APPLIED ZOOLOGY
PEST AND PEST MANAGEMENT Dinesh Kumar
Department of Zoology Centre of Advanced Studies
Banaras Hindu University Varanasi-221 005 CONTENT:
1. Introduction 2. Pest
2.1 Origin of pests
2.2 How pest problems arise 2.3 Key pests
2.4 Occasional pests 2.5 Potential pests 2.6 Migratory pests 2.7 Importance of pests
2.8 Types of phytophagous Insects 2.9 Insect Pests as vectors of Diseases 2.10 Pests of stored grain
2.11 Destructive stages of Insect Pests 2.12 Symptoms of Pest attack
3. Biological Control of Insect Pests 3.1 Predation
3.2 Parasitism 3.3 Insect Diseases
3.4 Insects that feed weedy plants 3.5 Implementation Method 3.6 Classical Biological Control
3.7 Biological control through Augmentation 4. Chemical Control of Insect Pests
4.1 Biochemical background
4.2 Organophosphates (OPs) and Carbamates insecticides inhibit Acetylcholinesterase 4.3 Organochlorines disturb the Axonal transmission
4.4. Third generation pesticides- includes juvenile hormones and its analogues
4.5 Fourth generation pesticides-includes plants’ derived chemicals and other biopesticides 5. Conclusion
6. Further Readiangs
1. INTRODUCTION
Ever since human race came to existence on this planet, they had to compete with other animals for their survival and propagation of the progeny. The humans are of the recent origin (60 m yrs) in the history of evolution of life. Insects are one of the greatest competitors of humans, which evolved much earlier ~300 m yrs in the Paleozoic-Missipian period along with the plants.
It appears that the insects that belong to phylum arthropoda, have dominated the earth after their origin. They are the biologically most successful animals, for they live in the greatest variety of habitats; exhibit diverse types of locomotion; have the widest range of structural variations; eat the greatest variety of food and include the greatest number of species (~60% of the total animals).
During the course of their evolution, the insects have largely adopted as phytophagous mode of feeding. However, some insects adopted as blood feeders along with other modes of feeding behavior. Insects have thus undergone a long and varied period of co-evolution and co- adaptation with their host plants under the evolutionary pressure. Various groups of insects have settled on various hosts so as to avoid competition among themselves. The net result of such evolution is that the insects exist as great threat for humans as they cause economic loss by way of consuming the economically important plants and the stored food grains and valuables like furniture, clothes etc. On the other hand, there are many insect diseases that are transmitted to humans during their feeding. The most common is Malaria, Filaria, Dengue, Chikungunya etc.
Consequently such insects pose a great health hazard for the human society and civilization.
Collectively such animals are known as insect-pests, which are the cause of great concern of the scientists in order to protect the humans form their menace.
2. PEST
Since the beginning of the civilization, man has concentrated on agriculture. About thousand plant species are of economic importance. Approximately 15 species of plants and 9 species of animals could provide almost all the requirements of food for the world’s population. As man was learning this process of life, his battle with his competitors mainly insects had already begun. The competition between man and vertebrate pests, however, is of relatively recent origin.
The word pest has been derived from Latin (pests-i.e. plague). Insects and disease causing organisms including some vertebrates fall in this category. A pest is an organism whose number crosses the threshold value so as to conflict with man’s welfare and economy. The organisms may be an insect, an arachnids, nematode, fungi, bacteria, viroids, viruses, mycoplasma, weeds, angiospermic parasites, rodents or birds etc.
2.1 Origin of pests
It appears that phytophagous insects were originally polyphagous thriving on a variety of food plants within the bounds of their ecosystem. In the Neolithic period, the vegetation and the animals (biomass) on the earth lived more or less in a natural balance. This very idea may gain weight by the fact that permanent forests today, unaffected by man’s influence, exhibit a balanced ecosystem. It may be said that the plants and animals live together having mutual understanding and no species multiplies out of proportion to cause a threat to others.
However, due to cosmic geological upheavals or interference by man in the name of advancing civilization, many species have become extinct or fall in the category of endangered species.
Thus the man made conditions have led certain species to increase to their population densities.
A few thousand species out of approximately 3 million species are pests. In India nearly 500 insect species have been reported to be pests of agricultural importance. Out of this, approximately 75 insect species have been classified as pests of economically important crops.
Cultivation of plants especially monoculture on large scale for economic benefit is the main reason between man and pest. Because this provides a ground for conflict of interests between them. One of the major factors that cause an innocuous population to change into a damaging one is the favourable condition created by abundant supply of food, shelter and breeding grounds for pests in a monotypic culture. Further, breeding of susceptible crop genotype also helps pest outbreaks. Nevertheless, this endeavour of man in his interest has helped the pest inadvertently by removing the parasitoids and predators of the pests from the crop fields that would otherwise have controlled the pest naturally. In other words it could be said that a complex ecosystem has a check on the outbreak of pests.
2.2 How pest problems arise
Agriculture constitutes one of the component parts of advancing civilization. This disrupts the
‘balance of nature’ by planting monotypic crops and storing them in large quantities after harvest. This stored food becomes the desired dens of certain insects to transfer into pest as they possess high fecundity.
The insects become sometimes pests when introduced to a new environment; or when exotic plants are introduced into an environment. For example, the cottony cushion Icerya purchasi, was and still is an insect pest, which uses native Australian shrubs as its hosts. After the introduction of new civilization (agriculture etc) by the Europeans in Australia, it has now several new varieties of crops, particularly citrus sp. (Lemon, orange etc.) Likewise, Leptinotarsa decemilineata (Colorado beetle) living on wild solanum sp. in a restricted area of Rocky mountain as unimportant insect, has become a serious pest of great economic importance after 1839 since the settlers started growing potatoes (in Mexico). In India, a number of insect pests have been introduced accidentally or inadvertently. Example Sanjose, Aspidiotus pernicious and the wooly aphis, Erisoma lanigerum.
Nevertheless, the disturbance of environment has helped promote some minor pests into major pests. Large scale changes brought about by such activities as deforestation, social forestry, large scale engineering, and unscrupulous exploiting of biodiversity have altered the environment in such a way as to help certain pest populations to increase rapidly. Felling of deciduous woodland reduces the number of nestling sites for small insectivorous birds and causes the migration of polyphagous pests of agricultural crops to other areas having suitable host crops.
The capacity of multiplying fast in a given time and space distinguishes a major pest from a minor pest, but not the nature of the damage. Thus the loss rendered by a pest is the function of population density. One becomes a serious pest, when its population reaches a critical level.
The high number of such pest is not required always. For example, for viruliferous insect species, the critical level can be much lower, since a very small population of infected individuals can spread the disease to the whole crop.
2.3 Key pest
Pests whose population is always above the damage threshold. They are also known as regular pests. Example-Coddling moths of apple. Such pests need a constant surveillance and control.
2.4 Occasional pests
Occasional pests or sporadic pests are those that cause economic damage only in certain places or at certain times. Such pests have low population density. However, sudden outbreaks of such pests occur due to various factors favoring population build up. Example-Locusts.
2.5 Potential pests
Which are in general considered as minor pests, because no apparent injury is caused to the crops under the prevailing conditions of the environment? However, under certain situations they may assume serious pest proportions. Example-Fruit tree red spider-mite.
2.6 Migratory pests
These are normally non-residents of a particular ecological habitat. They appear suddenly from habitats of their breeding, cause heavy damage to crops and migrate causing serious damages en route like locusts and army worms.
2.7 Importance of Pests
Pests are known since ages. References to locusts feeding on cereals dates back to the 6th dynasty in Egypt (2625-2475 BC). Pest outbreaks, leading to famine, pestilence, scarcity and economic ruin and then wide ranging economic and social problems, have affected the human race since the down of time. Rome was ruined by bubonic plague in the 2nd Century AD. The potato famine in Ireland between 1845 -1848 occurred as a result of Phytopathora infestans. The devastating incidence of coffee leaf rust (Hemileia vastatrax) in Ceylon around 1870 had far reaching economic and social effects. The calamitous outbreak of insect pests and grape diseases has frequently obliterated wine makers in Europe.
Of late, American cotton, (Gossypium hirsutum L) grown in India is severely being infected by the white fly, Bemisia tamaii and the American Boll Worm, Heliothis sp. leading to complete destruction of cotton. The brown plant hopper ( Nilaparvata lugens) caused extensive damage to the rice crop in Kerala during 1974-75. This insect pest has also caused havoc in other states of India and other south-Asian countries.
“The wide occurrence of insects, mites, nematodes, birds, rodents etc. diseases incited by fungi, bacteria, viruses, mycoplasma etc and various kinds of weeds deprive the cultivator of the potential yield. In recent years, due to ecological disturbances coupled with other causal factors, the incidence of pests has increased and this has underscored the importance of strengthening the production-protection axis on scientific principles. Farmers across the world depend heavily on pesticides to check the pests. But this situation is proving to be non-effective due to the development of resistance in the pests on one hand, and pollution of the environment on the other hand”.
2.8 Types of phytophagous insects
Some common insect pests of economically important crops are listed in table 1.
TABLE 1 SOME COMMON INSECT PESTS
S.No. Name of insect Host plant
1. Sesamia inferens (Pink borer, Lepidoptera) Wheat, paddy, maize, sorghum, sugar cane 2. Chrotogonus sps. (Grasshoppers, Orthoptera) Wheat
3. Schistocerca gregaria (Desert locust, Orthoptera) Wheat 4. Macrosiphum granarin (Wheat aphid, Hemiptera) Wheat 5. Microtermes obesi (Termite, Isoptera) Wheat 6. Chilo partellus (Lepidoptera)
Chilo suppressalis (Lepidoptera)
Wheat
7. Pyrilla perpusilla (Leaf hopper, Hemiptera) Maize, sugar cane 8. Heliothis armigera (Gram pod borer, Lepidoptera) Sorghum, pulses, tomato 9. Nymphula depenctalis (Rice case worm, Lepidoptera) Paddy
10. Leptocorisa varicornis (Gandhi bug, Hemiptera) Paddy
11. Scirpophaga novella (Borer, Lepidoptera) Paddy, sugar cane
12. Clavigrala gibbosa (Hemiptera) Arhar
13. Epilachna vigintioctopunctata (Hadda beetle,
Coleoptera) Potato, tomato
14. Spodoptera litura (Tobacco caterpillar, Lepidoptera) Tomato, cabbage, onion 15. Dysdercus koenigii (Red cotton bug, Hemiptera) Lady’s finger
16. Trichopusia ni (Cabbage semilooper, Lepidoptera) Cabbage
17. Pieris brassicae (Cabbage butterfly, Lepidoptera) Cabbage 18. Myzus pesicae (Aphid, Hemiptera) Mustard
19. Dacus cucurbitae (Fruit fly, Diptera) Dacus dorsalis (oriental fruit fly, Diptera) Dacus zonatus (Fruit fly, Diptera)
Pumpkin
Guava, banana, pear
Custard apple, pomegranate, pear
20. Amritodus atkinsoni (Mango hopper, Hemiptera) Mango
21. Papilio demoleus (Lemmon butterfly, Lepidoptera) Lemmon, oranges 22. Icerya purchasi (Cottony cushion scale, Hemiptera) Lemmon, oranges 23. Quadraspidiotus perniciosus (Sanjose scale,
Hemiptera) Apple, pear
24. Cydia pomonella (Codling moth, Lepidoptera) Apple
25. Sitophilus sp. (Weevil, Coleoptera) Stored grains
26. Corcyra cephalonica (Rice moth, Lepidoptera) Stored grains 27. Tribolium sp. (Flour beetle, Coleoptera) Flour
28. Tenebrio molitor (Meal worm, Coleoptera) Stored grains 29. Callosobruchus chinensis (Gram dhora, Coleoptera) Stored pulses
30. Cockraoches (Dictyoptera) All kinds of food, books etc.
31. Lepisma saccharina (Silver fish) Fabrics, libraries etc.
2.8.1 Monophagous: Such insects are confined to a single species of plants like silk worm capitalizing on mulberry. Nevertheless, monophagous insects usually feed on a group of closely related plants.
Oligophagous: Such insects feed on a group of broadly related plants; usually confined to a single plant family like potato moth (Phthorimea operculella) that feeds on potato, tobacco and some other plants. These plants fall within family Solanaceae. The diamond-back moth (Plutella xylostella) is confined to plants of cabbage family, Cruciferae.
Polyphagous: Those insects that thrive on many plants belonging to diverse families. Such insects prefer to feed on a few species. However, they can feed on others if the preferred plants’
species are not available. Example-Locusts, grasshoppers, hairy caterpillars, gram cutworm, termites, Dysdercus etc.
2.9 Insect Pests as vectors of diseases
2.9.1 Animal diseases: There are insects that act as vectors of human diseases, several of them are serious even fatal. There are more than 150 arthropod species that harbor and transmit
pathogens causing human diseases. Some of these are transmitted during their feeding, others mechanically and still others accidentally. These pathogens belong to nearly 8 classes (Bartonettacea, Rickettsia, Viruses, Spirochetes, Bacteria, Protozoa, Helminthes and Acanthocephala).
Plant diseases: Insects cause diseases by transmitting microorganisms (fungi, bacteria, and viruses) in animals and plants as well. The association of insects and microorganisms are of 3 types: accidental or casual, symbiotic and obligatory.
In accidental association, the pathogens are transmitted while insects are feeding; or when it comes in physical contact with the plants through its body surface. In symbiotic association, both microorganisms and insects mutually benefit from each other. e.g. association between some species of fungi and various bark beetles. In obligatory association, the microorganisms are entirely dependent on the insects for their transmission from plant to plant. E.g. some Hemiptera (aphids) and viruses. Aphids are the main transmitters of viruses to the plants.
2.10 Pests of stored grain
Grains, cereals and pulses are to be stored for considerable long time for human consumption.
During storage, the grains including other agricultural commodities are subjected to various biotic and abiotic stresses. The biological stresses include the agencies like insects, rodents, birds, mites and microbes. The abiotic stresses are moisture, temperature, storage sanitation, types of storage bins or receptacles etc. The scope of the present chapter is limited to insect pests that render great loss to the stored grains. There are more than hundred species of insects, which are responsible for the losses. Some of these insects that are found in India are: Rice Weevil, Lesser grain borer, Khapra beetle, Rice moth, Angoumois grain moth, Rust-red flour beetle, Pulse beetle, etc.
2.11 Destructive stages of insect pests
In Hemimetabola all stages (nymphs and adults) feed on their hosts and damage them. On the other hand, in Holometabola ( for example Lepidoptera) either larval stages or adults or both (Coleoptera, Diptera etc) cause the damage.
2.12 Symptoms of pest attack
Symptoms of insect pest attack differ with their mode of feeding. A phytophagous (leaf feeder) pest will nibble the leaves wither on the margins or on the surface that is easily visible. A leaf- sap sucker will produce pale specks at the points where it is punctured. Such insects often excrete or secrete substance or honeydew that attracts growth of fungus on the leaf surface. This obstructs the sunlight thereby retarding photosynthesis resulting in reduction of yield. On the contrary, a stem borer will produce stunted growth, bunchy tops and dead hearts. A root borer, on the other hand, renders the mooring of the plants so weak that they can easily be pulled out by a speedy wind. Fruit borers are not easily detectable since the holes made by them on the surface are soon healed up removing all traces of their existence. Such insects are only seen after cutting open the fruits. However, a technique has been developed by which the sounds produced by the growing insects could be heard leading to their detection.
3. BIOLOGICAL CONTROL OF INSECT PEST
Biological control is a form of pest control wherein a living organism is used to suppress the pest. This involves the use of natural enemies against the pest. Examples: Parasitoids (Hymenoptera and Diptera mainly) are used against scale pest. Herbivorous insects are used to control the weeds. Pathogens (bacteria, viruses or fungi) are used naturally or applied artificially
(as microbial pesticides) to control certain pests. Biological control agents are living organisms that increase in number through reproduction in response to pest so that in nature the balance between pests – parasitoids is maintained.
To avoid the hazards of insecticides, efforts are being made to use the natural enemies to control the pest populations. Broadly, there are 4 ways to increase the populations of natural enemies: 1.
incorporation of natural enemies; 2. augmentation; 3. conservation and 4. application of microbial pesticides. Biological control has its own advantages as it does not require further capital input and being self-sustaining is a cheaper than pesticides. However, it may be costly in the beginning as the natural enemies might have to import from the other countries. On the contrary, other forms of Biological control may be costly as every year capital has to be put in so it is not permanent.
Biological control as a scientific approach has its long history since 1880s and over the periods news skills and techniques are being added to maximize it. This control mechanism has now been fully conceptualized that has taken long years of study and research. This approach requires detailed knowledge of the biology of pests and their natural enemies. This provides the practical manipulation in the field to keep the animal and plant growth under control. Third could be achieved through predation, parasitism and disease etc.
3.1 Predation
This biological phenomenon is known since ages. However, this became more prominent through observation of lady bird beetle predating upon aphids. Linnaeus in 1752 observed that almost every insect has its predator. He was of the opinion that such predatory insects be caught and used for disinfesting crop plants. Such observation and suggestions by others also formed the fundamental basis for the modern use of augmentative biological control in greenhouses and fields too.
3.2 Parasitism
This is a biological phenomenon wherein an insect especially wasp lays its eggs inside the body of its host (insect pest). The eggs develop therein and come out as adults after killing the host.
Such insects are known as parasitoids. Such observations were first made around 1602 in Europe. Martin Lister (1685) for the first time correctly observed that Ichneumon wasps emerged out from a caterpillar. It was Asa Fitch (1855), who proposed the importation of parasitoid from Europe to America to control wheat midge Sitodiplosis mosellana. The first practical use of the said phenomenon was adopted in 1880s when Cotesia glomerata was brought from Europe to America to control Pieris rapae – a pest of Cabbage that had invaded N.
America in 1860.
3.3 Insect diseases
Study of pathogens of the domesticated insects like silkworm and honey bees initiated the insect pathology. The diseased insects were first demonstrated by Italian scientist, Agostino Basi in 1835, who studied silk worm larvae suffering from disease caused by fungus Beauveria bassiana . Elie Metchnikoff (1884) - a Russian entomologist used for the first time a pathogen (fungus) to control insect pests (sugar beet curcolis). Berliner (1911) -a German scientist observed a bacterial disease of larva of flour moth, and by 1938 this bacterium Bacillus thuringiensis was available in the market as microbial pesticide. These efforts formed the basis of future research of culture of the microbes artificially to be used in the field.
3.4 Insects that feed weedy plant
It was ASAFitch in 1855 who found that some European plants had invaded N. America (like toad fix), which was fed by any insect. He suggested importation of insects from Europe to
suppress these plants. In 1983 a scale insect was brought from Northern India to Southern India to destroy an invasive cactus species (Opuntia vulgaris). Two related cacti (O. stricta and O.
inermis) became havoc in Australia. In 1926, a moth Cactoblastis cactorum was released to destroy these ornamental cacti. Thus these plants were killed by insects by 1932. These results reveal many plants are limited (consumed) by specific insects.
3.5 Implementation method
Present day synthetic insecticides are the gift of Second word war. These chemicals became popular as they were killing insects (including pests) instantaneously and the use of insecticides on crop became a routine by the farmers. This indiscriminate use of chemicals led to destruction of natural predators too. Nevertheless, this rendered many insect pests becoming resistant against the commonly used chemicals. Consequent upon this, one was forced to have the idea of restoration of natural control, while making judicious use of insecticides, which could form the basis of the IPM (Integrated Pest management) in late 1950s. Thus there was a demand for natural enemies that were reared and sold to farmers. This type of management is called Augmentative biological control. In certain crops like apples, wherein the larvae of codling moth (Cydia pomonella) burrow deep in the fruit and cannot be controlled by natural enemies. In such cases a faster acting biological control i.e. microbial pesticides ( B. thuringiensis) can be used.
3.6 Classical biological control
People were interested in moving certain plants (ornamental) from one geographical area to another. This was infested with pest insect. In some case plants themselves may spread and become a problem. These lacked their natural enemies in the new area. Under such conditions, the natural enemies were transported to the new regions to control the insect pests and plants both. Examples are given below.
Cassava Mealybug: Cassava (Manihot esculenta) is a staple food source (rich in starch). This is a native of America, now a basic crop in all tropical countries (Asia to Africa). In 1970, an unknown species of mealy bug infested cassava in W. Africa and spread rapidly throughout the tropical Africa rendering damage to great extent of this crop. Cassava mealy bug was believed to be from the America. The pest was initially an unknown species. With International funding, a cassava mealy bug project was organized. Its parasitoid Encyrtid was Epidinocarsis lopezi was found in Paraguay. This parasitoid was released in the affected areas, which could result in ~95%
permanent control, with no recurring cost without use of pesticides and no damage to native plants.
Water Hyacinth: E. crassipes is a plant used in fish (ornamental) ponds and the world’s worst aquatic weed. This chokes the ponds, rivers etc. Such mats of weed were controlled by two weevils Neohetina eichhorniae and N. bruchi. Such weevils produced dramatic results on a water hyacinth infestation in Kenya in only a few months in 1999.
3.6.1 Natural enemy conservation
All insects and plants are attacked by their enemies. Such natural control is not sufficient to check any invasive species. This may be achieved by importing the natural pest, which later becomes the part of the fauna for that region. However, people disrupting natural control by using pesticides that may harm particularly the natural enemy. The aim of the conservation of Biological Control is to check this loss of natural control due to indiscriminate use of pesticides or habitat simplification.
3.6.2 Effects of pesticides on natural enemies
Before Second World War, a few synthetic pesticides were used. Most available chemicals were inorganic (stomach poisons) based on heavy metals as lead and arsenic, which
could kill the pests after being eaten. Nevertheless, some botanical extracts (rotenone and pyrethrum) were also used. After IInd world war, several synthetic chemicals became available that could kill the insects by physical contact. DDT became one of the choicest insecticides. The use of such chemicals resulted in mass destruction of beneficial insects (natural enemies too).
Rather the natural enemies were more vulnerable to these chemicals as compared to the pests.
The reason of this vulnerability was due to the smaller body size, greater surface area, and lower level of detoxifying enzymes possessed by (parasitic Hymenoptera) natural enemies compared with the pests.
3.6.3 Pest resurgence
Occasionally, farmers found that pests for which they had applied pesticides were more numerous than they had been before application of insecticides. This is known as pest resurgence. The steps in this phenomenon are as follows:
The pest population is reduced by insecticides.
The same insecticide destroys most of the natural enemies that were partially suppressing the pests before the application of the insecticides.
Natural enemies increase their number slowly than the pests after the pesticides residue from the application has degraded to levels unable to kill insects.
In the absence of pesticide and with few remaining natural enemies, the survival and fecundity of the pest population increase, leading to higher densities.
3.6.4 Secondary pest outbreak
A related population process occurs when insecticides applied to suppress a primary pest induce a different species, formerly not damaging, to become a pest. This is called a secondary pest outbreak.
3.6.5 Seeking pesticides compatible with natural enemies
To reduce the destruction of natural enemy populations caused by insecticides, there are two potential suggestions: A. using pesticides that have intrinsically selective action or B . using application systems that are ecologically selective.
3.6.5.1 Selective pesticides
Three kinds of insecticides show compatibility with natural enemies: stomach poisons, systematic pesticides, and insect growth regulators (IGRs). Stomach poisons including Bt and minerals like kryolite are taken along with the foliage by the pest. However, natural enemies walking on treated foliage are not affected. Systemic poisons are applied through the soil and are absorbed by the root. The pest feeding on the crop is affected but the natural enemies escape as they are not feeding on such plants. IGRs are chemicals that mimic or disrupt insect hormones, preventing normal molting. Such materials can be selective if only the pest is likely to be exposed in a susceptible stage.
3.6.5.2 Ecologically selective methods of pesticide use
Manipulation of a pesticide’s formulation, timing or method of application is another method for achieving selectivity in control of pests.
Natural control is ubiquitous and contributes extensively to pest control in most conditions.
Conservation of natural enemies under IPM is the main themes. Farmers prefer this practice as compared to chemicals being safe and cheap.
3.7 Biological control through augmentation
Entomologists and farmers, working together, have developed methods to rear some species of predators and parasitoids that attack insect pests in the fields and greenhouses.
3.7.1 Microbial pesticides
Insects suffer from diseases caused by pathogens (bacteria, viruses, fungi, nematodes and protozoa). Sometimes natural outbreaks of diseases occur that control the pest population.
Microbial control aims to use pathogens as tools to suppress pest. This involves finding of pathogens specific to pest: its easy production; storage of pathogens in ineffective state;
application of pathogens in effective manner. This also involves knowledge of the biology of pathogen to use it as effective control tool.
3.7.1.1 Bacteria
Many of the bacterial that infect insects are lethal only in stressed insects. Because the bacteria, lacking effective means of escaping from the hosts gut after ingestion, are unable to enter its body cavity. Out of four Bacillus thuringiensis, B. sphaericus, B. popililae and Serrati entomophila, only Bacillus thuringiensis has been used widely as pesticide. This produces toxic crystalline proteins inside its spores. These crystals bind to the gut membrane and degrade it, allowing bacteria to penetrate the body cavity and kill the host. New strains of the said bacteria can be used against several insect species. These pathogens are commercially produced in fermentation media without any living hosts rendering it cheaper. Application of Bacillus thuringiensis is advantageous in forests, where residues of insecticides are objectionable as it is harmful to wild life. This is also suitable in IPM, wherein natural enemies are required to stay in the field. Genes from Bacillus thuringiensis that code for toxic proteins, have been isolated and inserted into plants, where they are expressed and produce insecticidal proteins in plant tissues and pollen. Transgenic varieties of major crops like corn, Soya bean and Cotton (Bt) now exist.
Other species of bacteria (B. sphaericus) are used against some species of mosquito larvae. B.
popillae has been used for Japanese beetle. This bacterium requires a living host for its production. S. entomophila can be produced in fermentation media apart from a living host.
4. CHEMICAL CONTROL OF INSECT PESTS
As the human population and civilization advanced, the problem of insect pests also increased. The natural control was, however, not sufficient to check the hazards of pests.
Therefore, chemicals were used to control the harmful insects. Documentation of the ancient use of chemical control of insects appears in Homer’s writings before 1000 BC, wherein a “pest averting Sulphur” was identified as an insecticide. Cato (200 BC) suggested fumigation of grape vines by heating bitumen (mineral pitch). Pliny (AD 79) recommended the use of arsenic as an insecticide. Greek physician Dioscorides (AD 40-90) and the Chinese recommended the use of mercury and arsenic for body louse control and white arsenic was used to protect rice plants from insect attack as early as AD 400. These are a few examples of the many diverse concoctions and substances, which man has used against insect competitors over the centuries. The best available control for many insects of agricultural importance at that time was still hand picking and attempts to frighten insects away.
Fortunately, the chemical tools for insect control have evolved rapidly in the last 100 years or so, starting with inorganic agents such as lime sulphur and arsenicals, and augmented by natural products like pyrethrum, rotenone and nicotine; whose agricultural uses were beginning to be established by 1870. But heavy use of these inorganic substances started showing health hazards.
On the contrary the modern use of insecticides dates back to 1867 when Paris green was first used to control the Colorado potato beetles.
The first recorded use of a synthetic organic insecticide, dinitro-O-cresol, occurred in 1892 and by the 1930s a range of such compounds had been discovered and had found limited use.
From 1920s on, the increasing power of insecticides as tools for insect control was realized.
After 1939 with the spectacular success of DDT (dichloro diphenyl trichloroethane) revealed by Paul Muller (Nobel Laureate), the synthetic insecticides became the most popular chemical tools with unprecedented power and range of activity. This led to the chemical approach to insect control. In 1941-42, English and French scientists discovered BHC ( benzene hexachloride or Gammexane). During the same time Germans developed organophosphates as one of the very strong chemical warfare means. Since then the various chemical companies started manufacturing a varieties of chemicals and almost every day a new chemical is being synthesized to meet the challenges posed by the insects. Nevertheless, scientists have been putting efforts to synthesize effective chemicals against the insect pests. These insecticides could be categorized into the following four categories:
First generation pesticides- includes inorganic insecticides
Second generation pesticides-includes organophosphates, organochlorines etc.
Third generation pesticides- includes juvenile hormones and its analogues
Fourth generation pesticides -includes plants’ derived chemicals and other biopesticides.
Normally in India, the second generation insecticides are used indiscriminately by the farmers as they are very cheap and effective. A cursory knowledge is required to understand the mode of action of these insecticides. To understand this, one has to have a biochemical background.
4.1 Biochemical background
All most all the commercially available insecticides work by interfering with the passage of impulses in nervous system. Nervous transmission is a suitable target for a pesticide since it is the basis of co-ordination system, which the animal cannot do without even for short periods.
The basic molecular events in the nervous system of all animals including mammals seem to be rather similar. The nerve impulse passes down the long axon of a nerve cell as a result of sequential changes in permeability of the axon membrane to Na+ & K+ ions. Under resting conditions the electrical potential inside the membrane is –ve with respect to the outside, and the concentration of Na+ inside the nerve is low, while the concentration of K+, relative to the outside is high. A nerve impulse is a wave of changing polarity of the axon so that polarity is completely reversed. Thus the potential becomes +ve on the inside of the axon. This nerve impulse is caused by two factors: 1. the sodium gate in the axon opens so that Na+ passes in and 2. the potassium gate opens so that K+ passes out from inside of the axon with a restoration of the electrical status quo.
When the impulse has passed, the Na+ must be pumped out from inside of the axon, and the K+
pumped in from outside to restore the resting condition of the axon. This is performed by a single pump situated in the axonal membrane, which ejects Na+ and simultaneously takes in K+, the so called sodium/potassium exchange pump. During this process two kinds of ion movement takes place: 1-a fast downhill (passive) flux through gates or channels and 2-a slower uphill (active) movement by pumps. The passage of this wave of changing polarity down the axon can be regarded as an essentially electrical process in which the current is carried by ions. When the axons meets either another nerve cell or an effector cell, there is a junction usually some 10-20 nm wide, known as synapse.
Fig. 1. Diagrammatic representation of an uni-polar neuron showing synapse with another effector organ or neuron.
Action potentials arise in the axon hillock and are conducted towards the ax0n terminal.
Transmission of the nerve impulse across the synapse is, in almost all cases, chemical in nature.
Where as the axonal conduction is electrical. On reaching the synapse the wave causes the release of chemical transmitter from pre-synaptic membrane of the axon. This transmitter is usually acetylcholine, which combines with acetylcholine receptor or with the enzyme acetylcholinesterase being present on both the pre- and post synaptic membrane.
Fig. 2 Diagram of a Nerve Impulse
The combination of acetylcholine with the receptor causes the post synaptic cell either to pass an impulse through a nerve cell, or to do work by an effector cell. The enzyme acetylcholinesterase has the function of destroying the acetylcholine so that stimulation of receptor stops, and synapse becomes available for the release of a new transmitter. There is a general agreement that acetylcholine is a major, but not the sole chemical transmitter across synaptic junctions in vertebrates. There are other transmitters like L-glutamate, and Y aminobutyric acid and etc.
The majority of the commercially available insecticides have their effects on the nervous tissue in the following manner:
Acetylcholinesterase- most important insecticides in current use have this as their site of action
Axonal transmission- A few insecticides like organochlorines disturb the flow of the Na+ and K+ ions across the axons.
Acetylcholine Receptor- a few insecticides bind with the receptor on the post-synaptic membrane so that the axonal transmission is affected.
Mechanism of action of insecticides that act on acetylcholinestrase 4.2 Organophosphates (OPs) and Carbamates insecticides inhibit Acetylcholinesterase
These insecticides owe great economic significance. The discovery of the insecticidal action of these compounds was made in Germany during the 2nd world war. A number of such compounds, often referred to as Nerve gases, were prepared. These were known as tabun, sarin and soman etc. and were available in tank-cars by the end of the 2nd world war. These constitute a very large class of compound and more than 100,000 of them have been made. Nevertheless, new ones are being prepared every month. These are one class of compounds whose action is known. To understand their action, one should know their nomenclature and chemistry.
4.2.1 Nomenclature and chemistry of organophosphate insecticides
The pesticides Manual (Martin, 1972) lists ~502 pesticides in use at present- 92 are Ops and 22 are carbamate. Nomenclature for such pesticides was given by Martin in 1972.
Fig. 3 Nomenclature of the Ops and Carbamates depends upon the chemistry of the phosphorus atom in the compound
During the action of Ops two non-enzymatic reactions take place as below:
1. Ops are susceptible to alkaline hydrolysis due to attack of OHֿ ion on the relative +ve phosphorous. A similar reaction of this type occurs during Acetylcholinesterase inhibition, where OH¯ group of a serine amino acid residue at the active site of the enzyme attacks the phosphorous of the organophosphate (OP). It causes withdrawal of electron from phosphorous atom of OP. This results in more +ve phosphorous that promote the following reactions:
hydrolytic instability
reaction with acetylcholinesterase
Fig. 4 Diagram showing attraction of electron (e) by the active site of the enzyme from the phosphorus of organophosphate (Op).
Ops may undergo isomerizations from thiono to thiolo as shown below.
Fig. 5 Reaction showing isomerization of thiono (Parathion) to thiolo (S-ethyl Parathion) organophosphate
The effect of this isomerization is to convert P=S to P=O, which is more electron attracting than
=S. Therefore the phosphorous atom becomes more electron deficient. Thus thiono to thiolo conversion results in a compound that has following two properties.
The compound is more susceptible to hydrolysis and It is more active inhibitor of acetylcholinesterase
Due to the above reasons phosphorothionates ( Parathion etc.) are the latent or indirect inhibitors of acetylcholinesterase, and only act directly on the enzyme after conversion to P=O form in the body of the animal or pest. The above chemical isomerization is slow at normal rate. But this may take place fast enzymatically in the body of the organism. This constitutes an example of Lethal Synthesis. Since organophosphates are manufactured at elevated temperature, phosphorothionates are likely to be contaminated with thiolo isomers causing direct inhibition of acetylcholinesterase.
4.2.2 Properties of acetylcholinesterase
The site of action of all organophosphates and carbamates acting on the adult insect is the enzyme acetylcholinesterase (EC 3.1.1.7) that hydrolyses the synaptic transmitter- acetylcholine as shown below.
Fig. 6 Reaction showing hydrolysis of acetylcholine into choline and acetic acid by acetylcholinesterase
This enzyme is the principal enzyme responsible for the hydrolysis of acetylcholine in insects and vertebrates. However, it differs from acetylcholinesterase (EC 3.1.1.8) found mainly in vertebrates’ blood plasma. This particular enzyme has no known function, and is not the target for organophosphates or carbamates. Electron microscopic studies reveal that acetylcholinesterase (EC 3.1.1.7) is located on the pre- and postsynaptic membranes of the insect synapse (Smith and Treherne, 1965). Therefore, it is difficult to solubilize and renders its purification difficult. However, the purified enzyme could be obtained from a variety of sources like head of housefly, intact honey bees and boll weevils etc.
4.2.3 Mechanism of acetylcholinesterase reaction
Hellenbrand and Krupka (1970) proposed a mechanism of action for house fly head acetylcholinesterase, which is essentially the same as that of the vertebrate enzyme.
Fig. 7 Diagrammatic representation of the reaction of acetylcholinesterase and acetylcholine.
(EH= Enzyme, ACh = acetylchoiline, EH.ACh = intermediate reversible complex between enzyme and substrate, EA= acetylated enzyme, ChH= choline, AOH= acetic acid)
The enzyme initially forms a complex (the Michaelis complex) with the substrate and then acetylates with the release of choline. Deacetylation occurs by the reaction of water with acetylated enzyme forming acetic acid and the original free enzyme. This can be shown in a more pictorial form as follows:
Fig. 8 Pictorial form of the reaction showing comparison of enzyme with acetylcholine and
The acetylcholinesterase has 2 binding sites as shown below.
Fig. 9 Diagrammatic representation of two sites (anionic and esteratic) of the acetylcholinesterase that binds with acetylcholine.
Anionic site- has glutamate residue, which interacts with positively charged N atom of acetylcholine
Esteratic site- is responsible for the cleavage of the ester link of acetyl. It contains a serine residue, whose nucleophilicity is enhanced by hydrogen binding to the imidazole group of a neighboring histidine residue.
4.2.4 History of the mechanism of inhibition of acetylcholinesterase by organophosphates and carbamates
It is interesting to trace the history of the discovery of the site of action. Eserine (Physostigmine) is a naturally occurring carbamate. This chemical has long been known to cause constriction of the pupil of the eye. In 1930 it was demonstrated that eserine inhibits acetylcholinesterase. This provided the clue for the site of action of the insecticidal carbamates that were discovered during 2nd world war. Discovery of the site of action of organophosphates stemmed from the secret work on their use as potential warfare agents. In 1941, diisopropyl phosphorofluoridate was synthesized by Saunders group in Cambridge. This compound was shown to cause the constriction of the eye pupil too. Thus by analogy it was suggested to inhibit acetylcholinesterase, which was later proved the same experimentally.
4.2.5 Effect of organophosphates and carbamates on the insect
These chemicals kill the insects by inhibiting acetylcholinesterase. This causes building up of acetylcholine levels as high as upto 260% of normal condition. Eventually this condition causes a block in nervous transmission. Organophosphate and carbamate poisoned insects show initial hyperactivity, followed by convulsive and uncoordinated movements. This leads to paralysis and finally death occurs. Further these insecticides cause release of neurohormones that are responsible for the control of urinary output and the plasticity of the cuticle. Thus these insecticides kill the insects by causing a general release of hormones from the nervous system.
Further, organophosphates are commercially significant as ovicides that control the outbreak of the pest at the egg stage. In this case the organophosphates inhibit the egg lipase, which is required for the development of the egg. However, acetylcholinesterase does not appear at the egg stage.
4.3 Organochlorines disturb the axonal transmission
Dichlodiphenyl trichloroethane (DDT) and related compounds constitute this class of compound. The empirical formula of DDT is C14H9ClS.
Fig. 10 Structure of DDT
It was first described by a German chemist, Othmar Zeidler in 1874 and its insecticidal properties were discovered in 1939 by Paul Muller of Switzerland who was awarded Nobel Prize in 1948. The actual mode of action of DDT is not yet known. It shows its effect by binding to the nerve membrane, which interferes with ion movement into and out of the axons. DDT also affects other membrane linked functions like the Hill reaction in chloroplast and the oxidative phosphorylation in mitochondria. DDT exerts a general effect on membranes but due to its peculiar molecular structure it is particularly active on the axonal membrane. It is believed that it has “molecular wedge” and the apex of the wedge is -CCl3 moiety. The base of wedge binds to the protein portion of axonal membrane and –CCl3 keeps open a gate situated in axonal membrane for Na+ ions, thus interfering with the ionic basis of normal conduction. It has been further found that DDT affects a specific ATPase of the nerve, as this enzyme is perhaps involved in the transport of ions in the nerve.
4.4 Third generation pesticides- includes juvenile hormones and its analogues
Since its discovery by Wiggelsworth (1934, 1936), juvenile hormone (JH) has been known to be critical for the regulation of both metamorphosis and reproduction in insects.
Fig. 11 a. Natural JH in insects. b JH analogues used as insect growth regulators
Although initially thought to be released by the brain, later it was shown to be secreted by corpora allata (CA), a pair of glands in the head just behind the brain. JH was isolated by Williams in 1956 and chemically identified as a sesquiterpenoid (Roller et al, 1967). There are four types of JH. JH O, JH I and JH II being found in Lepidoptera and JH III in most other insects. JH III is found in higher Diptera. The action of these natural hormones can be mimicked by a number of small lipophilic molecules as shown in above figure.
Juvenile hormone analogues act as insect growth regulators. Williams (1956) reported use of these hormones as a pesticide, since it could be absorbed through the insect’s cuticle resulting in death. Since it is natural to insects, it would presumably be refractory to develop resistance.
Slama and Williams (1965) discovered the “paper factor”, a JH analogue, found in balsam trees, indicated their use in selective control of insects. Thus they were regarded as “third generation
pesticides (Williams, 1967). Many such compounds were synthesized and were found to be biologically active against the growth of insects. They were termed as insect growth regulators (IGRs). Amongst the JHAs, the most active are fenoxycarb and pyriproxyfen. These are aromatic compounds with little apparent resemblance to JH. However, the biological effects of various JHAs are similar to those of natural JH suggesting their common mode of action. It has been now found that the insects have become resistant to JHAs and therefore these compounds are no more suitable for the insect pest control. Further, these compounds are costlier than the organophosphates and Organochlorines making them beyond the reach of poor Indian farmers.
Some commonly used synthetic insecticides are shown in table 2.
TABLE 2: SOME COMMONLY USED INSECTICIDES S. N0. Name of insecticide Nature of insecticide 1. DDT(dichlorodiphenyl trichloroethane) Organochlorine
2. Endosulfan “
3. Aldrin (now banned) “
4. Lindane “
5. BHC Organophosphate
6. Methyl Parathion “
7. Malathion “
8. Ethoprop “
9. Acephate “
10. Chlorpyriphos “
11. Carbaryl Carbamate
12. Carbofuran “
13. Allethrin Pyrethroids (attacks
Nervous system)
14. Fenavalerate “
15. Cartap Unclassified (blocks
ganglionic transmission)
16. Diflubenzuron IGR (inhibits chitin
formation)
17. Methoprene IGR (mimics JH)
18. Dicofol Acaricides
19. Oxythioquinox “
4.5 Fourth generation pesticides-includes plants’ derived chemicals and other biopesticides There are evidences that most phytophagous insects are inhibited from feeding the secondary compounds in non-host plants. This is a very valid step to exploit such substances for protection of our food crops. Indeed since the dawn of civilization man has used plant materials to combat insect pests or alleviate the damage they cause. Records show that before 1850, 20 plant species belonging to 16 different families were used for control of agricultural and horticultural pests in Western Europe and China. A resurgent interest in plant derived chemicals to control insect pests stems from the fact that these pesticide products have less negative environmental and health impacts than the highly effective synthetic insecticides mostly have.
Some insecticides of pant origin were used on a large scale before they were replaced by synthetic insecticides. Nicotine, rotenone and pyrethrins have been extensively used and were effective as they degrade rapidly and do not accumulate in the food chain. The natural insecticides are less toxic to mammals as compared to synthetic insecticides. However, they cause resistance in the target insects. In addition, non-target natural enemies are also at risk.
Compounds that modify the behavior of the target species and have primarily a non-toxic mode of action may in the long term provide the most dependable and environmentally safe method of chemical control. Such phyto-chemicals include attractants, repellants and deterrents. Some plant origin derivatives with insecticidal properties are listed in table 3.
TABLE 3 SOME PLANT ORIGIN DERIVATIVES WITH INSECTICIDAL PROPERTIES
S. No. Name of plant Name of compound
1. Neem (Azadirachta indica) Azadirachtin (antifeedant & IGR) 2. Ageratum houstonianum Precocenes (anti JH)
3. Ferns (Polypodium vulagre) Phytoecdysone (IGR) 4. Custard apple (Annona squamosa) Juvenoids 5. Tamarind (Tamarindus indica) “
6. Guava (Psidium guajava) “ 7. Amla (Phylanthus embica) “
8. Chrysanthemum cinerariaefolium Pyrethrins (contact insecticide or stomach poison)
4.5.1 Antifeedants
Feeding deterrents or ‘antifeedants’ are chemicals that when perceived, reduce or prevent insect feeding. Such compounds in plants reduce damage through feeding. The insect pests in response to the antifeedants reduce food intake, which may either lead to leaving the plant or to adverse effects on growth, development survival and reproduction. In the presence of such compounds, insect may starve to death and females do not lay eggs before they find an untreated host. Azadirachtin (derived from neem tree) is one of the strongest antifeedants. It inhibits feeding in the Polyphagous desert locust.
Such compounds should have several essential properties like very low toxicity to vertebrates, no phytotxicity, at lower concentrations, effective in several insect pests, harmless to natural enemies, easy penetration in host plants and long shelf life etc. So far ~400000 such plants’ compounds have been tested on a limited number of insect pests, and several compounds are yet to be tested and discovered. Some of the promising antifeedants are Meliacins (Meliaceae), Drimanes (Polygonum hydropiper), Limonoids (Citrus paradise) and Azadirachtin.
Amongst all these azadirachtin needs special mention.
4.5.1.1 Azadirachtin: It has been known for centuries that neem trees have many remarkable properties, including a strong repellency to many insects. With the advent of DDT and many other broad spectrum synthetic insecticides, neem as a potential source of chemicals to manipulate insects remained unnoticed until a German entomologist, H. Schmutterer, in 1970 stimulated researchers from all over the world to launch studies on the useful properties of neem.
The neem tree, Azadirachta indica (Meliaceae; mahogamy family), probably native to Burma, has been widely cultivated for a long time in tropical Asia and Africa and now in Central America.
Fig. 12 Structural formula of azadirachtin.
Azadirachtin, a potent antifeedant and insect growth regulator, is one of the triterpenes. It is a highly oxidized limonoid with many reactive functional groups in close proximity to each other. It is a very potential antifeedant to many lepidopterous larvae and several Orthopetera. It also has pronounced physiological effects as after ingestion it causes growth inhibition, malformation and mortality due to interference with insect’s endocrine system. The left half of the azadirachtin molecule is the antiendocrine part whereas the right half is responsible for antifeedant activity. It also affects the food utilization through the inhibition of digestive enzymes. It fulfills the requirements of an ideal antifeedant. There is compelling evidence that the neem tree has the potential to contribute to solve global problems’.
5. CONCLUSION
The indiscriminate use of synthetic insecticides on a large scale has created havoc in the environment thereby posing a great to humans and other organisms. Therefore, we must stop their use and look for the biological control of insect pests. The common principle for successful insect control based on biological principles is diversification of crop cultivation. Diversification practices like crop rotation and inter cropping reduce the risk of insect damage. Antifeedants are more effective when applied in mixtures that affect various behavioral and physiological mechanisms of insect pests.
Further Readings
1.Corbett. J. R. (1974) The biochemical mode of action of pesticides, Academic Press.
2. Wilkinson, C. F. (1976)Insecticide Biochemistry and Physiology, Plenum, New York.
3.Rockstein, M. (1978) Biochemistry of Insect, Academic Press.
4. Kerkut, G. A. and Gilbert, L. I. (1985) Comprehensive Insect Physiology, Biochemistry and Pharmacology, Vol. 12, Pergamon Press.
5. Resh, V. H. and Cade, R. T. (2003) Encyclopedia of Insects, Academic Press.
