Natural Pesticides

CHAPTER 30 

Natural Pesticides 

INTRODUCTION

Pest is any animal, plant or microorganism that causes trouble, injuries or destruction; therefore, pesticide may be defined simply as chemical agents used to control or eliminate pest. Many kinds of insects transmit serious diseases, such as malaria and typhus. Some insects destroy or cause heavy damage to valuable crops, such as corn, cotton, wheat and rice. Other common pests include bacteria, fungi, rats and such weeds as ragweed and poison ivy. An understanding of genesis of pest status is important in the design and execution of pest control strategies incorporating the substance of natural origin. Worldwide, the traditional pesticide represents the big business in which the majority of the synthetic pesticides are utilized for agriculture or other purposes.

METHODS OF PEST CONTROL

The methods used for the control of pest can be of natural or artificial controls, which are discussed below.

Natural Controls

Nature is full of the example of prey–predator relationships. Every pest is more or less hindered in its increase by other predacious organisms. Parasitic pests, predators and the diseases caused by the pest are usually the most important factors in natural methods insect controls. As the use of specific pesticides against a major pest on a crop might lead to a serious outbreak of a secondary pest due to the destruction of natural enemies, this may lead to an upset in the balance between destructive and useful insects.

Topographical influence of the season changes, changing temperatures, rainfall, soil, atmospheric humidity and other natural factors also shows their effect on insects and their hosts. However, in tropical, temperate and frigid climates, the pest control methods are generally adapted to the topographic conditions. 

Artificial Control

Artificial controls of pest have been developed by man. These methods can be categorized as agriculture, chemical and biological controls as discussed below. 

Mechanical control: It employs manual labour as well as mechanical devices for collection or destruction of pest. Techniques, such as handpicking, pruning, trapping and burning are employed for the destruction of eggs, larvae, pupae and adult insects

Agricultural control: Agriculture control is the oldest in its approach. Deep plugging for the eradication of weeds and early stages of insects, alternate crop rotation or changing environmental conditions are some methods that leads to obstruction of the life cycle of pests. Nowadays advanced plant breeding techniques like hybridization, mutation, polyploidy and biotechnological manipulations are greatly used for the production of pest resistant species.

Chemical controls: Chemical agents are the major pesticides agents used for the control of pest throughout the world. These are the materials used for the purpose of killing pests or for protecting crops, animals or other properties against the attack of the pest. Insect repellents, attractants, fumigants like insecticides, parasiticides are used for killing mites, ticks, and sterilizing agents which employ radioisotopes or chemicals to interfere with reproductive capabilities are nowadays widely used.

New groups of compounds called as insect growth regulators (IGR) pesticides or bio-insecticides consists of the natural chemicals presents in the insects that control their developments. For example, methoprene prevent the pupate stage which develops the reproductive adults. In such cases, larvae grow larger, molt repeatedly and eventually die. Bio-pesticide of this type is very specific for their toxicity and safety

New groups of compounds called as insect growth regulators (IGR) pesticides or bio-insecticides consists of the natural chemicals presents in the insects that control their developments. For example, methoprene prevent the pupate stage which develops the reproductive adults. In such cases, larvae grow larger, molt repeatedly and eventually die. Bio-pesticide of this type is very specific for their toxicity and safetyinvertebrates and living organisms are used for controlling the pest or their biological activities. All these are refereed to as ‘Bio-rational pesticides’. Microorganisms may be used to kill by causing fatal disease in insects. For example, Bacillus thuringiensis selectively kills only larvae of butterflies and moths and B. papillae kill the grubs of Japanese bettles. B. thuringiensis Var. israelensis is a new strain which specifically attack only mosquito larvae. Microbial controls are safer for most of the nontargeted organisms and also human and pets. Biologically derived pest control agents, such as pheromones, allomones and kairomones combinely known as ‘semio-chemicals’ and hormones also attract, retard, destroy or otherwise exert a pesticide activity.

CLASSIFICATION   

Pesticides are classified according to the pest they control. The four most widely used types of pesticide are briefly discussed below.

Insecticides

Farmers use insecticides to protect their crops from insect damage. In urban localities, public health officials use these chemicals to fight mosquitoes and other insects. Insecticides are widely utilized in homes and other outdoor conditions to control such pests as ants, moths, cockroaches and termites.

Herbicides

Three groups of pesticide agents control weeds or eliminate plants that grow where they are not wanted. Farmers use herbicides to reduce weeds among their crops. Herbicides are also used to control weeds in such public and recreational areas as parks, lakes and ponds. It is also used as garden pesticide or in yards to get rid of crab grass and dandelions.   

Fungicides

Many of the fungi are pathogenic and many infect both plants and animals including human beings. Fungicides are used to control fungal diseases of plants and food crops. Most disinfectants used in homes, hospitals and restaurants contain fungicides.

Rodenticides

These agents are used especially in urban areas where rats and other rodents are a major health problem. Rats carry many of the pathogenic bacteria that cause disease, such as rabies, rat-bite, fever, tularemia and typhus fever. Rats also destroy large amounts of food and grain, so rodenticides help protect areas where these products are stored. Other pesticides that help to control variety of organisms are given below.

It is difficult to classify all pesticide chemically or biologically. Some of the important natural pesticides have been categorized in Table 30.1 enlisting few important examples of each class.

Pesticides are often classified according to the type of action that results in destruction of the pest. Three broad categories, namely stomach poisons, contact insecticides and fumigants are recognized. Stomach poisons kill by being taken into the stomach, absorbed in the blood and leads to the death of the pest due to the toxic action. Contact insecticides kill by direct or indirect contact with the insect or sometimes it penetrate inside the body and causes oxidation and suffocate the insect. Fumigants can be applied only in enclosed areas where it surrounds the insect, enters their breathing pores and kills. Most of the pesticides are used in the form of dust preparations, spray preparations, suspensions or bait preparations. Sometimes they are dispensed in the form of aerosol or liquefied gas propellants. Equipments used for the application of pesticides might vary from small hand sprayers, paint brushes for use in home to large power sprayers for treating livestock and field crops. For the dispersal in vast areas of forms, airplanes and helicopters are also used.

ESSENTIALS OF A GOOD PESTICIDE

For an agent to be a good and ideal pesticide, it should bear certain important characteristics as given below.

• A pesticide should have a high margin of safety for plants and animal causing very little or no damage to the foliage or livestock, respectively. 

• It should be safer. 

• It should be easier to handle and easy for application. 

• It should not show toxicity in case of warm-blooded animals. 

• It should not have flammable or explosive character. 

• It should have safety and palatability of the food products exposed to insecticides and should not show the residual effects of pesticides. 

• It should be available easily at affordable cost.

PYRETHRUM FLOWERS

Synonym

• Insect flowers, Pyrethrum flowers.

Biological Source

• Pyrethrum consists of the dried flower heads of Chrysanthemum cinerariaefolium (Trev.) Vis., family

Geographical Source

• Pyrethrum is indigenous to Balkan areas of Dalmatia, Herzegovina and Montenegrow. The flowers were known as Dalmatian insect flowers as they were formally exported from Dalmatia. Nowadays it is principally cultivated and produced in Kenya, Tanzania, Rwanda, Ecuador and Belgium Congo. On similar scales, it is grown in Japan, Brazil, Yugoslavia, Switzerland, Spain and India. Before World War II, Japan produced almost 80% of the world’s production, but presently Kenya is the largest exporter of pyrethrum.

Cultivation and Collection

• Pyrethrum is a perennial plant propagated by seeds and pieces of stems bearing roots known as splits. The best and favourable condition for pyrethrum cultivation is at an attitude of 1,900–2,700 m and an annual rainfall of 76–180 cm. Seeds are raised in nurseries. About 4-month old seedlings are planted out in the fields in rows about 1 m apart during sunny days. Low night temperature about 5–15°C favours the maximum bud formation. First collection of flower heads is made after about 4 months, but the best time for collection is when two-third of the disc florets are open. Collection is generally done by hand pickers at roughly 3-week intervals. The flowers are dried immediately after collection in the sun or in drying chambers at a temperature not exceeding 50°C. In Kenya, after gradation, the flowers are compressed into bales and exported. Kenyan pyrethrum production is controlled by pyrethrum marketing board at Nakuru. The flowers are directly exported or made into powder or standard liquid extract.

Characteristics

The closed pyrethrum flower heads are about 6–9 mm in diameter while the open flowers are about 9–12 mm diameter. The flowers bear a short, longitudinally striated peduncle. The involucre consists of two or three rows of yellowish or greenish yellow lanceolate hairy bract. The flat receptacle bears a single row of 15–23 cream or strawcoloured ligulate ray florets. Lignite corollas are 10–20 mm length with about 17 veins and three-rounded apical teeth of which the central one is suppressed. Disc florets are about 200–300 with tubular corolla. Pyrethrum flowers have a slight aromatic odour and a bitter acrid taste. It is interesting to note that before drying the flower heads are not toxic to insect.

The powder of pyrethrum flowers shows the presence of parenchymatous tissues, with aggregate crystals, sclerenchymatous tissues, t-shaped hairs, spherical pollen grains and tracheas. Dried flowers or powder should be stored in wellclosed container, protected from air and light and should not be kept for more than two years. Pyrethrum extract is comparatively more stable than pyrethrum flowers. 

Chemical Constituents

Pyrethrum flowers owe its insecticide properties to two groups of esters: the first group consists of pyrethrin I, jasmolin I and cinerin I, which have chrysenthemic acid(chrysanthemum monocarboxylic acid) as their acid component and the second group of esters that consists of pyrethrin II, jasmolin II and cinerin II, which have pyrethric acid (monomethyl ester of chrysanthemum dicarboxylic acid) as their acid component. The alcohol component of the pyrethrin present in the form of keto-alcohol pyrethrolone and that of cinerine as cinerolone. The flowers of pyrethrum also consist of a triterpene alcohol pyrethrol and sesquiterpene lactones, pyrethrocin. Other constituents includes pyretol, pyrethroloxic acid, chrysenthemine and chrysathemumic acid.

Various pyrethrins are called as pyrethoids. These components occur in the oleoresin secretion of certain floral parts, known as achenes. Pyrethrum extract contains up to about 50% active constituents. Pyrethrum extract (BP) consists of about 25% of pyrethrins. These commercial extracts are generally obtained by supercritical fluid extraction techniques at 100–250 bar pressure and 50°C temperature conditions. The concentrated extracts are usually diluted with kerosene to a pyrethrin strength of about 0.2%. Pyrethrin assays are based on the total chrysanthemum acid and the total pyrethrin acid; therefore, the content of pyrethrums is in effect the content of pyrethrins, cinerins and jasmolins.

Uses

Pyrethrum flowers are mainly used for the preparation of pyrethrum extract which is the form of pyrethrin usually employed in the compounding of pyrethrum preparation. Pyrethrum brings about its pesticide activity by an instantaneous knock down action on insects within few seconds. It has no appreciable effect on insects as a stomach poison but acts by contact, producing a characteristic effect on the nervous system that results in muscular excitation, convulsion and paralysis. However, it is less persistent and less stable. The compounds like piperonyl butoxide, bucarpolate, sesamin and DDT act synergistically and potentiate the insecticidal properties of pyrethrin.

The synthetic pyrethrin-like compound, allethrin, also shows the potential insecticidal activity but its activity is not synergistic when combined with synergists available at present.

Pyrethrum is widely used in domestic and agricultural insecticidal sprays, dusting powders and aerosol preparations for controlling a variety of garden pests and fleas, lice and ticks on pets. A noninflammable preparation is used as a spray in aircraft to kill insect vectors and so prevent the transmission of insect borne diseases.

The toxic effects of pyrethmm include irritation to the eyes and mucosa. Maximum permissible atmospheric concentration is up to 5 mg per m3 .

DERRIS ROOTS

Synonym

• Derris roots, Tuba roots, Tauba.

Biological Source

• Derris root consists of the dried rhizomes and roots of Derris elliptica (Roxb.) Benth. and D. malaccensis Prain, family Leguminoseae.

Geographical Source

• Derris is indigenous to Malaya. It is cultivated in Burma, Thailand, East Indies and tropical African countries. It is also produced in Singapore, Borneo and in Sumatra.

Characteristics

Derris roots are slender pieces of about 2-m long with a diameter of about 8–10 mm. Externally the root bark is dark reddish brown in case of D. elliptica and greyish brown in cash of D. malaccensis, while the inner wood is yellowish and porous in both the cases. It shows the fine longitudinal furrows on the outer surface of roots. It is flexible, tough and hard and breaks in the fibrous fracture. It shows slightly aromatic odour and bitter and numbingtaste. Derris rhizomes constitute a very small proportion of the commercial drug. It consists of short but thick brown coloured pieces with numerous longitudinal wrinkles, transverse cracks and circular lenticels. The transversely cut surface of roots shows thick cork followed by rings of sclerenchyma containing strands of phloem.

Microscopy

The transverse section of the derris root consists of the layers of cork cells followed by phelloderm and pericyclic parenchyma with numerous groups of lignified sclereids and starch grains. The phloem is stratified by the altering bands of phloem fibres and sieve tissue. Phloem parenchyma contains longitudinal small cells and prismatic crystals of calcium oxalate. Two types of xylem vessels of larger and smaller size are present in groups of two to six. Groups of xylem fibres and vessels are usually embedded in cellulosic parenchyma. The fibres of both phloem and xylem are unlignified except the middle lamella. One to six cell thick medullary rays are radially arranged.

Chemical Constituents

Derris roots contains about 3–10% of a flavone derivative rotenone. It is colourless to brownish crystalline compound or a white to brownish white odourless tasteless crystalline powder. It is insoluble in water but soluble in alcohol, acetone and other organic solvents. Rotenone is incompatible in the alkalies and oxidizing agents. Derris in powder should be protected from light. The overall evaluation of drug depends both on rotenone content and on the amount of chloroform extractive that the root yields. Rotenone is rapidly degraded by sunlight, lasting a week or less.

Uses

Derris is a widely used agricultural and horticultural insecticide and larvicide. It is a contact and stomach poison. Its action is more persistent but less rapid than pyrethrum. The insecticidal preparations of rotenoids at concentrations ranging from 0.75 to 1.0% are effective against a wide range of insects, such as Mexican bean beetle, cabbage worms, leafhoppers and other insects attacking a variety of vegetable. It is especially useful for application to vegetable near the time of harvest when certain other insecticides cannot be used because of potentially excessive residues. Derris and rotenoids are also useful in controlling insect parasites of animals such as cattle, grubs, lice, fleas and ticks on pets and live stocks.

hoppers and other insects attacking a variety of vegetable. It is especially useful for application to vegetable near the time of harvest when certain other insecticides cannot be used because of potentially excessive residues. Derris and rotenoids are also useful in controlling insect parasites of animals such as cattle, grubs, lice, fleas and ticks on pets and live stocks.

LONCHOCARPUS ROOTS

Synonym

• Lonchocarpus roots, Cube roots, Timbo, Barbasco

Biological Source

• Lonchocarpus consists of the dried roots of Lonchocarpus utilis, L. urucu and other species of Lonchocarpus, family Leguminoseae.

Geographical Source

• Lonchocarpus is indigenous to Peru and Brazil. It is also produced in British and Dutch Guiana.

Characteristics

• Lonchocarpus roots usually occur in pieces to 30-cm long and 12–25 mm in diameter. Outer surface is brownish-gray with longitudinal reticulated wrinkles. Microscopically it resembles Derris but may be distinguished by the abundant starch grains, comparatively larger and lignified xylem. The freshly cut transverse surface of lonchocarpus roots appears grayish-green under UV light, and its ethereal extract shows bright blue fluorescence.

Chemical Constituents

• Lonchocarpus roots contain about 3–10 % rotenone.

Uses

• Lonchocarpus roots owe its action to the presence of constituents similar to those of Derris and are used for the same-purpose.

TOBACCO

Synonym.

• Tobacco

Biological Source

• Tobacco consists of the dried leaves of Nicotiana tabacum, family Solanaceae, known as Virginian tobacco and N. rustica referred to as Turkish tobacco.

Geographical Source

• Tobacco is indigenous to tropical America. It is cultivated on large scale in China, United States and India. It is also produced in Brazil, Turkey, Russia and Italy. In India tobacco is mainly cultivated in Andhra Pradesh, Karnataka, Tamil Nadu, Orissa, Gujarat, Bihar, West Bengal and Assam.

Cultivation, Collection and Preparation

• Although tobacco is tropical in origin and thrives best in the warm climate, it is grown under wide range of conditions. Tobacco is an annual crop attaining 1–3 m height. It tears about 20 large leaves. Tobacco is generally propagated by seeds. Seedlings are developed in the seedbeds during early spring and the seedlings of about 12 weeks are transplanted in the field. Cutting of the flowering tops encourages the growth of the foliage. The crop is harvested after about three to three–and-a-half months. Tobacco is subjected to curing by any of the three procedures, namely flue curing, fire curing and air curing, which modifies the aroma and flavour characteristics of tobacco. Loss of nicotine during flue-curing is negligible but sun-curing causes considerable loss of nicotine content.

Characteristics

• Tobacco is a crop which is mostly used for the preparation of Cigarettes, bidi, cigar, cherrot, hookah, snuff and chewing tobacco. Nicotine is the characteristic alkaloid prepared commercially from waste material of tobacco industry. Nicotine is a colourless to pale yellow, very hygroscopic, oily liquid with an unpleasant pungent odour and a sharp burning persistent taste. It gradually becomes brown on exposure to air or light. It is soluble in water, alcohol, chloroform, kerosene and fixed oils. It should be stored in airtight containers.

Chemical Constituents

• Levels of nicotine content in various tobacco types vary drastically, but it is in the range of about 1–10%. Nicotine is a pyridine–pyrolidine group of alkaloid mainly responsible for its pesticidal activity. Along with nicotine, tobacco leaf has several other alkaloids especially narcotine and anabasine which may have some insecticidal properties in association with nicotine.

• Most of the tobacco waste materials obtained from the tobacco industry are used as the potential source of raw material for production of commercial grade insecticidal nicotine. Nicotine is isolated by mixing tobacco waste with lime and extracting with water. Aqueous extract is further extracted with kerosene and the subsequent kerosene extract is treated with sulphuric acid to obtain nicotine sulphate solution from which it is separated

Uses

• Insecticidal use of nicotine dates back to the 17th century. During 18th century, aqueous tobacco extract or dust was employed as an insecticide in the vegetable gardens of Europe, and the commercial preparation of nicotine sulphate was put into market by 1910 as a potential insecticidal agent. Nicotine acts by its triple action insecticidal property, acting as stomach, contact and fumigant poison. Its free base is more toxic than the sulphate or hydrochloride salt. It is mostly effective against minute soft bodied insects, such as aphides and also against white flies, red spidermites, leaf rollers, moths, fruit tree borers, termites, cabbage butterfly larvae, etc. Nicotinoids are active as a spray solution containing 0.04–0.05% active ingredients.

• One of the advantages of the insecticidal use of nicotine is its high margin of safety for plants. Nicotine preparations are safer, easier to handle and much less toxic to warmblooded animals. Due to the volatile nature of nicotine, it disappears quickly leaving no residue on treated plants. The above properties make nicotine preparation a very ideal insecticide

NEEM

Synonym

• Neem, Margosa, Azadirachta.

Biological Source

• Neem consists of almost all parts of the plants which are used as drug. Some important morphological parts are the dried stem bark, root bark, leaves and fruits of Azadirachta indica also, known as Melia azadirachta, family Meliaceae

Geographical Source

• Neem is native of the arid region of India and Pakistan. Neem is found abundantly in India, Pakistan, Bangladesh, Sri Lanka, Thailand, Malaysia, and Mauritius, countries of East and South Africa and in tropical Australia.

Characteristics

• Neem is large subtropical shade tree. It is known for centuries as being free of insects, disease and nematodes. All the parts of the tree, such as bark, leaves, fruits and especially the seeds are resistant. The bark is grey to reddish brown with numerous furrows. Leaves are imparipinnate, alternate or opposite and bluntly serrate. Flowers are white to pale yellow which gives green drupacious fruits turning yellow on ripening. Fruit contains single exalburainous seed.

• Neem oil or margosa oil is a fixed oil expressed from seed kernels. It gives about 10% of the oil which is yellow in colour with garlic-like odour and bitter taste. It is soluble in organic solvents and practically insoluble in alcohol and water.

• The cake left after the expression of oil is used as such. It may be subjected to alcoholic extraction to yield neem cake extract. 

Chemical Constituents

• Neem has been found to possess several types of chemicals that could be exploited for pest management. Neem seeds mostly contain the complex tetranorterpenoid lactones azadirachtin, Nimbin, nimbidin, salanin and nimbolin B out of which azadirachtin is the most active component responsible for the antifeedant activity of neem. Other antifeedent components identified are meliantrol a triterpenoid alcohol and salanin. Neem oil obtained from seeds also shows the presence of these constituents along with other compounds such as nimbolides, olichinolide B and azadiradione. The leaves also contain azadirachtin, meliantrol, salanin, β-sitosterol, stigmasterol and flavonoids such as nimatone, quercetin, myrecetin and kaempferol.

• The bark shows the presence of riimbin, nimbidim and nimbinin like antiviral agents and margolone and margolonone like antibacterial principles.

Uses

The pest control usage of neem and neem products can be properly exploited depending upon the nature of the pest. The various reported pest control activities are given in Table 30.2 along with the neem and neem products used on specific pests.

Neem seeds can be directly extracted to yield neem seed extracts. The oil expressed from the seed is known as neem oil, while the residual marc is called as neem cake which may be extracted using alcohol to obtain neem cake extractives. Neem oil extractive is a resinous dark byproduct of neem oil refining. It is well known that neem possesses low- to medium-contact toxicity which is restricted to soft body insects, and its use as an insecticide alone does not carry much conviction with the user.

CEVADILLA

Synonym

• Cevadilla, Sabadilla, Caustic Barley.

Biological Source

• Cevadilla consists of the dried ripe seeds of Schoenocaulon officinale, family Liliaceae.

Geographical Source

• Cevadilla is a tall, herbaceous plant found in Mexico to Venezuela. It also grows in Guatemala

Collection and Preparation

• Sabadilla plant is 3–4 feet high. The seeds and fruits are collected from the plant after maturity. It is not quite certainwhether the seeds are obtained from Veratrum sabadilla or from Veratrum officinale which differs slightly in appearance and morphological characteristics.

Characteristics

• Dried mature seeds of cevadilla are dark brown to blackish about 6-mm long but narrow and sharply pointed at the ends. The seeds are bitter and acrid in taste. The seed powder is sternutatory and causes violent sneezing.

Chemical Constituents

• Cevadilla seeds contain about 2–4% of mixed alkaloids known combinely as ‘Veratrine’. The major alkaloids cevadine (crystalline veratrine), sabadine, sabadilline and veratridine shows the close relationship to the ester alkaloids of veratrum. Sabadilla is less toxic to mammals than rotenone or pyrethrin. The acute oral LD50 is greater than 4,000 mg/kg

Uses

Cevadilla was formerly used as pesticide especially for pediculosis coptis in the form of ointment. Powdered cevadilla is used for killing house flies, thrips and vegetable attacking bugs in the form of sprays or dust in overdoses capable of producing fatal results. It is also used as a taenicide.

RYANIA

Synonym

• Ryania.

Biological Source

• Ryania consists of the roots and stems of Ryania speciosa, family Flacourtiaceae.

Geographical Source

• Ryania is indigenous to South America. It is mostly found in Trinidad.

Collection

• The roots and stem of Ryania are collected after flowering and fruiting. It is the most expensive material and is not as readily available as rotenone or pyrethrin

Chemical Constituents

• Ryania contains about 0.16–0.2 % of alkaloids. Ryanodine is the major pyrrole alkaloid that is esterified with a complex polyhydroxy diterpenoid.

Uses

• Ryania extracts containing the alkaloid ryanodine are used as insecticide for various lepidopterous larvae which attacks fruits. It is also used for controlling moths and corn borer. Ryanodine is formulated as a wettable powder and is labelled to be used against the codling moth in apples. It is more persistent than rotenone or pyrethrin and more selective. It is generally not very harmful to pest predators and parasites. It may also be used up to 24 h before harvest. It is toxic to fish.

RED SQUILL.

• Red squill and white squill are the two important varieties of Urginia maritima, family Liliaceae. Red squill either the whole bulb or dried scales and powder is distinguished from that of white variety on the basis of its reddish colour. Red variety of U. maritima contains in addition to cardiac glycosides, an active principle, scilliroside, which is very toxic to rats. It acts on the central nervous system (CNS). Unlike other mammals rodents do not regurgitate the red squill and death follows convulsions and respiratory failure. Red squill was not considered acceptable to animals other than rodents but poisoning has been reported in catties, sheep, chicken and dogs. It is incorporated as a pesticide in rat pastes. Since it is extremely irritating to the skin it should be handled with rubber gloves. Its use as a poison for animals is prohibited in England and is considered as a cruel poison. WHO expert committee on insecticides had endorsed its use from the standpoint of safety.

STRYCHNINE

• Strychnine is an alkaloid obtained from the dried seeds of Strychnous nuxvomica, family Loganiaceae. It occurs as odourless translucent, colourless crystalline powder with a very bitter metallic taste. The symptoms of poisoning by strychnine are mainly those arising from stimulation of CNS. The first signs are tremors and slight twitching of limbs followed by sudden convulsions quickly involving all muscles. The jaw is rigidly cramped. Respiration is arrested and death occurs from asphyxia or medullary paralysis. Strychnine was used as rodenticide in olden days, and it was recommended that strychnine should only be used by trained pest control operators in areas to which access by unauthorized persons and useful animals could be completely prevented. In England use of strychnine for killing of animals is prohibited under the Animal (Cruel poisons) regulations 1963. Strychnine is traditionally used for the extermination of moles. Its toxicity and painful poisonous action do not make it a rodenticide of choice.

MOLLUSCICIDES

• A number of organisms from the Mollusca class cause a variety of diseases in human beings and animals. The pharmaceutical interest in molluscicides is concerned primarily to kill such parasitic organisms. The blood flukes, Schistosoma haemotobium and its other species causes intestinal and bladder damage in South America and Africa. Fresh water snails act as an intermediate host to produce numerous cercaria which emerge into fresh water and causes infection to humans by penetrating through the skin into the blood. The berries of Phytolacca dodecandra, family Phytolaccaceae is an Ethiopian plant which have shown molluscicidal activity against fresh water snails which works as a host for Schistosoma. The berries contain a triterpenoid saponin glycoside oleanolic acid which is responsible for its poisonous effect on snails.

• The pods of Swartzia madagascariensis and S. simplex, family Leguminoseae, contain oleanolic acid glycosides and certain other saponins. Locally these are used as insecticidal and piscicidal agents. Another leguminous plant Tetrapleura tetraptera which consists of saponins is used as a potential piscicidal and molluscicidal agents in Nigeria.

CITRONELLA OIL

• Citronclla oil is a pale to deep yellow oil with a pleasant characteristic odour. It is obtained by distillation from Cymbopogon nardus and C. wintcrianus, family Graminae and also from other varieties and hybrids of these species. The commercial oil is found in two types which are known as Ceylon oil and Jawa oil. Both the oils contain a monoterpene aldehyde citronellal as a major component of essential oil. Both citronella and citronellal oil have been used as a constituent of insect repellant products. However, it shows lesser effectivity than diethyltoluamide or dimethyl phthalate. It is used as an outdoor insect repellantSome of the other Indian medicinal plants which have shown the promising results in several pesticidal studies are mentioned

FACTORS INFLUENCING DEVELOPMENT OF NATURAL PESTICIDES

Discovery

• The secondary compounds of plants are a vast repository of compounds with a wide range of biological activities. This diversity is largely the result of coevolution of hundreds of thousands of plant species with each other and with an even greater number of species of microorganisms and animals. Thus, unlike compounds synthesized in the laboratory, secondary compounds from plants are virtually guaranteed to have biological activity and that activity is highly likely to function in protecting the producing plant from a pathogen, herbivore or competitor. Thus, knowledge of the pests to which the producing plant is resistant may provide useful leads in predicting what pests may be controlled by compounds from a particular species. This approach has led to the discovery of several commercial pesticides such as the pyrethroid insecticides. Isolation and chemical characterization of the active compounds from plants with strong biological activities can be a major effort compared to synthesizing a new synthetic compound. However, the assurance of biological activity and improvement in methods of purification and structural identification is shifting the odds in favour of natural compounds. 

• Considering the probability of plant secondary products being involved in plant–pest interactions, the strategy of randomly isolating, identifying and bioassaying these compounds may also be an effective method of pesticide discovery. Biologically active compounds from plants will often have activity against organisms with which the producing plant does not have to cope. Many secondary compounds described in the natural product, pharmacological and chemical ecology literature have not been screened for pesticidal activity. This is due, in part, to the very small amounts of these compounds that have been available for screening. 

• The discovery process for natural pesticides is more complicated than that for synthetic pesticidesTraditionally, new pesticides have been discovered by synthesis, bioassay and evaluation if the compound is sufficiently promising, quantitative structure–activity relationship-based synthesis of analogues is used to optimize desirable pesticidal properties. The discovery process with natural compounds is complicated by several factors.

• First, the amount of purification initially conducted is a variable for which there is no general rule. Furthermore, secondary compounds are generally isolated in relatively small amounts compared to the amounts of synthesized chemicals available for screening for pesticide activity. Therefore, bioassays requiring very small amounts of material will be helpful in screening natural products from plants. A number of published methods for assaying small amounts of compounds for pesticidal and biological activities are available in the allelochemical and natural product literature. At some point in the discovery process, structural identification is a requirement. This step can be quite difficult for some natural products. Finally, synthesis of the compound and analogues must be considered. This is generally much more difficult than identification. Despite these difficulties, modern instrumental analysis and improved methods are reducing the difficulty, cost and time involved in each of the above steps.  

Development

• Few pesticides that are found to be highly efficacious in testing are ever brought to market. Many factors must be considered in the decision to develop and market a pesticide. An early consideration is the patentability of the compound. A patent search must be done for natural compounds as with any synthetic compound. Prior publication of the pesticidal properties of a compound could cause patent problems. Compared to synthetic compounds, there is a plethora of published information on the biological activity of natural products. For this reason, patenting synthetic analogues with no mention of the natural source of the chemical family might be safer than patenting the natural product in some situations.

• The toxicological and environmental properties of the compound must be considered. Simply because a compound is a natural product does not ensure that it is s The most toxic mammalian poisons known are natural products and many of these are plant products. Introduction of levels of toxic natural compounds into the environment that would never be found in nature could cause adverse effects. However, evidence is strong that natural products generally have a much shorter half-life in the environment than synthetic pesticides. In fact, the relatively short environmental persistence of natural products may be a problem, because most pesticides must have some residual activity in order to be effective. As with pyrethroids, chemical modification can increase persistence.

• After promising biological activity is discovered, extraction of larger amounts of the compound for more extensive bioassays can be considered. Also, analogues of the compound should be made by chemical alteration of the compound and/or chemical synthesis. Structural manipulation could lead to improvement of activity, toxicological properties, altered environmental effects, or discovery of an active compound that can be economically synthesized. This has been the case with several natural compounds that have been used as a template for commercial pesticides (e.g. pyrethroids).

• Before a decision is made to produce a natural pesticide for commercial use, the most cost-effective means of production must be found. Although this is a crucial question in considering the development of any pesticide, it is even more complex and critical with natural products. Historically, preparations of crude natural product mixtures have been used as pesticides. However, the potential problems in clearing a complex mixture of many biologically active compounds for use by the public may be prohibitive in today’s regulatory climate. Thus, the question that will most probably be considered is whether the pure compound will be produced by biosynthesis and purification or by traditional chemical synthesis. 

• Before considering any other factors, there are two advantages to the pesticide industry to industrial synthesis. They have invested heavily in personnel and facilities for this approach. Changing this approach may be difficult for personnel trained in disciplines geared to use it. Secondly, in addition to the patent for use, patents for chemical synthesis often further protect the investment that a company makes in development of a pesticide. 

• However, many natural products are so complex that the cost of chemical synthesis would be prohibitive. Even so, more economically synthesized analogues with adequate or even superior biological activity may tip the balance towards industrial synthesis. If not biosynthesis must be considered.

• There are a growing number of biosynthetic options.The simplest method is to extract the compound from field-grown plants. To optimize production, the species and the variety of that species that produce the highest levels of the compound must be selected and grown under conditions that will optimize their biosynthetic capacity to produce the compound. Genetically manipulating the producing plants by classical or biotechnological methods could also increase production of some secondary products. For instance, low doses of diphenyl ether herbicides can cause massive increases in phytoalexins in a variety of crop species. 

• Another alternative is to produce the compound in tissue or cell culture. With these methods, cell lines that produce higher levels of the compound can be rapidly selected. However, genetic stability of such traits has been a problem in cell culture production of secondary products. Cells that produce and accumulate massive amounts of possibly autotoxic secondary compounds are obviously at a metabolic disadvantage and are thus selected against under many cell or tissue culture conditions. A technique, such as an immobilized cell column that continuously removes secondary products can increase production by decreasing feedback inhibition of synthesis, reducing autotoxicity, and possibly increasing generic stability. Other culture methods that optimize production can also be utilized. For instance, supplying inexpensively synthesized metabolic precursors can greatly enhance biosynthesis of many secondary products. Also, plant growth regulators, elicitors, and metabolic blockers can be used to increase production. Genetic engineering and biotechnology may allow for the production of plant-derived secondary products by gene transfer to microorganisms and production by fermentation. This concept is attractive because of the existing fermentation technology for production of secondary products. However, it may be prohibitively difficult for complex secondary products in which several genes control the conversion of several complex intermediates to the desired product.

• Genetic engineering might also be used to insert the genetic information for production of plant-produced pesticides from one plant species to another species to be protected from pests. However, such transgenic manipulation of the complex metabolism of a higher plant might be extremely difficult. A simpler alternative might be to infect plant-colonizing microbes with the desired genetic machinery to produce the natural pesticide, as has been done with bacterial-produced insecticides. 

THE FUTURE

• Plants contain a virtually untapped reservoir of pesticides that can be used directly or as templates for synthetic pesticides. Numerous factors have increased the interest of the pesticide industry and the pesticide market in this source of natural products as pesticides. These include diminishing returns with traditional pesticide discovery methods, increased environmental and toxicological concerns with synthetic pesticides, and the high level of reliance of modern agriculture on pesticides. Despite the relatively small amount of previous effort in development of plant-derived compounds as pesticides, they have made a large impact in the area of insecticides. Minor successes can be found as herbicides, nematicides, rodenticides, fungicides and molluscicides. The number of options that must be considered in discovery and development of a natural product as a pesticide is larger than for a synthetic pesticide. Furthermore, the molecular complexity limited environmental stability, and low activity of many biocides from plants, compared to synthetic pesticides, is discouraging. However, advances in chemical and biotechnology are increasing the speed and ease with which man can discover and develop secondary compounds of plants as pesticides. These advances, combined with increasing need and environmental pressure, are greatly increasing the interest in plant products as pesticides.