In this article we will dicuss about how to control pests without using chemicals.
Introduction to Botanical Pesticides:
Insects and plants originated almost simultaneously about 500 million years ago, and ever since both these groups of organisms have been fighting for their survival. This fight is still on between the attacker (insect) and the attacked (plant). During this long evolutionary history of attacker and attacked, plants have developed ways and means to combat attacker.
Plants cannot run away nor can they fight physically, hence nature during this evolutionary period provided plants with highly sophisticated in-built defense mechanisms to resist/desist attack by insects and other pests.
The greatest testimony that they are not helpless is that even the plant species considered most susceptible survive well in the nature. Of the various defense mechanisms developed, chemicals elaborated by the plants are the most important for defense. Plants are nature’s ‘chemical factories’, providing the nature’s richest source of chemicals on earth.
Botanical pesticides are either naturally occurring plant materials or products derived rather simply from such plant materials. The chemicals that plants produce to protect themselves against insect attack belong to a group that includes compounds known as secondary plant substances. These chemicals are a subset of what are known as phytochemicals (plant-based chemicals). Within the context of pest control, they are referred to as botanical pesticides.
Secondary plant substances are produced as byproducts of major biochemical pathways and chemically, they include alkaloids, terpenoids and phenolics as well as a number of other compounds. These chemicals repel approaching insects, deter feeding and oviposition on the plants, disrupt behaviour and physiology of insects in various ways and even prove toxic to different developmental stages of many insects.
Promising Pesticidal Plants:
Plants are biochemists par excellence. During their long evolution, plants have synthesized a diverse array of chemicals to prevent their colonization by insects and other herbivores. It is estimated that there are about 2,50,000 to 5,00,000 different plant species in the world today.
Only 10 per cent of these have been examined chemically indicating that there is enormous scope for further work. Over the years, more than 6000 species of plants have been screened and more than 2500 plant species belonging to 235 families were found to possess biological activity against various categories of pests.
The highest number of pesticidal plants belong to Meliaceae (>500), followed by Fabaceae (157), Asteraceae (147), Myrtaceae (72), Euphorbiaceae (63), Leguminosae (60), Verbenaceae (60), Cryptogams (58), Ranunculaceae (55), Labiatae (52) and Solanaceae (52).
This number seems to be far less than the actual number of naturally occurring pesticidal plants as this is just 0.77 per cent of the total 3,08,000 species of plants or 0.87 per cent of 2,75,000 species of flowering plants. It is thus likely that novel and potent molecules that can be used for pest suppression remain to be discovered from many plant species.
Plants are known to produce a diverse range of secondary metabolites such as terpenoids, alkaloids, polyacetylenes, flavonoids, unusual amino acids, sugars, etc. Various isolated chemically from plants include 350 compounds that are insecticidal and more than 900 isolates that are feeding deterrent alone.
The structures of more than 600 alkaloids, 3000 terpenoids, several thousands of phenylpropanoids, 1000 flavonoids, 500 quinones, 650 polyacetylenes, and 4000 amino acids have already been elucidated. Many of these chemicals protect the plants from pests and pathogens.
But in addition to high insecticidal activity, plant species must possess some other characteristics for development into an ideal botanical insecticide, viz., safety to plant and animal life, biodegradability with sufficient residual action, ready availability of the plant or capability for cultivation with a reasonably short gestation period, economic isolation procedures for the active component(s) or capacity for formulation of crude extracts obtained from plant parts, and yield products of consistent quality.
Some of the pesticidal plants are:
1. Neem:
Neem, Azadimchta indica A. Juss. (Fam. Meliaceae) is indigenous to India from where it has spread too many Asian and African countries. For centuries, the tree has been held in esteem by Indian folk because of medicinal and insecticidal value. A breakthrough in the insecticidal application of neem was made by Pradhan et al. (1962) who successfully protected the standing crops at Indian Agricultural Research Institute, New Delhi, by spraying them with 0.001 per cent neem seed kernel suspension during a locust invasion.
Due to its legendry insect-repellent and medicinal properties, it has been identified as the most promising of all plants by the National Research Council, Washington, USA. Neem has assumed the status of an international tree which is evident from the fact that it has been a subject of discussion at several global conferences, viz. Rottach-Egern, Germany (1980), Rauischolzhausen, Germany (1983), Nairobi, Kenya (1986), Bangalore, India (1993), Queensland, Australia (1996), Vancouver, Canada (1999), Mumbai, India (2002), Kunming, China (2006) and Coimbatore, India (2007).
All parts of the neem tree possess insecticidal activity but seed kernel is the most active. Neem bark, leaf, fruit and oil as well as extracts with various solvents especially ethanol have been found to exhibit activity against insect pests. Neem products exhibit almost every conceivable type of activity against insects.
In addition, neem possesses fungicidal, nematicidal, bactericidal, molluscidial, diueretic, and antiarthritic properties. It also exhibits immunomodulatory, anti-inflammatory, antihyperglycaemic, antiulcer, antimalarial, antiviral, antioxidant, antimutagenic, and anticarcinogenic effects. Azadirachtin has systemic effects in certain crop plants, greatly enhancing its efficacy and field persistence.
Neem products also affect insect vigour, longevity and fecundity. Females of E. varivestis and Leptinotarsa decemlineata (Say) were sterilized by neem compounds, while reproductive maturation was inhibited in N. lugens males. At higher concentrations, most females did not emit normal male eliciting signals. During the last decade, neem products have been found to act as ovipositional deterrents for Bactrocera cucurbitae (Coquillett), H. armigera, Spodoptera litura (Fabricius), Callosobruchus spp., etc.
Ovicidal activity of neem products has been reported in Corcyra cephalonica (Stainton), Earias vittella (Fabricius) and S. litura. Strong contact effects of neem oil leading to transformation of gregarious nymphs to intermediate and solitary forms have been recorded in locusts. Direct contact toxicity of neem products has been demonstrated against termites, Macrotermes sp. and aphids, Lipaphis erysimi (Kaltenbach), Rhopalosiphum nymphae (Linnaeus), etc.
Neem biopesticides have been extensively used in IPM modules for the management of insect pests of various crops. Studies have indicated that in rice IPM, neem treatments did not allow the occurrence of brown planthopper, green leafhopper, yellow stem borer and gall midge to exceed economic injury levels.
On an average, rice yield was 20.5 per cent more under IPM treatments using neem than under farmers’ practice treatment. In one of the cotton IPM modules, neem oil mixed with detergent (2.5 litres/ha) has been found effective for the management of jassids, whitefly and aphids. Use of neem oil 0.5 per cent and teepol 0.1 per cent and neem seed kernel extract (NSKE) 5 per cent have been effective against cotton bollworms and sucking pests.
An IPM module for groundnut resulted in 24-46 per cent control of major insect pests (white grub, leaf miner, tobacco caterpillar, gram pod borer, jassids, thrips and aphids) and diseases from 24-48 per cent. It also resulted in average increase in yield by 19 per cent and higher monetary returns by 40 per cent as against farmers’ practice.
Nearly 100 protolimonoids, limonoids or tetranor-triterpenoids, pentanortriterpenoids, hexanortriterpenoids and some non-terpenoids have been isolated from various parts of the neem tree and still more are being isolated Azadirachtin, the most important biologically active component of neem shows phagorepellent and toxic effects at 0.1 to 1000 ppm when incorporated into diets of different insect species.
Azadirachtin is safe to mammals-the rat oral acute LD50 is > 5000 mg kg-1. A 90-day oral feeding of rats with 10,000 ppm of azadirachtin did not show chronic toxicity. Azadirachtin was synthesised in the laboratory by scientists of the University of Cambridge (UK) in 2007, after 22 years of research when structure of azadriachtin was determined in 1985.
Inspite of these impressive strides, large scale practical utilization of neem in pest management still faces several hurdles. There is an urgent need to characterize best ecotypes of neem tree for different environmental conditions. Intensive breeding and selection work needs to be undertaken for economic production of various high quality raw materials required for insecticide production. The development of new techniques for harvesting, depulping, drying and storage of neem seeds is a necessary pre-requisite for large scale production of neem-based pesticides.
Ironically, the neem tree itself is attacked by 60 species of insects besides mites, nematodes, mammals and 16 phytopathogens. Some of these like Pulvinaria maxima Green, Aonidiella orien talis (Newstead), Helopeltis antonii Signoret and Rhizoctonia solani Kuhn are already causing serious damage in some areas.
As commercially high quality genotypes are introduced on a large scale, the pest and pathogen problems may be aggravated. It is, therefore, imperative that the neem ecosystem is thoroughly studied to manipulate various biological components and cultural practices to keep the damaging organisms in check.
Another problem facing the utilization of neem is the phytotoxic effects recorded in several crops, viz. cabbage, onion, muskmelon, potato, tobacco, tomato, etc. Dose-response relationships need to be worked out for all the crops where neem products appear to be useful for use in IPM.
Commercialization of neem-based pesticides is expanding at a rapid rate. Worldwide, there are over 100 commercial neem formulations available. In India many products are either being marketed or are awaiting commercialization. However, lack of standardized and reliable formulations is an important constraint in increased, use of neem products for pest control.
To overcome these difficulties, the Central Insecticides Board in 1991 approved the guidelines and data requirements for registration of neem based pesticides. Formulated neem products are required to contain at least 1500 ppm of a.i. azadirachtin in kernel based formulations and 300 ppm in neem oil based ones. Recently, neem formulations with 10,000 and 50,000 ppm of azadirachtin have been introduced.
Surveys conducted in India have shown that farmers usually buy commercial products, which are also difficult to obtain in the local market. Improved local production units, which incorporate inexpensive equipment and chemicals, would probably make neem pest control attractive to many more farmers.
Many times neem products have shown inconsistent results in the field. The products available in the market and/or prepared locally vary widely in quality and quantity of different active components. A survey of neem oil samples in Canada revealed that 2 samples completely lacked detectable amounts of azadirachtin while remaining samples contained 200 to 4000 ppm (0.02-0.4%) azadirachtin.
In another study, one sample containing 6800 ppm was also detected. There is an urgent need to monitor market samples of neem products so that farmers are able to get consistent and reliable results while using neem based pesticides.
2. Chinaberry:
Melia azedarach Linnaeus, commonly known as Dharek, Chinaberry tree, China tree, Persian lilac or ‘Pride of India’ is a close relative of neem tree. Very strong antifeedant effects of the tree against locusts, Schistocerca gregaria (Forskal) were discovered during the locust invasion of Palestine in 1915. It was observed by an adolescent, Rachel Shpan-Gabrielith, that while all other vegetation was nearly completely devoured, the Persian lilac was almost undamaged.
Subsequently, laboratory studies by her established that the foliage remained untouched by locusts even after one week of starvation. The hot water extracts of the plant applied to wheat bran also prevented feeding by the locusts. She subsequently successfully used this technique to save several crops during later invasions (1945, 1951) by locusts.
The work on active principles established that several tetranortriterpenoids (limonoids) related to azadirachtin were present in M. azedarach. The compounds isolated included meliacarpinins, azedarachins, ammorastatins, amoorastatone, trichlins and nimbolidins.
Among the meliacarpinins, 1-cinnamoyl-3, acetyl-11-methoxymelia-carpinin with antifeedant index (AI50) of 50 ppm was the most active. Among the four azedarachins, azedarachin-A was twice as active (200 ppm) as other azedarachins against Spodoptera eridania (Cramer) larvae.
A wide range of behavioural, physiological and toxic effects were observed against several insect species including Epilachna varivestis Mulsant, Nilaparvata lugens (Stal), Mythimna separata (Walker), Plutella xylostella (Linnaeus), etc. Extracts in different solvents as well as pure compounds were found to exhibit phagodeterrent, oviposition deterrent, fecundity and longevity reducing, development disrupting and toxic effects. Limited work on natural enemies also revealed that Chinaberry products are comparatively safe to coccinellids and predatory mites, Amblyseius sewsami Evans.
At least 36 limonoids including toosendanin have been isolated from stem and root back of Melia toosenden Siebold & Zucc. as antifeedant constituents. Based on toosendanin, a product has been developed and registered in China for use against fruit and vegetable pests.
A number of toxic tetranortriterpenoids namely meliantriol, 12-hydroxy amoorastatone, 12-hydroxy amoorastatin and 12-acetoxy amoorastatin have been isolated from the dried stem bark. Since these meliatoxins exhibit mammalian toxicity, it needs to be ensured that any potential product based on M. toosendan must be devoid of such harmful toxicants.
3. Chrysanthemum:
Pyrethrum, derived from the dried flowers of Tanacetum (Chrysanthemum) cinerariaefolium (Treviranus) (Fam. Asteraceae), has been used as an insecticide since ancient times. The original home of the plant is Middle and Near East. Its commercial use originated in Persia from where it was introduced to Europe, America and Japan in the nineteenth century.
After the First World War, its cultivation was taken up in Africa and majority (>75%) of the world supply of pyrethum was produced in Kenya and Tanzania. However, its production began in Tasmania (Australia) in 1996 and it produces almost one-half of the world supply. A related plant, Persian insect flower, T. roseum (Adams), is the basis of the pyrethrum industry in Papua New Guinea. Worldwide annual production of pyrethrum now averages 30,000 tonnes.
Pure pyrethrins are moderately toxic to mammals (rat oral acute LD50 values range from 350 to 500 mg kg-1) but technical grade pyrethrum is considerably less toxic (almost 1500 mg kg-1). Pyrethrum is a highly effective insecticide against common household insects like house flies, mosquitoes, fleas and lice.
It is safe to mammals and is easily broken down to non-toxic metabolites. The insecticidal principals in pyrethrum comprise pyrethrins I and II, cinerins I and II, and jasmolins I and II. Pyrethrin I is the most effective. Pyrethrins act quickly on the insect central nervous system, causing a knockdown effect.
Addition of sesame oil or synergists like piperonyl butoxide enhances the insecticidal activity of pyrethrins, which causes most flying insects to drop almost immediately upon exposure. Because of its low toxicity to vertebrates, pyrethrum finds wide acceptance worldwide. Like other natural pesticides, pyrethrins have limited stability and shelf-life, and under field conditions are rapidly degraded by sunlight and heat.
The use of pyrethrum in agriculture is mainly restricted to some vegetable and fruit crops due to its high cost and photo-instability. Efforts were, therefore, made from 1940s onwards to develop photostable analogues resulting in the development of synthetic pyrethroids.
4. Tobacco:
Tobacco, Nicotiana tabacum Linnaeus, has been cultivated by the American Indians for at least 1000 years and it remained a part of their religious ceremonies. Long before people knew of nicotine alkaloid in tobacco, the latter was being used as a dust or water extract to control phytophagous insects, some three hundred years ago.
Nicotine has by now been isolated from at least 18 species of plants. N. rustica Linnaeus containing 18 per cent nicotine is a better source than the more familiar N. tabacum containing 6 per cent nicotine. Many of these plants also contain the related alkaloids, nornicotine and anabasine. N. glauca Graham grown in Argentina and Uruguay contains a higher amount of anabasine.
Systematic use of nicotine sulphate started with the introduction of a standardized pesticide formulation containing 40 per cent actual nicotine around 1910. Before the Second World War, nicotine sulphate was a very popular insecticide around the globe. With the ady6nt of synthetic insecticides, it lost its ground due to less persistence and high cost. Now that interest in botanicals is being revived, nicotine sulphate has also resurged as a preferred pesticide.
On a world-wide basis, about 600,000 kg of nicotine sulphate and 75,000 kg of pure nicotine were being produced annually till recently, mostly in UK, India, Germany and the Netherlands. India manufactures around 800 tonnes of nicotine sulphate annually and exports the entire quantity to Japan and Europe.
The cost may be a prohibitive factor in its use in India. But the pesticide could be obtained from 35-40 million kg of tobacco waste produced in the country annually. Recently, cost effective methods have been developed for extraction of nicotine sulphate from tobacco waste.
Nicotine sulphate is effective against a wide range of pests. Its efficacy against soft bodied insects like aphids is well known, but it has also been found effective against whitefly, thrips and bollworms in cotton; brown planthopper and green leafhopper in rice; grubs in brinjal, potato and cauliflower. Recently, nicotine sulphate (0.2 and 0.4% a.i.) has been found highly toxic to eggs and neonate larvae of H. armigera and S. litura. It was also found highly effective against Bemisia tabaci (Gennadius) under field conditions.
Nicotine sulphate is safe to coccinellids but toxic to chrysopids. It does not leave any residue on the crop. However, its low persistence necessitates a repeated number of sprays for effective control of insects. It may prove very useful for use on fruit, vegetable and edible oilseed crops, where residues of pesticides are not acceptable.
However, because of its high mammalian toxicity (oral human lethal dose is 60 mg) and detrimental effect on human health, the use of nicotine as an insecticide has decreased tremendously. As a result, it is seldom used today in North America or Europe, although it continues to be used in China and crude tobacco extracts are used in Africa.
5. Rotenone Plants:
Rotenone and related alkaloids occur in the roots of tropical legumes, Derris and Lonchocarpus plants and in leaves and seeds of Tephrosia plants, all belonging to family Fabaceae. Derris is native to East Asia, Lonchocarpus to American tropics and Tephrosia to Eastern and Southern Africa.
Traditionally, all these plants were used as fish poisons by the people for centuries. The commercially important species Derris elliptica (Wallich) Benth contains 4-5 per cent rotenone while Lonchocarpus nicou (Aubl.) DC contains 8-10 per cent rotenone in dried roots.
The vogel tephrosia, Tephrosia vogelii Hook f. is regarded as a more promising source of rotenone than Derris and Lonchocarpus. Rotenone is commonly sold as dust containing 1 to 5 per cent active ingredients for home and garden use, but liquid formulations used in organic agriculture may contain as much as 8 per cent rotenone and 15 per cent total rotenoids.
In addition to rotenone, these plants contain a number of other flavonoids like deguelin, tephrosin, elliptone, sumatrol, toxicarol, malaccol, etc., which show toxic as well as various behavioural and physiological effects on the insects. Rotenone is mainly active as a contact poison with some action as a stomach poison. It mainly acts as a site I respiratory inhibitor.
Its rapid degradation in sunlight has limited its utility in crop protection. Synthetic rotenoids have so far proved very costly because of the complex chemical structure. Its use has also been discontinued due to high fish and/or mammalian toxicity. Pure rotenone is quite toxic to mammals; the oral acute LD50 in rats is 132 mg kg-1.
6. Pongram:
Pongamia pinnata Linnaeus (Syn. P. glabra Vent.), variously known as karanja, puna oil tree, Indian beech or Pongram is also indigenous to India. Karanj seed oil is rich in karanjin, a furanoflavonoid and a host of other polyphenolics like pongamol, pongapin, glabrin, karanj ketone, karanjone and pongaglabrone. Karanjin has been found effective against mites, scales, chewing and sucking insect pests.
It is a potent deterrent to many different genera of insects and mites, and is effective against whiteflies, thrips, leafminers, caterpillars, aphids, jassids, beetles, mealybugs, etc. on a wide range of crops. Karanjin has a dramatic antifeedant and repellent effect with many insects avoiding treated crops. Insect antifeedant, growth reduction and miticidal activity of karanj oil have been attributed to the presence of high concentration of karanjin and pongamol in the oil.
Karanja oil applied as a surface protectant effectively checked the infestation of pulse beetles, Callosobruchus maculatus (Fabricius) and C. chinensis (Linnaeus) and other storage pests like Rhyzopertha dominica (Fabricius) and Sitotroga cerealella (Olivier).
A concentration of 1 per cent afforded complete protection even after 150 days and did not alter taste and smell of the grains. Pongamia cake was found effective in controlling the attack of ground beetles on tobacco. It also did not leave any of harmful residues in the soil. Pongamia cake water extract was found effective for protecting tobacco seedlings from S. litura damage.
The oil of karanja repelled brown planthopper in rice and significantly reduced its ingestion and assimilation of food. Both brown planthopper and whitebacked planthopper suffered heavy mortality but green leafhopper was less susceptible. Other pests which have been reported susceptible to powders or extracts of Pongamia are Henosepilachna vigintioctopunctata (Fabricius), Amsacta moorei (Butier), Chilo partellus (Swinhoe), Papilio demoleus Linnaeus, Leucopholis lepidophora Blanchard, etc. Various types of bioactivity observed were antifeedant, fecundity curtailing and toxicity.
7. Custard Apple:
Custard apple, Annona squamosa Linnaeus and other Annona species are well known for their pesticidal activity. Annonins (acetogenins) and related compounds namely squamocin, asimicin and annonacins occur widely in twigs and branches, unripe fruits and seeds of several Annona species.
Most of the acetogenins performed better than the conventional insecticides. Powdered seeds applied to wheat and rice grains act as a protectant against Sitophilus oryzae (Linnaeus) and C. chinensis. The plant extracts act as a feeding deterrent against A. moorei, Oncopeltus fasciatus (Dallas), N. lugens, Dicladispa armigera (Olivier), Nephotettix virescens (Distant), S. litura and H. vigintioctopunctata.
As with other botanical insecticides, disruption of growth, reduced oviposition, reduced adult emergence and moderate toxicity has also been observed in different species. Annonine, an alkaloid found in the stems and leaves of custard apple, has been found effective in checking the infestation by termites, root grubs, etc.
8. Sabadilla:
Sabadilla formulations were originally obtained from Sabadiila officinarum Brandt, a lily that grows wild in Central and South America. The commercial source of sabadiila was however a related plant cevadilla or caustic barley, Schoenocaulon officinale (Schitdl. & Chem.) (Liliaceae), which subsequently became widely cultivated in Venezuela. S. officinale and several other plant species including the false hellebore, Veratrum album (Verat) (Melanthaceae) produce insecticidal veratrine alkaloids.
The veratrine is commonly sold under the trade names ‘Red Devel’ or ‘Natural Guard’. This compound gained popularity during Second World War, when other botanicals like pyrethrum and rotenone were in short supply. The veratrine alkaloids comprise about 0.3 per cent of the weight of aged sabadiila seeds; of these alkoloids, cevadine and veratridine are most active insecticides. Other alkaloids present in the seed and in insecticidal extracts include sabadinine, sabadiline and sabadine.
Sabadilla was used historically for the control of insect pests on crops, animals and humans. Since the advent of synthetic insecticides, sabadilla’s use has declined and organic gardeners currently provide the major market for sabadilla products. Sabadilla acts as a contact and stomach poison for the control of a variety of pest species. Grasshoppers, house flies, jassids, lice, thrips and various caterpillars have been reported to be controlled with crude extracts.
The pests for which sabadilla, in particular, is considered effective including hemipteran bugs such as the squash bug, Anasa Tristis (DeGeer); chinch bug, Blissus leucopterous (Say); and the stink bugs. Although not persistent, sabadilla is known to be toxic to some soil bacteria and honey bees. The active ingredient veratrine with an oral LD50 of 4000-5000 mg/kg is considered among the least toxic of botanical pesticides.
9. Ryania:
The compound ryanodine has been derived from the woodly stem tissue of the shrub, Ryania speciosa Vahl. (Flacourtiaceae), a plant native to South America and is sometimes referred to as ryania. Ryania represents one of the first examples of a commercially successful natural insecticide discovered during 1940s, by randomly screening plant extracts for activity. It began to be marketed for pest control from 1945 onwards. Among the 11 compounds identified with insecticidal activity, the most active constituents were ryanodine and 9, 21-dehydroryanodine.
Ryanodine is a contact and stomach poison that is more stable than many other botanical pesticides. The residual toxicity of ryanodine has been reported to be for over one week after application. It is used mostly for the control of caterpillar pests of fruits and foliage.
The codling moth, Cydia pomonella (Linnaeus) in apples and pears; citrus thrips, Scirtothrips citri (Moulton) in citrus; and the European corn borer, Ostrinia nubilalis (Hubner) in corn are among the most common targets of ryania used by organic farmers. However, high costs associated with large scale production and processing of extracts are now becoming prohibiting except for home gardeners and organic producers.
10. Quassia:
Quassia was originally extracted from Quassia amara Linnaeus (Simaroubaceae), a Central American tree with a characteristically bitter bark and wood. However, in the eighteenth century, the commercial extracts of quassia were obtained from the related shrub, Aeschrion excelsa (Sw.) Kuntze.
The yellowish white wood is the source of quassia chips from which a bitter extract with insect killing activity is prepared. The active component within extracts is quassin, a water soluble molecule that acts as a contact and stomach poison. Quassin has been shown to possess systemic activity; it is a potent aphicide and toxic to a number of lepidopteran pests.
In India, farmers have been seen to use extracts from Picrasma excelsa (Sw.) Planchon (shrub closely related to A. excelsa) for pest control. At least 31 quassinoids from P. ailanthoides Planchon have been reported to be potent antifeedant and insecticidal compounds against Plutella xylostella (Linnaeus). The quassinoids reported from Simaba multiflora A. Juss. and Soulamea soulameoides (Gray) Nooteboom are feeding deterrents against Heliothis virescens (Fabricius) and Spodoptera frugiperda (J.E. Smith), and induce toxic and bioregulatory effects in H. virescens.
11. Essential Oil Bearing Plants:
Essential oils are volatile oils that have strong aromatic components which give distinctive odour, flavour or scent to a plant. These are the by-products of plant secondary metabolites. Essential oils are found in glandular hairs or secretory cavities of plant cell wall and are present as droplets of fluid in the leaves, stems, bark, flowers, roots and/or fruits in different plants. Plant essential oils are produced commercially from several botanical sources, many of which are members of the mint (Lamiaceae), carrot (Apiaceae), myrtle (Myrtaceae) and citrus (Rutaceae) families.
Among higher plants, there are 17,500 aromatic plant species and approximately 3,000 essential oils are known out of which 300 are commercially important for cosmetics, perfume, and pharmaceutical industries, apart from pesticidal potential. The oils are generally composed of complex mixtures of monoterpenes, biogenetically related phenols, and sesquiterpenes.
There are several examples of essential oils, which are known for their pest control properties. Eugenol from cloves, Eugenia cryophyllus (Sprengel) Bullak & Harr. (Myrtaceae); 1, 8-cineole from eucalyptus, Eucalyptus globulus Labill. (Myrtaceae); citronellal from lemon grass, Cymbopogon nardus (Linnaeus) Rendle; pulegone from pennyroyal, Mentha pulegium Linnaeus (Labiatae); thymol and carvacrol from thyme, Thymus vulgaris (Linnaeus) (Labiatae) are among the most active constituents against insects. In addition to their toxic effects, most of these compounds act as larval growth inhibitors, antifeedants and repellents to a wide range of insects, mites and even nematodes.
Some terpenoids like thymol and carvacrol were more effective for deterring oviposition by Aedes aegypti (Linnaeus) mosquitoes than N, N-diethyl-m-toluamide (DEET). The essential oil from the rhizomes of sweet flag, Acorus calamus Linnaeus (Araceae) is known for its insecticidal and antigonadal actions associated with its most abundant constituent β-asarone Vulgarone B, isolated from Artemisia douglasiana Besser; apiol, isolated from Ligusticum hultenii Fernald; and cnicin isolated from Centaurea maculosa Lamarck, exhibited high termiticidal activity against the Formosan subterranean mite, Coptotermes formosanus Shriaki.
Ginger oleoresin extracted from fresh rhizomes of Zingiber officinale Roscoe (Zingiberaceae) is a complex mixture of several closely related phenolic alkalones such as gingerols, shogaols, gingerones, paradols, gingerdiols and diarylheptanoids. Ginger based products have been found to exhibit insect growth regulatory and antifeedant activity as well as antifungal activity.
Turmerone and (ar) turmerone (dehydroturmerone), the major constituents of turmeric, Curcuma longa Linnaeus (Zingiberaceae) rhizome powder oil, are strong repellents to stored grain pests. The turmeric leaves, the unutilized part of turmeric plant, on hydrodistillation, yielded oil rich in 2-phellandrene that inhibited the growth of Spilarctia obliqua (Walker), Plutella xylostella (Linnaeus) and several stored product beetles. The active compounds in garlic, Allium sativum Linnaeus (Amaryllidaceae) have antibacterial, antifungal, nematicidal, amoebicidal, insecticidal and insect repellent properties.
It is the storehouse of a large number of bioactive molecules which include allin, allicin, garlicin, opine and allyl sulfides. The garlic extract has been found to be toxic to house fly, mosquitoes and storage pests, and highly repellent to adults of cockroach, Blatella germanica (Linnaeus). It has also proved to be oviposition deterrent, and toxic to eggs and larvae of P. xylostella.
Today, essential oils represent a market estimated at US$ 700 million and a total world production of 45,000 tons. Almost 90 per cent of this production is focused on mint and citrus plants. Several private companies produce essential oil-based insecticides for controlling greenhouse pests and diseases, and for controlling domestic and veterinary pests. In general, essential oils and their major constituents are relatively non-toxic to mammals, with acute oral LD50 values in rodents ranging from 800 to 3000 mg kg-1 for pure compounds and > 5,000 mg kg-1 for formulated products.
Effect on Non-Target Organisms:
1. Effect on Natural Enemies:
The application of NSKE for the control of S. litura did not affect the emergence of the egg parasitoid, Telenomus remus Nixon, and predator, Brinckochrysa scelestes (Banks). However, longevity of the parasitoid was reduced in case of oviposition on pretreated egg masses. Topical application of seed oil of neem, Chinaberry and custard apple showed no adverse effect on predatory spider, Lycosa pseudoannulata (Bosenberg & Strand) and was only slightly toxic to the mirid bug predator, Cyrtorhinus lividipennis Reuter at and above 10 µg/female.
Neem formulations, Repelin and Neemguard, were relatively safe at lower concentrations to the egg, larval and pupal parasitoids, viz. Trichogramma australicum (Girault), Bracon hebetor Say and Tetrastichus israeli (Mani & Kurian) of Opisinia arenosella Walker. Repelin, Neemark and nicotine sulphate were safe to the predatory coccinellid, Menochilus sexmaculatus (Fabricius) but highly toxic to its hyperparasitoid, Tetrastichus coccinellae Kurdyumov. Neemark and Repelin worked as good acaricides against Tetranychus macfarlanei Baker & Pritchard on okra, but were safer to predatory mites.
In laboratory trials with NSKE on bean leaf discs, predatory phytoseiid mite, Phytoseiulus persimilis Athias-Henriot was less affected than the casmine spider mite, Tetranychus cinnabarinus (Boisduval). In the same trials, predatory clubionid spider, Chiracanthium mildei Koch was unaffected.
Neem formulations, viz. Neemark, Repelin, Wellgro, neem seed kernel suspension (NSKS) and Neemrich did not show toxic and ovicidal effects against the green lacewing, B. scelestes but acted as oviposition repellents against the females. On the other hand, nicotine sulphate was toxic to adults and showed ovicidal activity against the eggs of the lacewing. Parasitization of whitefly, Bemisia tabaci (Gennadius) nymphs by Eretmocerus spp. was not affected by application of various botanical insecticides, viz., mineral oil, neem oil, nicotine sulphate, saradine oil, etc.
There have been reports indicating moderate to high adverse effects of neem products on natural enemies. Coccinellid predator, Delphastus pusillus (LeConte) preferred untreated eggs of B. tabaci. The emergence of aphelinid parasitoid, Eretmocerus californicus Howard from treated B. tabaci was reduced by more than 50 per cent. Untreated whiteflies were attacked at a rate three times more than that for treated ones.
Emergence of eulophid parasitoid, T. howardi from pupae of M. patanalis treated with 1000 ppm neem seed bitters decreased. A dose of 50 µg/female of neem seed bitters proved lethal to the parasitoid, Goniozus triangulifer Kieffer. The emergence of parasitoids from treated leaf folder, M. patanalis hosts as well as the fecundity of such parasitoids were reduced. Treatment of cocoons at 2.5 per cent or higher concentrations of neem oil reduced emergence of braconid parasitoid, Apanteles plutellae Kurdyumov in the laboratory.
2. Effect on Man:
Neem preparations have been used since ancient times in unani medicine for the treatment of a wide range of human ailments due to their anti-inflammatory, concoctive, blood purifying, anti- leprosy, anti-arthritic, anti-pyretic, anti-microbial and anthelminthic actions. Injections of sodium nimbidinate at 1 g and its oral administration up to 7 g to human beings did not produce any harmful effects.
Cases of neem seed oil intoxication have, however, been reported. The intake of oil resulted in vomitting, drowsiness, metabolic acidosis, etc. It has been reported that the oil uncouples mitochondrial oxidative phosphorylation, thus inhibiting respiratory chain. Normal human cells in culture were not affected by neem extracts at 5 mg/ml while tumor-originated cells degenerated.
In USA, Margosan-O underwent comprehensive toxicological tests prior to registration by US Environmental Protection Agency (EPA). Tests for skin irritation, inhalation, mutagenicity, and immune response were low enough to allow EPA registration. There have been a number of reports of death of children and adult human beings after consuming neem oil in South India.
These poisonings apparently resulted because the neem seeds from which the oil was extracted had been contaminated with aflatoxin producing fungus, Aspergillus flavus Link. When extracted from clean and fungal-free seed kernels, neem oil did not cause any oral toxicity in laboratory rats at 500 mg/ kg body weight.
Neem preparations have been found to have no teratogenic and carcinogenic effects. In the standard Ames mutagenicity test, azadirachtin showed no mutagenic activity on strains of Salmonella thyphimurium (Loeffler) castellanichalmers. There are extensive records of human exposure and response to pyrethrum used as an insecticide and these establish the negligible hazards to the users of pyrethrum products.
On the other hand, several other plants are known to be highly toxic to humans and other mammals. A single leaf of yellow oleander, N. oleander, is potentially lethal to humans. Affected persons become dizzy and drowsy; heart beat becomes progressively weaker and irregular leading to dyspnoea and coma.
3. Effect on Other Organisms:
Recent studies have shown that neem leaf extract completely stopped aflatoxin production by Aspergillus flavus Link and A. parasiticus Speare. These fungi which grow widely on various foods are one of the most deadly organisms on earth and the aflatoxins produced by them are highly carcinogenic. The use of neem for inhibiting aflatoxin production may open the door to a simple, inexpensive method for protecting stored foods using locally produced materials, even in the remotest villages.
Neem oil extracts at 0.005 per cent used as a mosquito larvicide were non-toxic to insectivorous fish, Gambusia sp. Concentrations upto 0.01 per cent were essentially non-toxic to the fish. However, at 0.04 per cent concentration, 80 per cent tadpole mortality was obtained within 24 hours and 100 per cent in 48 hours.
The LC50 values of Margosan-0 to rainbow trout, Salmo gairdneri Richardson and blue sunfish, Lepomis macrochirus Rafinesque, 96 hours after treatment were 8.8 and 37 mg/1 of water, respectively. The 96 hours no effect concentrations for the two species were 5 and 20 mg/litre, respectively. Young guppies, Lebistes reticulatus (Peters) tolerated 100 ppm AZI-VR-K/litre of water.
It has been reported that toxicity of Margosan-0 to fish and other aquatic organisms is caused by its petroleum oil content (15%) or probably another compound used for its formulations. Margosan-0 is also toxic to the water flea, Daphnia magna Straus and other invertebrates that inhabit stagnant water.
The feeding of water extract of neem berries to poultry birds resulted in toxicity symptoms like sluggish movement, dropping head, etc., and in many cases even death. The liver underwent degenerative changes with focal congestions, retention of bile in gall bladder and congestion of kidney with localized haemorrhages. The hepato- and nephro-toxic effects may prove lethal.
Trial feeding of neem seed to starter chicken caused severe heptatitis with necrotic patches, mild to severe nephritis with congestion and slight inflammation in intestine. However, compared to synthetic insecticides, the toxic effects were produced at higher dosages. The acute LD50 of Margosan-0 to mallard duck, Anas platyrhynchos Linnaeus is 16.0 mg/kg. Acute oral LC50 of this compound to bobwhite quail, Colinus virginianus (Linnaeus) and mallard duck is in excess of 7000 ppm in a 5-day test period.
Traditionally, neem-based preparations have been used for the treatment of a wide range of disorders in domestic animals and livestock. Neem leaves contain appreciable amounts of protein, minerals and carotene, and adequate amounts of trace minerals except zinc. Incorporation of 20 per cent neem cake in sheep diet increased the growth rate.
But neem leaves have also been reported to cause toxic effects on sheep, goats and guinea pigs. The acute oral LC50 of Margosan-O against rat, Rattus sp. is 5.0 ml kg-1 and acute inhalation LC50 < 43.9 mg/hours. Lantana camara Linnaeus is toxic to sheep when given orally at 60 mg/kg body weight. Affected animals show depression and anorexia, a few days after ingesting. Death occurs a few weeks later and may be due to renal failure.
Environmental Impact of Botanical Pesticides:
The use of botanical pesticides for plant protection has assumed greater importance in recent years all over the world due to environmental deterioration and health hazards associated with the use of synthetic pesticides. It is hoped that extensive use of plant-based pesticides in integrated pest management will help in conserving environmental quality.
The neem tree has answers to several environmental problems such as the rehabilitation of degraded ecosystem and wastelands, reduction in the use of agro-chemicals such as fertilizers and synthetic pesticides, and generation of income for all small farmers with limited resources. The neem-based pesticides are relatively safe and do not leave any residues on agricultural produce.
As the pesticidal preparations using neem seeds, leaves or cake are very simple, farmers may make direct use of these products locally. The use of neem products for plant protection will help in minimizing atmospheric pollution and prevent food poisoning. It will also reduce the demand for costly chemical pesticides.
Neem oil has found a wide range of industrial applications in India. Due to its antiseptic properties, it has been a major ingredient in soaps for atleast 50 years. In addition, pharmaceutical preparations like emulsions, ointments, poultices and liniments as well as cosmetics such as creams, lotions, shampoos, hair tonics and gargles have been prepared. Neem toothpaste and neem sticks are also widely used for cleaning of teeth and as a mouth freshener. Neem oil is non-drying and is, therefore, used to grease cast wheels.
Neem is a valuable forestry species in India. Being a hardy species, it is ideal for reforestation programmes and for rehabilitating degraded, semiarid and arid lands and coastal areas. During a severe drought in Tamil Nadu during June-July, 1987, it was witnessed that neem trees remained luxuriant while all other vegetation dried up.
Neem is useful as windbreaks and in areas of low rainfall and high wind speed, it can protect crops from desiccation. Neem is also a preferred tree along avenues, in markets and near homelands because of the shade it provides. It can grow in and even neutralize acid soils that plague much of the tropics. It may also act as an important source of fuel.
Neem oil is burned in lamps. The wood has long been used in firewood. The husk from seeds produced as a waste during pesticide manufacture can also be used as fuel. The neem timber is durable and resistant to attack of termites and other pests. It can be used for making fence posts and poles for house constructions.
It is, thus, clear that all parts/products of the neem tree find a variety of uses in agriculture, and as raw materials for the manufacture of household articles, pharmaceutical preparations and other products. Even the industrial by-products have found application in agriculture and as fuel. Due to its successful cultivation even under adverse environmental conditions and utilization of all its products so that no waste material is left, neem growing can be an important component in environmental protection programmes.
Pest Resistance to Phytochemicals:
The naturally occurring phytochemicals exert a wide range of behavioral and physiological effects on insects and there is, thus less likelihood of development of resistance to these pesticides. Ironically, the neem tree itself is attacked by 60 species of insects besides mites, nematodes and 16 phytopathogens.
Some of these like Pulvinaria maxima Green, Aonidiella orientalis (Newstead), Helopeltis antonii Signoret and Rhizoctonia solani (Kuhn) are already causing serious damage in some areas. This indicates that some insects might adapt to limonoids in the future but in laboratory tests two genetically different strains of the diamondback moth, Plutella xylostella (Linnaeus), treated with neem seed extract showed no signs of resistance in feeding and fecundity tests up to 35 generations.
In contrast, deltamethrin-treated lines developed resistance factors of 20 in one line and 35 in the other. There was no cross resistance between deltatamethrin and neem seed extract in the deltamethrin resistant lines. However, two lines of Myzus persicae (Sulzer) of the same origin, when treated with pure azadirachtin, developed a 9-fold resistance to azadirachtin compared to non-selected control line after 40 generations.
Some resistance to pyrethrins has been reported among a few agricultural pests, particularly those with resistance to organochlorines, organophosphates and carbamates. However, the diversity of neem compounds and their combined effects on insects seem to confer a built-in resistance prevention or delay mechanism in neem.
Even then, the farmers should refrain from exclusive and extended application of single bioactive materials such as azadirachtin. Also, for durability sake, even novel neem-rich insecticides should be applied within the framework of integrated pest management programmes.
Integration with Other Tactics:
The use of pesticides of plant origin for the control of agricultural pests has a long history but has assumed greater importance in recent years due to environmental deterioration and health hazards associated with the use of synthetic pesticides. Botanical pesticides exert a range of behavioural and physiological effects on the colonization, development, growth, survival and multiplication of insects.
In view of their environmental safety, these pesticides offer an attractive alternative to synthetic pesticides for use in IPM. Botanical pesticides can be integrated with the use of parasitoids and predators, microbial pesticides and even synthetic chemical pesticides to achieve greater efficiency in pest control.
The application of neem seed kernel extract (NSKE) for the control of S. litura did not affect the emergence of the egg parasitoid, Telenomus remus Nixon, and predator, Brinckochrysa scelestes (Banks). Topical application of seed oil of neem, Chinaberry and custard apple showed no adverse effect on predatory spider, Lycosa pseudoannulata (Bosenberg & Strand) and was only slightly toxic to the mirid bug predator, Cyrtorhinus lividipennis Reuter at and above 10 µg/ female in rice crop.
An active neem seed kernel fraction evaluated against sorghum pests was found to be safe to midge, Contarinia sorghicola (Coquillett) parasitoid, Tetrasictus sp. and predator, Otitis sp. It was, however, toxic to Apanteles ruficrus (Haliday), a larval parasitoid of Mythimna separata (Walker). Field trials with neem oil for the control of sorghum aphid, Melanaphis sacchari (Zehntner) did not show any adverse effect on syrphids and coccinellids.
Botanical pesticides have been found to be compatible with synthetic chemical insecticides. The use of neem formulations in combination with conventional insecticides resulted in better control of pink bollworm and spotted bollworm on cotton. The use of neem pesticides and alphamethrin on cotton proved as good as the sprays of alphamethrin alone in checking the menace of bollworms and increasing the seed cotton yield.
Similarly, alternate application of high potency neem-based insecticides and synthetic insecticides led to the effective management of cotton whitefly. Neem and other plant products have also been found to increase the efficacy of microbial control agents like baculoviruses, B. thuringiensis and fungi.