piperine, capsaicin, etc.
1.2 Terpenes
This type of compounds include pinene, monoterpenes, sesquiterpene, diterpenes and triterpenes and mostly exist in the form of ester derivatives. There are various aspects of pest control effects, e.g. stomach poisoning, contact poisoning, deterrence, narcotic, growth and development inhibition, etc. This type of compounds are mainly toosendanin, celastrus angulatus ester I, celastrus angulatus ester II, celastrus angulatus ester III, celastrus angulatus ester IV, triptolide, rhodojaponin, daphnetoxin, grayanotoxin, asebotoxin (Zhu Zhengfang, 1992), α-pinene, β-pinene, etc.
1.3 Flavonoids
The flavonoid compounds mostly exist in the state of glucosides or aglycones, biglucosides or triglucosides and those having pest control effects are mainly rotenone, elliptinone, etc.
1.4 Volatile oils
The essential volatile oils have not only poisoning, deterrent, antifeedant and growth and development inhibiting effects on the pests, but also insect sex pheromone and attracting effects (Xu Hanhong and Zhao Shanhuan, 1994). There are various volatile oils that can be used for pest control, mainly including eucalyptus oil, peppermint oil, thyme oil, turpentine oil, dianthus chinensis oil, black pepper oil, coriander oil, tanacetum vulgare oil, citronella oil, calamus oil, garland chrysanthemum essential oil, santalol, anethole, tanacetum vulgare oil, rue essential oil, cinnamon essential oil, star anise essential oil, etc.
1.5 Phototoxicity
This type of substances have a lethality on the pests that will be enhanced by several or even thousands of times under light and exist widely in the plants. They are mainly thiophenes, e.g. α-trithiophene; polyacetylenes, e.g. capillene; quinones, e.g. hypericin; coumarins, e.g. xanthotoxin.
1.6 Others
For example, pyrethrin (carboxylic acid esters), diethyl ether acyl phrymarolin (lignans), inokosterone (steroids), tomatine (glycosides), etc.
2 Overview of studies on botanical insecticides
It is well known that the botanical insecticidal active substances have a great potential to be used for pest control, therefore, the domestic and foreign studies are also very active. The studies on botanical insecticides in our country began in the early 20th century and as early as 1903, Shen Zhuwen introduced the cultivating and manufacturing methods of pyrethrum and began to study the pyrethrum in 1918, while a large amount of study work was concentrated in the period of 1932～1936 (Zhao Yucheng, 1932; Zheng Ningyuan, 1934; Li Shichang, 1935; Yang Ningzhen, 1936). Meanwhile, some study work was also carried out for the chemical compositions and insecticidal effects of tripterygium wilfordii (Chen Tongsu, 1933; Gu Xuan, 1934) and derris trifoliata (Zheng Naitao, 1935).
The meliaceae plants mainly studied in our country are Melia toosendan Sieb. et Zucc. and Melia azedarach L. The study on toosendanin began in 1953 in order to explore the active ingredient in Melia toosendan Sieb. et Zucc. for ascarid expelling and a kind of active ingredient was separated from the phloem in 1955 and was named toosendanin (Zhao Shanhuan et al., 1987). This active ingredient is a kind of furan triterpene compound with its chemical structural formula defined (Zhong Chichang, 1975; Shu Guoxin and Liang Xiaotian, 1980). The insecticidal effect of melia azedarach is recorded in ancient books, e.g. “The flowers placed under the terpene can kill the lice and fleas” in Compendium of Materia Medica. In 1980, the scholars in South China Agricultural University began to study the insecticidal effect of toosendanin. It was found in the bioassay for the larvae of tryporyza incertulas that the toosendanin had strong antifeedant and positioning effects (Zhao Shanhuan and Zhang Xin, 1982) and then pest control tests for toosendanin were widely carried out in different crops, vegetables and fruit trees. These tests show that toosendanin has a good control effect on various pests of these crops, e.g. cabbage worm (Wang Wenlu et al., 1992; Zhang Xing and Zhang Shanhuan, 1992a, 1992b; 1989; Zhao Shanhuan et al., 1985), plutella xylostella, mamestra brassicae, aulacophora femoralis (Zhang Xing et al., 1993), citrus mites (Zhao Shanhuan et al., 1986; Wei Xikui, 1989), archips fuscocupreanus, fenusasp, trichiosoma bombifouma (Zhang Xing et al., 1993), chilo suppressalis, rice planthopper, cnaphalocrocis medinalis guenee (Hu Jianzhang et al., 1983; Du Zhengwen, 1986), ostrinia nubilalis (Wang Wenlu et al., 1992; Zhao Shanhuan et al., 1984; 1985), armyworm (Liao Chunyan et al., 1986), etc. The Azadirachta indica A.Juss. was introduced in our country in 1983 and then in Wanning Country, Hainan Province on a large scale in 1986 (Zhao Shanhuan et al., 1989). According to the studies on domestic Azadirachta indica A.Juss., it is primarily found that azadirachtin is contained in the bark of domestic Azadirachta indica A.Juss., which is not recorded in the literatures (Zhang Xing and Zhao Shanhuan, 1992a). The studies show that all the insecticidal active substances contained in the meliaceae plants like azadirachtin are tetracyclic triterpenoids (Zhang Xing and Zhao Shanhuan et al., 1992a). Azadirachtin is internationally recognized as the most important insect antifeedant and many studies show that azadirachtin has antifeedant, contact positioning, stomach positioning, growth and development inhibiting and ovarian development inhibiting effects on various kinds of pests.
There is a long history in the agricultural application in our country that the velamen powder, leaves powder and bark of Celastrus angulatus Maxim. are used for vegetable pest control (Ke Zhiguo et al., 1993). In 1930s, the scientists in our country carried out insecticidal effect tests for celastrus angulatus (Huang Ruilun, 1957) and proved that the velamen powder of celastrus angulatus had contact positioning, stomach positioning and deterrent effects on malacosoma neustria testacea in 1950 (Zhao Shanhuan, 1950; Ke Zhiguo et al., 1993). In 1950s, the Pesticide Teaching and Research Group of Beijing Agricultural University studied the chemical composition of the velamen of celastrus angulatus and proved that all the insecticidal active ingredients of celastrus angulatus could be leached in ethanol. According to the studies on the main pest control effects of the velamen powder and seed oil of celastrus angulatus on the vegetables and stored grains, it is primarily ascertained that the velamen powder and seed oil of celastrus angulatus can obviously control the formation of population of the pests like sitophilus zeamais (Ke Zhiguo et al., 1987; 1992). Currently, tens of new compounds have been separated and identified from the velamen or seed, especially the 4 crystals obtained from the seed oil, i.e. celastrus angulatus esters I～IV (Nan Yusheng and Lu Lingxian, 1995) and the 5 natural substances obtained from the velamen, i.e. celastrus angulatus I～V (Wu Wenjun et al., 1993). The structures of the above active ingredients have been identified and all of them are dihydroagarofuran sesquiterpene esters or alkaloids. These substances have positioning, antifeedant and narcotic effects on the insects. Celastrus angulatus I has an antifeedant effect on the pests (Wu Wenjun et al., 1988, 1989), celastrus angulatus II and celastrus angulatus III have a direct poisoning effect on the insects (Wu Wenjun et al., 1993) and celastrus angulatus IV has a strong narcotic effect on the armyworms (Wu Wenjun et al., 1992). The structures of the above 4 active ingredients have been identified (Wu Wenjun, 1989; 1993). A large amount of tests show that celastrus angulatus has a good control effect on cabbage worm, aulacophora femoralis, phaedon brassicae baly, armyworm, parnara guttata, turnip sawfly, semiothisa cinerearia, sitophilus zeamais, locust, plutella xylostella, etc.
Rhododendron molle G.Don (also called Chinese azalea flower) contains various kinds of toxins such as andromodotoxin, rhododendrin, ericolin and rhodojaponin. (Feng Xia and Zhao Shanhuan, 1990a, 1990b). The rhodojaponin is a kind of tetracyclic diterpenoids (Zhu Zhengfang, 1992). The studies show that the extractive of rhododendron molle G.Don has significant contact positioning, stomach positioning and antifeedant effects on the insects and is mainly used for the control of aphid, cabbage worm, plutella xylostella, armyworm, borer, housefly, louse, flea, etc. Therefore, it can be widely applied in the agricultural industry and the control of sanitary insect pests.
Alkaloids are the botanical insecticidal active substances known to people in the earlier period. As early as the 5th century AD, the Ancient Mediterranean residents used the extractive of veratrum nigrum to treat the seeds for pest control (Smith, 1966). In the early 1940s, the substances like veratrine had become commercial insecticides in Venezuela and been used for citrus pest control. Meanwhile, the laboratories like Merck separated ryanodine from the roots and stems of flacourtiaceae ryania speciosa plants for the control of such pests as ostrinia nubilalis and sugarcane borer. Due to the unique acting mechanism, this alkaloid drew the wide attention of people (Casida et al., 1987). Tobacco was used for pest control 200 years ago in our country and now nicotine sulfate and nicotine oleata have been supplied in the market. There is a wide range of the studies carried out for the alkaloids. The camptothecin extracted by Hunan Institute of Forestry (1987) from the camptotheca acuminata and the cephalotaxin extracted by Hunan Institute of Forestry (1987) from the cephalotaxus fortunei have various biological activities for dendrolimus punctatus and the camptothecin and venoterpine have a special insecticidal effect (anti-fertility effect) (Xu Li et al., 1985; Chen Jisheng et al., 1987). The piperine in pepper fruits has a stronger lethality on the housefly than that of the extractive of pyrethrum. Recently, Xu Meijuan et al. (1994) have studied the alkaloids in tomato plants and found that the tomatidine has not only an obvious antifeedant effect on cabbage worm, but also an obvious oviposition deterrent effect on pieris rapae. In addition, we have also carried out deep studies for tropane alkaloids (Hao Naibin and Ge Qiaoying, 1998) and matrine (Zhang Laolao et al., 1997). In short, there are many kinds of alkaloids that have been studied to be used for pest control, e.g. hyoscyamine, berberine, bocconine, brucine, wilforine, stemonine, etc. (Luo Wanchun, 1995) There are various effects of these alkaloids on the insects and they are mainly used for the control of aphid, cabbage worm, armyworm, gypsy moth, beetle, pieris rapae, etc.
Some essential oils or their components also have poisoning, deterrent, antifeedant and growth and development inhibiting effects on the insects, so they are studied widely. The studies on pest control by essential oils in our country mostly focus on the warehouse pests and a large amount of study work has been carried out for crop pests in recent years. Ding Desheng (1993) found that the cauline leaf essential oil of mentha arvensis had a deterrent effect on aedes albopictus. Li Guangcan et al. (1985) studied the fumigating effect of several kinds of essential oils on confused flour beetle and proved that α-pinene, fir needle oil and lobular spearmint oil had a good effect. Yao Kang (1982) used litsea cubeba oil for the control of bruchus rufimanus boheman and also obtained an obvious effect. Xu Hanhong et al. (1993) found that capillene, the main component of artemisia scoparia essential oil, had significant biological activity for various important agricultural pests like prodenia litura. Wu Zhaohua (1994) pointed out that garland chrysanthemum essential oil had antifeedant activity for the larvae of pieris rapae linnaeus and the further study showed that the active ingredient of antifeedant was mainly eugenol. The eugenol also has a repelling effect on sitophilus zeamais. In addition, cinnamon oil (Xu Hanhong et al., 1994), cinnamomum micranthum essential oil (Xu Hanhong et al., 1996) and rue essential oil (Xu Hanhong et al., 1994) have strong biological activity for grain insect pests. In particular, 2-undecanone, the main component of rue essential oil, is a kind of good leechcide nematicide and vermicide. Unlike other botanical insecticides, many essential oil components like eugenol, geraniol, anethole, α-pinene, β-pinene, etc. also have insect sex pheromone and attracting effects on the insects, so they can be used as attractants. Due to the distinct advantages on aspect of stored grain insect pest and sanitary insect pest control and the obvious effects on agricultural pest control, the essential oils have broad application prospects.
In recent years, a significant progress has been achieved for the studies on the phototoxicities contained in the plants. As this type of substances have a lethality on the pests that will be enhanced by several or even thousands of times under light, they draw the great attention of people. The studies on photoactivated insecticides are mainly concentrated in Canada and the USA and have just been started in China (Xu Hanhong, 1995). Berenbaum (1978) first reported the photoactivated toxicity of some botanical compounds on phytophagous insects. The lethality of berberine on the larvae, pupas and adults of aedes is enhanced significantly under light (Philogone et al., 1984). Xu Hanhong et al. (1993) pointed out that the biological activity of the capillene in artemisia scoparia essential oil for prodenia litura was enhanced with the excitation of light. Le Haiyang et al. (1997) proved that the methanol extract from the root of tagetes had a very obvious phototoxic effect on the fourth instar larvae of aedes albopictus and culex quinquefasciatus. Phototoxicities widely exist in at least 30 families of plants and this type of compounds are mainly furocoumarins, polyynes, thiophenes, quinones and the alkaloids derived from tryptophan and tyrosine (Xu Hanhong, 1995). The recent studies show that α-terthienyl has obvious photoactivated toxicity on manduca sexta and heliothis virescens (Le Haiyang, 1997). As the photoactivated insecticides are efficient for the pests, harmless to human and livestock and liable to be decomposed in the environment, they are new-type botanical insecticides with great potential.
The studies on botanical insecticides in our country develop very fast and a lot of study work has proceeded deeply into the aspects of chemical structure analysis and acting mechanism. In order to develop the unique new-type insecticides in our country, the studies and findings of botanical lead compounds must be paid attention to so as to carry out the artificial synthesis. This work has just been started in our country. Wu Wenjun et al. (1998) studied the structure-activity relationship of celastrus angulatus Ⅰ～Ⅴ, Hao Naibin et al. studied the structure-activity relationship of tropane alkaloids and Li Xiaodong et al. (1997) studied the secondary compounds of meliaceae plants and their structure-activity relationship. Some progress had been achieved for the above study work.
3 Modes of insecticidal effects and mechanisms of botanical insecticides
The measurement of a series of indoor biological activity and physiological and biochemical indexes, and the studies on the changes in the behavior of test insects after application and the toxic symptoms show that there are various effects of the botanical insecticides on the insects, which can be generally grouped as follows:
3.1 Poisoning effect
Various kinds of botanical insecticides have a poisoning effect on the insects, e.g. nicotine, bocconine, toosendanin, celastrus angulatus II and celastrus angulatus III, rhododendrin FC-22, rhodojaponin III, etc. The acting mechanisms of these active substances are different from each other. For example, the positioning mechanism of concentrated nicotine for the pests is to develop the desensitizing inhibition to acetylcholine receptors, i.e. the nerve impulse conduction is blocked while the poisoning effect of sanguinarine, the main toxic ingredient of bocconine, mainly lies in the inhibition to acetylcholinesterase (Luo Wanchun, 1995). The positioning effect of toosendanin is considered to damage the midgut tissue and block the central nervous, thus causing paralysis, coma and death. The physiological studies show that the main cause is the repression of presynaptic transmission (Zhao Shanhuan et al., 1987). It should belong to a nerve poison according to the toxic symptoms of insects caused by rhododendrin FC-22, but the studies show that it has no inhibiting effect on the acetylcholine esterase of armyworm and the larvae of cabbage worm. Therefore, the acting mechanism of FC-22 may be affecting the permeability of K＋ and Na＋ on the neurilemma (Feng Xia and Zhao Shanhuan, 1990a). For stomach poisoning, the acting mechanism is generally that the digestive system of a pest is affected after the pest takes a certain amount of insecticide and then the toxic symptoms appear. For example, the larvae of cabbage worm will be in a coma and rigid after taking a certain amount of toosendanin and the residues of food will be stuck and caked in the midgut with symptoms of diarrhea and severe superficial dehydration. They finally die from the perforation of midgut, leakage of food and decomposition (Zhang Xing, 1993). In addition, the nutritional problems or the disturbance of endogenous hormone balance may lead to malformed insects. In short, there are various aspects of the contact poisoning and stomach poisoning mechanisms of botanical insecticides and further thorough studies are still required.
3.2 Deterrent and antifeedant effects
Within a certain concentration range of insecticide, the feeding amount of an insect will significantly decrease with the increase of concentration and the insect will no longer feed (the antifeeding is completed basically) when the concentration reaches a certain value and finally die from hunger. But the antifeeding degree varies with the insect species and the concentration of insecticide. For example, as to the antifeedant concentration for the fourth instar larvae of prodenia litura, the azadirachtin is 1.1 mg/L, the rhodojaponin-III is 40.5 mg/L and the antifeedant concentration of toosendanin for the larvae of cabbage worm is over 1?200 mg/L (Li Xiaodong et al., 1995). Celastrus angulatus has high antifeedant activity for locusta migratoria manilensis, plutella xylostella larvae and fall armyworm (Wu Wenjun, 1991). Thodojaponin III also has a strong antifeedant effect on spodoptera frugiperda and leptinotarsa decemlineata (Hu Meiying et al., 1992). Toosendanin also has a good antifeedant effect on tryporyza incertulas, cabbage worm, armyworm and prodenia litura (Zhang Xing et al., 1993). There are many reports about the antifeedant activity of alkaloids for the insects. Miller et al. (1983) carried out tests for fall webworm, spodoptera eridania and gypsy moth with 6 kinds of benzyl isoquinoline alkaloids and the tests showed that all these alkaloids had an antifeedant effect but the alkaloid containing methylene dioxy phenolic groups had the highest activity. Blades et al. (1986) studied the antifeedant effects of 9 kinds of alkaloids (quinine, papaverine, solanine, brucine, cytisine, tomatin, caffeine, arecoline and atropine) on calliphoridae and the studies showed that all these alkaloids had an antifeedant effect and the inhibiting degree varied with the type of alkaloid. Essential oils also have strong deterrent and antifeedant effects on the insects. Ding Desheng et al. (1983) found the deterrent effect of the cauline leaf essential oil of mentha arvensis on aedes albopictus. Tanacetum vulgare essential oil has a strong deterrent effect on leptinotarsa decemlineata while artemisia capillaris essential oil (Katsumi Yano, 1983) and garland chrysanthemum essential oil have a strong antifeedant effect on pieris rapae and larvae of pieris rapae linnaeus respectively (Wu Zhaohua et al., 1994). The alkaloids in a tomato and the extractive of rhododendron molle have an obvious deterrent effect on pieris rapae (Feng Xia and Zhao Shanhuan, 1990a, Xu Meijuan and Guan Zhihe, 1994). Due to the deterrent effect, many essential oils can be used for mosquito repellent manufacturing. The studies show that the main reason for the antifeeding of insects is that these insecticides inhibit the chemoreceptors of the mouthparts of an insect, e.g. the inhibition of toosendanin to the basiconic receptors in the maxillary palpus and the styloconic receptors in the maxillary nodular body of the larvae of armyworm, thus stopping the nerve conduction and blocking the transmission of feeding stimulus information and making the insect lose the taste sense and show antifeedant reaction (Zhao Shanhuan et al., 1987). In fact, according to the antifeeding features of insects, people can adopt the insecticide solution with a low concentration to increase their feeding amounts, thus achieving the purpose of poisoning the insects and improving the insecticide effect.
3.3 Narcotic effect
Celastrus angulatus IV has a narcotic effect on cabbage worm, armyworm, larvae of parnara guttata, semiothisa cinerearia, etc. (Wu Wenjun et al., 1993) The symptoms are that the insects are rigid and extremely limp and after several hours to tens of hours, the anesthetized insects revive and then lives and feed normally (the low instar larvae taking a large amount of insecticide will no long revive) (Wu Wenjun, 1991). This kind of narcotic active substance in the velamen of celastrus angulatus has no destructive effect on the midgut cells, but has a significant inhibiting effect on the secretion of peritrophic membrane. The oxygen consumption during the respiration of anesthetized test insects is only 47.68%-50.24% of the normal consumption and the rate of cardiac activity is only 40%-65% of the normal rate with a symptom of cardiac arrhythmias, which indicates that the narcotic ingredient works on the neuromuscular junction and obviously inhibits the excitatory junction potential (EJP), so the conduction of excitation is blocked, the insects are limp with a symptom of flaccidity and the motions controlled by nerve-muscle (e.g. respiration, cardiac impulse, etc.) are affected (Liu Huixia et al., 1992). In addition, under the effect of the narcotic ingredient, the detoxifying enzyme system in the insects, e.g. the esterase isoenzymes in the midgut and hemolymph are inhibited, so the exogenous toxicants cannot be decomposed in the insects, thus leading to the poisoning of insects (Liu Huixia et al., 1992).
3.4 Growth and development inhibiting effect
After the feeding, the insects have such symptoms as the growth and development are delayed, the eggs cannot be hatched normally, the larvae cannot pupate and emerge normally or pupate into malformed pupas. Although some test insects can pupate, they cannot shell and emerge normally and then become malformed ones (Zhang Xing et al., 1993), thus the number of insect population can be reduced. Azadirachtin and toosendanin have an inhibiting effect on the growth and development of the insects. Zhang Xing et al. (1992a) pointed out that after the injection of azadirachtin and the extractive of melia azedarach bark, the possibility of the fifth instar larvae of cabbage worm becoming malformed pupas and malformed insects was up to 80.6%-94.4%. After the fourth instar larvae of ostrinia furnacalis were fed with the above active substance, the larvae couldn't pupate and became “permanent” larvae and finally died from hunger and dehydration. Azadirachtin and rhodojaponin-III have an inhibiting effect on the growth and development of prodenia litura (Li Xiaodong and Zhao Shanhuan, 1995). The seed oil of celastrus angulatus also has effects of inhibiting the oviposition and incubation of the insects, and controlling their growth and development (Ke Zhiguo, 1991). Some alkaloids like lupinine and cytisine also have an inhibiting effect on the growth and development of acyrthosiphon pisum and melanoplus bivittatus. Some essential oils also have a growth and development inhibiting effect. For example, karanja oil can interfere with the normal propagation and development of tribolium castaneum herbst and make most of the larvae cannot pupate and then die, or develop into malformed pupas and malformed adults (Xu Hanhong and Zhao Shanhuan, 1994). The studies show that the effect of azadirachtin on the growth and development of the larvae of prodenia litura is mainly the hindering and inhibiting effect on the synthesis, secretion and transmission to target of such hormones important and related to the growth and development as prothoracico-tropic hormone (PTTH) and ecdysteroids, and rhodojaponin-III seems to have the similar effect (Li Xiaodong and Zhao Shanhuan, 1995).
3.5 Other effects
In addition to the above effects, botanical insecticides also have an attracting or anti-fertility effect. For example, some essential oils or their components have the effect of insect sex pheromone. Gulati et al. (1982) found that the flowers of a kind of catasetum cycynoches plant contained the compound of 12 kinds of monoterpenes, which was attractive to the male exaerete smaragdina and led to the sexual excitation. Ladd (1980) pointed out that eugenol could attract Japanese beetles and anethole could lead to the strong tropism of bees. Many essential oils show the activities of juvenile hormones. Calamus oil can inhibit the development of insect gonads and European calamus oil can obviously hinder the development and maturation of the ovary of large milkweed bug (Xu Hanhong and Zhao Shanhuan, 1994). After the male moths of dendrolimus punctatus are treated with camptothecin, infertility may be caused when they mate with normal female moths. Azadirachtin also has an anti-fertility effect on some pests.
4 Application and development prospects of botanical insecticides
As early as in the stage without chemical synthetic pesticides, many of the pesticides are extracted from the plants and some plants have been developed into commercial insecticides, e.g. nicotine sulfate, etc. However, since the 1940s, the chemical synthetic pesticides have substituted the botanical pesticides and captured the market, which leads to the trough of the studies on botanical pesticides (Zhou Hongxi, 1994). Even so, there are some interesting successful examples of natural botanical insecticides. For example, the pyrethrum extracted from the pyrethrum plant is very prominent in the botanical insecticides. This is because it is not only the source of the synthesis of pyrethroid insecticides, but also nontoxic, residue-free and pollution-free, so it is accepted by more and more countries in the world and the natural ethrin industry has become one of the pillar industries in such countries as Kenya and Ecuador. In recent years, the remarkable natural botanical insecticide is azadirachtin and the most active ingredient in the azadirachtin is melia azedarach, which was separated successfully for the first time in 1968 and the structure of which was completely defined in 1988. The USA has developed it into a commercial preparation “Margosan-O” as the control agent for household flower pests and another kind of preparation “AZT-VR-K” is used for the control of aedes aegypti. A kind of azadirachtin preparation “Neemmark” is also put into the market in India. It is reported that the azadirachtin has antifeedant, deterrent and growth and development inhibiting effects on at least 71 kinds of insects with a wide insecticidal range and a special insecticidal mechanism and is the internationally recognized most promising insecticidal plant variety. There are many outstanding achievements with regard to the direct utilization of insecticidal plants in our country and currently nicotine oleata emulsifiable concentrate, hypertonic nicotine sulfate emulsifiable concentrate, toosendanin emulsifiable concentrate, rotenone emulsifiable concentrate, aqueous solution of garland chrysanthemum and stemonine and aqueous solution of matrine have been registered or temporarily registered and put into operation formally. These botanical insecticides are mainly used for the control of various kinds of pests such as aphid, cabbage worm, helicoverpa assulta, plutella xylostella, prodenia litura, aulacophora femoralis, red spider, ostrinia nubilalis, rice planthopper, euproctis pseudoconspersa, ectropis obliqua, euproctis pseudoconspersa strand, phyllonistis citrella stainton and predatory mite. In addition, some botanical insecticides have been in the pilot test stage and will be available soon, which fully shows that our country has an abundant strength with regard to the studies on botanical insecticides.
Besides the direct utilization, the insecticidal active substances can also be used as the lead substances for the synthesis of new-type pesticides. Currently, in the three pillars of insecticides in the world, two of them are made creatively by finding the lead compound in the botanical insecticidal active substance and then taking it as the template for chemical simulation. The pyrethroid insecticides are synthesized with the pyrethrin of composites as the template while the carbamate insecticides are synthesized with the physostigmine of leguminous plant as the template. In addition, the isobutyramide insecticides are obtained from the structural transformation of the natural compound of black pepper while the nicotine nitroolefin insecticides are thought to be a kind of promising new-type insecticides synthesized with the inspiration of nicotine template. Undoubtedly, the botanical insecticide is an important treasure-house for the research and development of pesticides.
In a word, in nowadays when “green pesticides” are strongly advocated, environmental production is strengthened, “integrated pest management (IPM)” is implemented and sustainable agriculture is developed, it has a profound significance for ensuring the food safety of the 1.3 billion people in our country to research and develop botanical pesticides, especially to find the lead compounds with insecticidal activity in the plants and guide the chemists to synthesize nuisanceless pesticides with new targets for selectivity and environmental compatibility.
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