Comparative toxicity of crude extracts of plants (10%) to
Pesticides once having entry to an environment either get into the complex web of life through food chain or different components of the environment through physical passages like drifting by air and aquatic runways. Such facts were meticulously described by Rachel Carson  in her book ‘Silent spring’ where she advocated for choosing either the chemical control or biological control to avoid creation of endless problems to mankind owing to pesticide use. Looking back at the history of tremendous potentiality of synthetic chemicals to manage insect pests followed by subsequent cases of failure of chemical control due to development of insect resistance and pest resurgence, here we intend to cite examples of selective toxicity of insecticides and reiterate its importance in management of insect pests. Pesticidal pollution is a global problem. Use of synthetic insecticides to control pest around the world has resulted in disturbances of the environment, secondary pest resurgence, pest resistance to pesticides, lethal effects to non-target organisms as well as direct toxicity to users. It has been reported that about 2.5 million tons of pesticides are used on crops each year and the worldwide damage caused by pesticides reaches $100 billion annually. The reason behind this amount of cost is the high toxicity and non biodegradable properties of pesticides and the residues in soil, water resources and crops that affect public health . Hence search for the environment friendly, highly selective, newer biodegradable pesticides for pest management programme has been advocated to be essential for last several decades.
South East Asian countries are the hubs for production of raw silks produced by sericigenous insects namely
Components of Seri-ecosystem in Assam
2. Pesticides in environment
Introduction of pesticides into a crop system subjects it into a variety of physico-chemical and biological processes which determine the persistence, fate and the ultimate degradation product. Many workers have shown that only a portion of pesticides sprayed onto crops reach their targets, the rest enter the atmosphere by spray drift, volatilization from soil or water, surface runoff, biotransformation by microorganisms, plants, animals, biomagnification through food chain and photodecomposition . One of the major environmental aspect is the effect of sunlight that may lead to various photoprocesses and to photoproducts which are mostly different from parent pesticides in the environmental properties and toxicological significance. The quantum of light energy emitted from the radiation is absorbed by pesticides in environment and this raises the energy state of the molecule, causes excitation of electrons leading to formation or disruption of chemical bonds. Photolysis of pesticides have been studied in water, soil and plant surface. Various sensitizers present in environment such as riboflavin, humic substance etc. absorb light energy and serve as donor of energy to pesticide acceptor and bring about photodecomposition of pesticides. Most organophosphates whose photochemistry have been studied are phosphorothioate and phosphorodithiotate compounds. Although not highly susceptible to photodegradation by UV light, malathion degrades to different photoproducts such as malaoxon, malathion diacid, o,o-dimethyl phosphorodithioic acid,, o,o-dimethyl phosphorothioic acid and phosphoric acid [12, 13]. Some of the compounds viz. malaoxon are more toxic than the parent compound. Insecticides aimed against pest population may enter non-target arena through spillage at sublethal level but even these sublethal dosages may exert considerable damage on behaviour and activity of non-target population . Pesticidal effect of insecticides at sublethal dosages may have long term effect and they may be expressed at a later part of the insect’s life [15,6]. Continuous exposure to sublethal dosages on the environment may on the otherhand help a pest to develop resistance mechanism against the toxic compound. Troitskaya and Chichigina  showed that combined use of bacterial and chemical insecticides in silk-producing areas possess a real danger to
3. Insecticide mechanism of action
The major classes of synthetic pesticides are organochlorines, organophosphates, carbamates and pyrethroides. Preliminary survey revealed that organophosphates and pyrethroides are two of the most common pesticide classes used by common farmers against pests of paddy and other crops and vegetables in Assam. They also belong to the most commonly used pesticide groups in tea gardens. Organophosphates like malathion, phosphamidon and dimethoate even at sublethal dose have been reported to be highly toxic against the larvae of
4. Natural products
Natural products are excellent alternative to synthetic chemical pesticides. Plants exhibit enormous versatility in synthesizing complex materials which have no immediate obvious growth or metabolic functions. These complex materials referred to as secondary metabolites are produced as constitutive and induced defense as a result of co evolution arising out of millions of years of plant-herbivore interaction [36-41]. They may be exploited for the management of insect pests owing to their ability to act as toxicant, repellent, antifeedant and insect growth regulators. They are non phytotoxic, biodegradable and have little or no mammalian toxicity [42,43]. Plant extracts and essential oils come under the category of “ Green pesticides” as they are safe, eco friendly and more compatible with environmental components compared to synthetic pesticides. 20th century witnessed an increasing trend in use of botanicals with more than 2000 bioactive plant species identified for their insecticidal and anti - pathogenic properties [39,40,44,45]. Natural insecticides such as pyrethrum, rotenone and nicotine have been used extensively for insect control . Limnoids such as azadirachtin and gedunin, present in species from Meliaceae and Rutaceae are recognized for their toxic effects on insects and are used in several insecticide formulations in many parts of the world [46,47]. Neem formulations have been found effective against the mulberry hairy caterpillar,
Grasserie (viral), Flacherie (bacterial), Muscardine (fungal) and Pebrine (protozoan) are four common diseases of silkworm and they have been causing heavy loss to silkworm crops in silk producing countries like India and China. Herbal extracts have been tried for control of these diseases. Isaiarasu
4.1. Plants of ethnic importance
Indigenous knowledge (IK) is unique to a particular culture and society. IK is embedded in community practices, institutions, relationships and rituals . It forms the basis for local decision-making in agriculture, health, natural resource management and other activities and constitutes an important component in the global knowledge system. In most cases, IK is an underutilized resource in the development processes . Learning from indigenous knowledge of specific communities used for generations after generations can improve the understanding of their local conditions and saves time, effort and money besides constituting the foundation for activities designed to address regional and global problems. Thus, the natural products based on the indigenous use of botanicals could be one way of mitigating the problems associated with inappropriate use of synthetic chemicals .
4.2. Botanicals in sericulture field
In this chapter we restrict our discussions to candidate plants for being used against a parasitoids of silk worm. In sericulture field, farmers of Assam traditionally sprinkle extracts of tulsi (
Further fractionation of ethanolic extract of the effective plants using an eleutropic series of solvents viz. petroleum ether, chloroform, butanol and water followed by subsequent bioassay showed that the petroleum ether extract of
Life Cycle of
|Sl. No||Plant||Cold water|
4.3. Botanicals in paddy cultivation
Farmers in some locations of Brahmaputra valley of Assam use plant materials in rice fields for prevention of pest infestation. For instance, robub tenga(
Similarly with the discussions in context with pests and their control in sericulture field, here we limit our discussions regarding traditional use of plants and their scientific validation in context with a representative pest of paddy.
Twenty two plants were found to be used traditionally in paddy fields against
Life cycle of
|Scientific Name||Common Name||Family||Part Used||Farmers in %|
|Gundhua ban||Asteraceae||Leaf (LF)*||01.0|
|Mahaneem||Meliaceae||Leaf and Seed||05.5|
|Akan||Asclepiadaceae||Leaf and stem||04.0|
|Rebab and Bor Tenga||Rutaceae||Fruits and Fruit peel||11.5|
|Koni bih||Euphorbiaceae||Leaf and Seed||03.0|
|Bhoot ara||Euphorbiaceae||Leaf and Stem-Bark||03.0|
|Patharuabihlongoni||Polygonaceae||Leaf, Stem, Root (RT)*||07.5|
|Kuku mah||Fabaceae||Stem-Bark and Leaf||06.0|
|Rakta karabi||Apocynaceae||Stem-Bark, Seed||02.00|
|Ricom, Tezmui||Rutaceae||Root-Bark (RB)*||03.00|
|Ongare||Rutaceae||Leaf and Stem Bark||02.00|
4.4. Botanicals in tea gardens
5. Selective toxicity of plant products
Integrated pest management emphasizes on use of pesticides having selective toxicity as against the use of broad spectrum pesticides. This reminds us of classical examples of using different insecticides for controlling aphids and thrips in potted chrysenthemum in glass houses of Southern Britain in 70’s at the advent of development of the concept of integrated pest management . Apart from the aspects why plant products are considered better options for pest control as discussed above, if they can be categorized for having differential efficacy against different insects, the latter can give an added value to plant products for being used in pest control. However the efficacy also depends not only on the compound(s) present in the plant preparation but also on the ability of the insect to defend against the compounds they are subjected to.Less tissue susceptibility, presence of detoxifying enzyme, high immune response, development of alternative physiological pathways, reduced penetration of pesticides through the cuticle and intestine, lower transport of pesticides to the target sites, storage of pesticides in fat body or other inert organs, genetically determined modified behavior in response to pesticide etc. may be factors responsible for conferring resistance to certain insects against high susceptibility of other insect species [23, 88, 89].
Silk worms have a bitrophic relationship with the Dipteran,
Essential oils are volatile mixtures of hydrocarbons with diverse functional groups. Essential oils are defined as any volatile oil(s) that have strong aromatic components and that give distinctive odour, flavor or scent to a plant. These are the byproducts of plant metabolism and are commonly referred to as volatile 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 or fruits in different plants. The aromatic characteristic of essential oils attract or repel insect, protect plant from heat or cold and the chemical constituents of the oil act as defense material. Their probable diverse mode of action and use in food and pharmaceutical industry justify their status as possible environmentally benign candidate for pest management.
5.1. Identification of haemocytes of
A.assama by light microscopic and transmission electron microscopic studies (TEM )studies
In order to understand the possible effects of the extracts of
Plasmatocyte and Oeonocyte
Plasmatoytes are polymorphic cells with round, ovoid,elongate and spindle shaped structures with filopodia. The cells are small to moderate size. The long axis is 25 µm and the short axis is 14.74 µm.Surface of PLs is not smooth and have ridges much flatter than those of granulocytes. Nucleus is dense, round, ovoid or elongated with distinct membrane. The cytoplasm is generally abundant, granular and agranular. Cytoplasm contains a good number of mitochondria and extensive rough endoplasmic reticulum. Golgi body recorded was in less number. Vacuole is rare in normal cell. The nucleus is mostly centrally placed, prominent, usually round but changes shape with the change in shape of the cell.
They are rounded or oval in shape with long and short filopodia. The long axis is 8.382 µm. Two types of membrane bound granules were observed, electron dense and structured. Cytoplasm contains large number of mitochondria, endoplasmic reticulum, free ribosomes, lysosomes and small number of vacuoles. Binucleate granulocyte was also recorded. Nucleus generally small takes various shapes, round, ovoid or irregular. The cell surface contains many elevations and depressions giving a conspicuous pattern.
These are spherical or ovoid or elongated cells with long axis 7.21 µm. The cell surface contains pits.Wavy edges show presence of spherules. Nucleus is mostly centrally placed, sometimes obscured by spherules. Mitochondria, rough endoplasmic reticulum and golgi complex were observed in cytoplasm when spherule number was less.
They are round, oval in shape and long axis is 7.94 µm. The nucleus occupies major area of the cell. Plasma membrane is almost smooth.Cytoplasm is thin, almost homogeneous, contains few number of mitochondria, Golgi bodies and endoplasmc reticulum. SEM shows a spherical surface.
These are large rounded or ovoid cells with eccentric nucleus. Nucleus is small, nucleoli were prominent. The plasma membrane is smooth and regular. Cytoplasm uniformly distributed and contains extensive rough endoplasmic reticulum. Filopods are generally absent. Other cellular organelles like Golgi bodies and mitochondria are less in number.
5.2. TEM studies on effect of petroleum ether extract on haemocytes of late instar larvae
Transmission electron microscopic studies of haemocytes of fifth instar silk worm larvae was done after application of petroleum ether extracts of
Application of petroleum ether extract of
Cell membrane breakdown of PL was observed at 1h of treatment while mitochondria was intact. Filopods are less. But cell attachment was marked. GR cell was less affected and filopod like extension was many toward cell attachment site. Granules were released from the area of damage of the cell membrane. Dissolution of nuclear membrane was observed at certain points. OE, SP, PRO was less affected. At 6h of treatment PLwas less affected. Golgi body was observed at formative phase. Filopods are less in number. Export of material from the cell was observed. GR cell was also less affected. Cell membrane, mitochondria were well preserved. Nucleus in dividing stage was recorded. In some GR, granules release from the cell. Some GR attach to one another. Other cells are less affected. But at 12 h of treatment period, cell membrane breakage, mitochondrial damage vacuolization of PL cell was observed while GR cell was less affected. But again at 48h no effect on cell membrane, nuclear membrane, mitochondria was observed. Golgi was found to be in formative stage. Release of material was not recorded. GR cell was looking intact where cell membrane, mitochondria and nuclear membrane were not affected. Aggregation of cell was less. OE cell membrane was ruptured and released some cytoplasmic materials but mitochondria, nuclear membrane were well preserved. Attachment of OE with other cells was observed. SP and PRO were less affected.
At 1h of treatment of
At 1h of acetone treatment as control almost intact cell membrane, mitochondria, ER, nuclear membrane was observed. Cell attachment was less with few filopods in PL cell. Granules and vesicles number were less and release of cytoplasmic material were not observed in PL cell. But GR cell was affected and plasma membrane rupture and damage in mitochondria was observed. Plenty number of ER but few vesicles were observed. Both types of granules structured and electron dense were observed. Cell aggregation with release of flocculent material was noticed.OE was less affected than PL and GR. But at 6h of acetone treatment PL and GR were found intact with intact plasma membrane, nuclear membrane, normal mitochondria, few vesicles and very less cell attachment. A medium number of filopods were observed but release of cytoplasmic material was not observed. OE cell was also found intact with unaffected mitochondria. After 48h treatment of acetone also, the nuclear membrane and mitochondria of both PL and GR were found intact with less cell attachment. Cell membrane damage of PLwas less. Golgi body was found in formative phase and few number of filopods and vesicles were observed but release of flocculent material was not recorded. Nuclear division of PL cell was observed. Cell membrane rupture of some of the GR cell was recorded with discharge of granule while other cells were intact with almost absence of filopods. Plenty number of ER and large sized vesicles were seen. Both types of granules structured and electron dense were noticed.
A comparative assessment of the effects made showed that the rupture of plasma membrane (PM) in plasmatocyte (PLs) was maximum in treatment with
Plasmamembrane rupture in
Nuclear membrane ruptureA.
Pinocytic vesicle formation of PL at 12 h of treatment with
Cell clumping between PLs and GRs at 12 h of treatment with
PM rupture at 12 hr after treatment with
Normal PLs at 48hr of treatment with
GR at 1hr of
Mitochondria at 15min of
Granulocyte damage at 15min of
Granular material release at 12hr of treatment in
GR breakdown at 1hrof treatment in
GR mitochondria damage at 12hr of treatment in
5.3. Effect of Essential oil on economic characters of silk wrom
Study on impact of essential oils of
Economic characters of silkworm like cocoon weight and pupal weightof treated larvae did not vary significantly from that of control larvae. But significant decrease of shell weightwas recorded after application of essential oil of
6. Mechanism of action of botanicals
The efficacy of plant products depend on presence of specific organic compound (s) which may interfere with the body physiology of the target organism. The compounds may belong to different chemical groups of secondary metabolites of plants viz. Alkaloids, phenolics, flavonoids and terpenoids. The ethanolic extract of
The mechanism of toxic effect of essential oil and oil compounds on insect at present is not well known. Insects vary enormously in their response to different essential oils and oil compounds. Essential oils are liquid in room temperature and get easily transformed from a liquid to a gaseous state at room or slightly higher temperature without undergoing decomposition. The quantity of essential oil found in most plants is 1 to 2% but can contain amounts ranging from 0.01 to 10%. Most essential oils comprise of monoterpenes with 10 carbon atoms, sesuiterpenes with 15 carbon atoms and rarely diterpenes or higher terpenes. The most predominant groups are cyclic compounds with saturated and unsaturated hexacyclic or an aromatic system. Bicyclic and acyclic components are also present. Monoterpenes are common essential oil constituents and several hundred naturally occurring monoerpenes are reported. They are biosynthesized from geranyl pyrophosphate of the isoprenoid pathway. These can be classified into two major groups- monoterpene hydrocarbons and oxygenated monoterpenes. Monoterpene hydrocarbons include acyclic aliphatic, monocyclic aliphatic and dicyclic aliphatic while oxygenated monoterpenes include acyclic monoterpenoid, monocycic monoterpenoid and dicyclic monoterpenoids .Some of the major oil constituents of
The root-bark extract of
The structures of phenolic allelochemicals and their mode of action are diverse . In lepidopteran larvae, phenolic toxicity might occur in the form of oxidative stress . However,
7. The future of prospects of botanicals in seri-ecosystem
An important essence of integrated pest management is to consider the whole ecosystem as the management unit. A seri-ecosystem should necessarily include not only the sericulture field or the silkworm culture units, but also the agricultural field in the neighbouring areas. A consensus approach is to keep the natural ecosystem largely intact. While attempts are made to control pests and pathogens in the neighbouring agricultural fields, issues regarding their impact on sericulture activities must be taken into consideration or vice-versa. Emphasis is to be given on studies associated with efficacy of plant and plant products against pests and pathogens in the whole management unit, their mode and mechanism of action, identification of target sites and use of resistant strains. Life scientist and chemist need to act coherently to make effective use of botanicals. With the knowledge in hand we hypothesize that holistic approach involving in depth research for using botanical in both sericulture field and the other crop systems in the vicinity might be able to provide a green environment to the silk worms.
The authors are grateful to UGC, India for funding part of the work. Thanks to Mr. B.Deka for helping in statistical analysis.
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